text:
December 27

Pre-1600

 * 537 – The second Hagia Sophia in Constantinople is consecrated.
 * 1512 – The Spanish Crown issues the Laws of Burgos, governing the conduct of settlers with regard to native Indians in the New World.
 * 1521 – The Zwickau prophets arrive in Wittenberg, disturbing the peace and preaching the Apocalypse.

1601–1900

 * 1655 – Second Northern War/the Deluge: Monks at the Jasna Góra Monastery in Częstochowa are successful in fending off a month-long siege.
 * 1657 – The Flushing Remonstrance articulates for the first time in North American history that freedom of religion is a fundamental right.
 * 1703 – Portugal and England sign the Methuen Treaty which allows Portugal to export wines to England on favorable trade terms.
 * 1814 – War of 1812: The destruction of the schooner USS Carolina (1812) brings to an end Commodore Daniel Patterson's makeshift fleet, which fought a series of delaying actions that contributed to Andrew Jackson's victory at the Battle of New Orleans.
 * 1831 – Charles Darwin embarks on his journey aboard HMS Beagle, during which he will begin to formulate his theory of evolution.
 * 1836 – The worst ever avalanche in England occurs at Lewes, Sussex, killing eight people.
 * 1845 – Ether anesthetic is used for childbirth for the first time by Dr. Crawford Long in Jefferson, Georgia.
 * 1845  – Having coined the phrase "manifest destiny" the previous July, journalist John L. O'Sullivan argued in his newspaper New York Morning News that the United States had the right to claim the entire Oregon Country.

1901–present

 * 1911 – "Jana Gana Mana", the national anthem of India, is first sung in the Calcutta Session of the Indian National Congress.
 * 1918 – The Great Poland Uprising against the Germans begins.
 * 1918  – Ukrainian War of Independence: The Revolutionary Insurgent Army of Ukraine occupies Yekaterinoslav and seizes seven airplanes from the UPRAF, establishing an Insurgent Air Fleet.
 * 1922 – JAPANESE AIRCRAFT CARRIER Hōshō becomes the first purpose built aircraft carrier to be commissioned in the world.
 * 1927 – Kern and Hammerstein's musical play Show Boat, considered to be the first true American musical play, opens at the Ziegfeld Theatre on Broadway.
 * 1929 – Soviet General Secretary Joseph Stalin orders the "liquidation of the kulaks as a class".
 * 1932 – Radio City Music Hall, "Showplace of the Nation", opens in New York City.
 * 1935 – Regina Jonas is ordained as the first female rabbi in the history of Judaism.
 * 1939 – The 7.8 Erzincan earthquake shakes eastern Turkey with a maximum Mercalli intensity of XI (Extreme). At least 32,700 people were killed.
 * 1939  – Winter War: Finland holds off a Soviet attack in the Battle of Kelja.
 * 1945 – The International Monetary Fund is created with the signing of an agreement by 29 nations.
 * 1949 – Indonesian National Revolution: The Netherlands officially recognizes Indonesian independence. End of the Dutch East Indies.
 * 1966 – The Cave of Swallows, the largest known cave shaft in the world, is discovered in Aquismón, San Luis Potosí, Mexico.
 * 1968 – Apollo program: Apollo 8 splashes down in the Pacific Ocean, ending the first orbital crewed mission to the Moon.
 * 1968  – North Central Airlines Flight 458 crashes at O'Hare International Airport, killing 28.
 * 1978 – Spain becomes a democracy after 40 years of fascist dictatorship.
 * 1983 – Pope John Paul II visits Mehmet Ali Ağca in Rebibbia's prison and personally forgives him for the 1981 attack on him in St. Peter's Square.
 * 1985 – Palestinian guerrillas kill eighteen people inside the airports of Rome, Italy, and Vienna, Austria.
 * 1989 – The Romanian Revolution concludes, as the last minor street confrontations and stray shootings abruptly end in the country's capital, Bucharest.
 * 1991 – Scandinavian Airlines System Flight 751 crashes in Gottröra in the Norrtälje Municipality in Sweden, injuring 92.
 * 1996 – Taliban forces retake the strategic Bagram Airfield which solidifies their buffer zone around Kabul, Afghanistan.
 * 1997 – Protestant paramilitary leader Billy Wright is assassinated in Northern Ireland, United Kingdom.
 * 2002 – Two truck bombs kill 72 and wound 200 at the pro-Moscow headquarters of the Chechen government in Grozny, Chechnya, Russia.
 * 2004 – Radiation from an explosion on the magnetar SGR 1806-20 reaches Earth. It is the brightest extrasolar event known to have been witnessed on the planet.
 * 2007 – Former Pakistani prime minister Benazir Bhutto is assassinated in a shooting incident.
 * 2007  – Riots erupt in Mombasa, Kenya, after Mwai Kibaki is declared the winner of the presidential election, triggering a political, economic, and humanitarian crisis.
 * 2008 – Operation Cast Lead: Israel launches three-week operation on Gaza.
 * 2009 – Iranian election protests: On the Day of Ashura in Tehran, Iran, government security forces fire upon demonstrators.
 * 2019 – Bek Air Flight 2100 crashes during takeoff from Almaty International Airport in Almaty, Kazakhstan, killing 13.

Pre-1600

 * 1350 – John I of Aragon (d. 1395)
 * 1390 – Anne de Mortimer, claimant to the English throne (d. 1411)
 * 1459 – John I Albert, King of Poland (d. 1501)
 * 1481 – Casimir, Margrave of Brandenburg-Bayreuth, Margrave of Bayreuth (d. 1527)
 * 1493 – Johann Pfeffinger, German theologian (d. 1573)
 * 1566 – Jan Jesenius, Bohemian physician, politician and philosopher (d. 1621)
 * 1571 – Johannes Kepler, German mathematician, astronomer, and astrologer (d. 1630)
 * 1572 – Johannes Vodnianus Campanus, Czech poet, playwright, and composer (d. 1622)
 * 1584 – Philipp Julius, Duke of Pomerania (d. 1625)
 * 1595 – Bohdan Khmelnytsky, hetman of Ukraine (d. 1657)

1601–1900

 * 1622 – Teofil Rutka, Polish philosopher (d. 1700)
 * 1633 – Jean de Lamberville, French missionary (d. 1714)
 * 1636 – William Whitelock, English gentleman, Member of Parliament (d. 1717)
 * 1637 – Petar Kanavelić, Venetian writer (d. 1719)
 * 1645 – Giovanni Antonio Viscardi, Swiss architect (d. 1713)
 * 1655 – Abstrupus Danby, English politician (d. 1727)
 * 1660 – Veronica Giuliani, Italian Capuchin mystic (d. 1727)
 * 1663 – Johann Melchior Roos, German painter (d. 1731)
 * 1683 – Conyers Middleton, English priest and theologian (d. 1750)
 * 1689 – Jacob August Franckenstein, Encyclopedia editor, professor (d. 1733)
 * (baptised) 1692 – Francis Blake Delaval, British Royal Navy officer and Member of Parliament (d. 1752)
 * 1697 – Sollom Emlyn, Irish legal writer (d. 1754)
 * 1705 – Prince Frederick Henry Eugen of Anhalt-Dessau, German prince of the House of Ascania (d. 1781)
 * 1713 – Giovanni Battista Borra, Italian architect and engineer (d. 1770)
 * 1714 – George Whitefield, English preacher and saint (d. 1770)
 * 1715 – Philippe de Noailles, French general (d. 1794)
 * 1721 – François Hemsterhuis, Dutch philosopher and author (d. 1790)
 * (baptised) 1761 – Michael Andreas Barclay de Tolly, Russian field marshal and politician, Governor-General of Finland (d. 1818)
 * 1773 – George Cayley, English engineer and politician (d. 1857)
 * 1776 – Nikolay Kamensky, Russian general (d. 1811)
 * 1797 – Ghalib, Indian poet (d. 1869)
 * 1797  – Charles Hodge, American theologian (d. 1878)
 * 1803 – François-Marie-Thomas Chevalier de Lorimier, Canadian activist (d. 1839)
 * 1809 – Alexandros Rizos Rangavis, Greek poet and politician, Foreign Minister of Greece (d. 1892)
 * 1822 – Louis Pasteur, French chemist and microbiologist (d. 1895)
 * 1823 – Mackenzie Bowell, English-Canadian journalist and politician, 5th Prime Minister of Canada (d. 1917)
 * 1827 – Stanisław Mieroszewski, Polish-born politician, writer, historian and member of the Imperial Council of Austria (d. 1900)
 * 1832 – Pavel Tretyakov, Russian businessman and philanthropist, founded the Tretyakov Gallery (d. 1897)
 * 1838 – Lars Oftedal, Norwegian priest, social reformer, politician, and newspaper editor (d. 1900)
 * 1858 – Juan Luis Sanfuentes, Chilean lawyer and politician, 17th President of Chile (d. 1930)
 * 1863 – Louis Lincoln Emmerson, American lawyer and politician, 27th Governor of Illinois (d. 1941)
 * 1864 – Hermann-Paul, French painter and illustrator (d. 1940)
 * 1878 – Kalle Korhonen, Finnish politician (d. 1938)
 * 1879 – Sydney Greenstreet, English-American actor (d. 1954)
 * 1882 – Mina Loy, British modernist poet and artist (d. 1966)
 * 1883 – Cyrus S. Eaton, Canadian-American businessman and philanthropist (d. 1979)
 * 1888 – Thea von Harbou, German actress, director, and screenwriter (d. 1954)
 * 1892 – Alfred Edwin McKay, Canadian captain and pilot (d. 1917)
 * 1896 – Louis Bromfield, American author and theorist (d. 1956)
 * 1896  – Maurice De Waele, Belgian cyclist (d. 1952)
 * 1896  – Carl Zuckmayer, German author and playwright (d. 1977)
 * 1898 – Inejiro Asanuma, Japanese politician (d. 1960)
 * 1900 – Hans Stuck, German racing driver (d. 1978)

1901–present

 * 1901 – Marlene Dietrich, German-American actress and singer (d. 1992)
 * 1901  – Irene Handl, English actress (d. 1987)
 * 1904 – René Bonnet, French racing driver and engineer (d. 1983)
 * 1905 – Cliff Arquette, American actor and comedian (d. 1974)
 * 1906 – Oscar Levant, American pianist, composer, and actor (d. 1972)
 * 1907 – Asaf Halet Çelebi, Turkish poet (d. 1958)
 * 1907  – Sebastian Haffner, German journalist and author (d. 1999)
 * 1907  – Mary Howard, English author (d. 1991)
 * 1907  – Conrad L. Raiford, American baseball player and activist (d. 2002)
 * 1907  – Willem van Otterloo, Dutch conductor and composer (d. 1978)
 * 1909 – James Riddell, English skier and author (d. 2000)
 * 1910 – Charles Olson, American poet and educator (d. 1970)
 * 1911 – Anna Russell, English-Canadian singer and actress (d. 2006)
 * 1913 – Elizabeth Smart, Canadian poet and novelist (d. 1986)
 * 1915 – William Masters, American gynecologist, author, and academic (d. 2001)
 * 1915  – Gyula Zsengellér, Hungarian-Cypriot footballer and manager (d. 1999)
 * 1916 – Werner Baumbach, German pilot (d. 1953)
 * 1916  – Cathy Lewis, American actress (d. 1968)
 * 1917 – Buddy Boudreaux, American saxophonist and clarinet player (d. 2015)
 * 1917  – T. Nadaraja, Sri Lankan lawyer and academic (d. 2004)
 * 1917  – Onni Palaste, Finnish soldier and author (d. 2009)
 * 1918 – John Celardo, American captain and illustrator (d. 2012)
 * 1919 – Charles Sweeney, American general and pilot (d. 2004)
 * 1920 – Bruce Hobbs, American jockey and trainer (d. 2005)
 * 1921 – John Whitworth, English countertenor (d. 2013)
 * 1923 – Bruno Bobak, Polish-Canadian painter and educator (d. 2012)
 * 1923  – Lucas Mangope, South African politician (d. 2018)
 * 1924 – Jean Bartik, American computer scientist and engineer (d. 2011)
 * 1924  – James A. McClure, American soldier, lawyer, and politician (d. 2011)
 * 1925 – Michel Piccoli, French actor, singer, director, and producer (d. 2020)
 * 1926 – Jerome Courtland, American actor, director, and producer (d. 2012)
 * 1927 – Antony Gardner, English engineer and politician (d. 2011)
 * 1927  – Nityanand Swami, Indian lawyer and politician, 1st Chief Minister of Uttarakhand (d. 2012)
 * 1927  – Audrey Wagner, American baseball player, obstetrician, and gynecologist (d. 1984)
 * 1930 – Marshall Sahlins, American anthropologist and academic (d. 2021)
 * 1930  – Wilfrid Sheed, English-born American novelist and essayist (d. 2011)
 * 1931 – Scotty Moore, American guitarist and songwriter (d. 2016)
 * 1933 – Dave Marr, American golfer (d. 1997)
 * 1934 – Larisa Latynina, Ukrainian gymnast and coach
 * 1934  – Jeffrey Sterling, Baron Sterling of Plaistow, English businessman
 * 1935 – Michael Turnbull, English bishop
 * 1936 – Phil Sharpe, English cricketer (d. 2014)
 * 1936  – Eve Uusmees, Estonian swimmer and coach
 * 1939 – John Amos, American actor
 * 1940 – David Shepherd, English cricketer and umpire (d. 2009)
 * 1941 – Miles Aiken, American basketball player and coach
 * 1941  – Mike Pinder, English singer-songwriter and keyboard player (d. 2024)
 * 1941  – Nolan Richardson, American basketball player and coach
 * 1942 – Byron Browne, American baseball player
 * 1942  – Thomas Menino, American politician, 53rd Mayor of Boston (d. 2014)
 * 1942  – Ron Rothstein, American basketball player and coach
 * 1943 – Cokie Roberts, American journalist and author (d. 2019)
 * 1943  – Joan Manuel Serrat, Spanish singer-songwriter and guitarist
 * 1943  – Peter Sinfield, English songwriter and producer
 * 1943  – Roy White, American baseball player and coach
 * 1944 – Mick Jones, English guitarist, songwriter, and producer
 * 1946 – Lenny Kaye, American guitarist, songwriter, and producer
 * 1946  – Joe Kinnear, Irish footballer and manager (d. 2024)
 * 1946  – Janet Street-Porter, English journalist and producer
 * 1946  – Polly Toynbee, English journalist and author
 * 1947 – Bill Eadie, American wrestler and coach
 * 1947  – Doug Livermore, English footballer and manager
 * 1947  – Willy Polleunis, Belgian runner
 * 1948 – Gérard Depardieu, French-Russian actor
 * 1949 – Terry Ito, Japanese director, producer, and critic
 * 1950 – Haris Alexiou, Greek singer-songwriter
 * 1950  – Roberto Bettega, Italian footballer and manager
 * 1950  – Terry Bozzio, American drummer and songwriter
 * 1951 – Karla Bonoff, American singer-songwriter
 * 1951  – Ernesto Zedillo, Mexican economist and politician, 54th President of Mexico
 * 1952 – Jay Hill, Canadian farmer and politician
 * 1952  – David Knopfler, Scottish singer-songwriter, guitarist, and producer
 * 1954 – Kent Benson, American basketball player
 * 1954  – Mandie Fletcher, English director, producer, and production manager
 * 1954  – Teo Chee Hean, Singaporean politician and 5th Senior Minister of Singapore
 * 1955 – Brad Murphey, American race car driver
 * 1955  – Barbara Olson, American journalist and author (d. 2001)
 * 1956 – Doina Melinte, Romanian runner
 * 1958 – Steve Jones, American golfer
 * 1959 – Gerina Dunwich, American astrologer, historian, and author
 * 1959  – Andre Tippett, American football player and coach
 * 1960 – Maryam d'Abo, English actress
 * 1960  – Donald Nally, American conductor and academic
 * 1960  – Terry Price, Australian golfer
 * 1961 – Guido Westerwelle, German lawyer and politician, 15th Vice-Chancellor of Germany (d. 2016)
 * 1962 – Mark Few, American basketball player and coach
 * 1962  – John Kampfner, Singaporean journalist and author
 * 1962  – Bill Self, American basketball player and coach
 * 1962  – Sherri Steinhauer, American golfer
 * 1963 – Gaspar Noé, Argentine-French director and screenwriter
 * 1964 – Ian Gomez, American actor
 * 1964  – Theresa Randle, American actress
 * 1965 – Salman Khan, Indian film actor and producer
 * 1966 – Marianne Elliott, English director and producer
 * 1966  – Bill Goldberg, American football player, wrestler and actor
 * 1966  – Eva LaRue, American model and actress
 * 1969 – Jean-Christophe Boullion, French racing driver
 * 1969  – Chyna, American professional wrestler and actress (d. 2016)
 * 1969  – Sarah Vowell, American author and journalist
 * 1970 – Brendon Cook, Australian racing driver and rugby player
 * 1970  – Lorenzo Neal, American football player and radio host
 * 1970  – Naoko Yamazaki, Japanese pilot and astronaut
 * 1971 – Duncan Ferguson, Scottish footballer and coach
 * 1971  – Guthrie Govan, English guitarist and educator
 * 1971  – Savannah Guthrie, American television journalist
 * 1972 – Colin Charvis, Welsh rugby union player and coach
 * 1972  – Kevin Ollie, American basketball player and coach
 * 1972  – Matt Slocum, American guitarist and songwriter
 * 1973 – Wilson Cruz, American actor
 * 1973  – Kristoffer Zegers, Dutch pianist and composer
 * 1974 – Tomáš Janků, Czech high jumper
 * 1974  – Masi Oka, Japanese-American actor and visual effects designer
 * 1974  – Fumiko Orikasa, Japanese voice actress and singer
 * 1974  – Jay Pandolfo, American ice hockey player and coach
 * 1975 – Aigars Fadejevs, Latvian race walker and therapist
 * 1975  – Heather O'Rourke, American actress (d. 1988)
 * 1976 – Nikolaos Georgeas, Greek footballer
 * 1976  – Piotr Morawski, Polish mountaineer (d. 2009)
 * 1976  – Daimí Pernía, Cuban basketball player and hurdler
 * 1976  – Fernando Pisani, Canadian-Italian ice hockey player
 * 1977 – Jacqueline Pillon, Canadian actress
 * 1977  – Chris Tate, English footballer
 * 1978 – Deuce McAllister, American football player
 * 1978  – Lisa Jakub, Canadian actress
 * 1979 – Pascale Dorcelus, Canadian weightlifter
 * 1979  – David Dunn, English footballer and manager
 * 1979  – Carson Palmer, American football player
 * 1980 – Bernard Berrian, American football player
 * 1980  – Claudio Castagnoli, Swiss wrestler
 * 1980  – Dahntay Jones, American basketball player
 * 1980  – Meelis Kompus, Estonian journalist
 * 1981 – David Aardsma, American baseball player
 * 1981  – Emilie de Ravin, Australian actress
 * 1981  – Jay Ellis, American actor
 * 1981  – Moise Joseph, American-Haitian runner
 * 1981  – Patrick Sharp, Canadian ice hockey player
 * 1982 – Erin E. Stead, American illustrator
 * 1983 – Anthony Boric, New Zealand rugby union player
 * 1983  – Cole Hamels, American baseball player
 * 1983  – Jesse Williams, American high jumper
 * 1984 – Andrejs Perepļotkins, Ukrainian-Latvian footballer
 * 1984  – Gilles Simon, French tennis player
 * 1985 – Logan Bailly, Belgian footballer
 * 1985  – Jérôme d'Ambrosio, Belgian racing driver
 * 1985  – Paul Stastny, Canadian-American ice hockey player
 * 1986 – Torah Bright, Australian snowboarder
 * 1986  – Jamaal Charles, American football player
 * 1986  – Shelly-Ann Fraser-Pryce, Jamaican sprinter
 * 1987 – Lily Cole, English model
 * 1988 – Jorge Gutiérrez, Mexican basketball player
 * 1988  – Hera Hilmar, Icelandic actress
 * 1988  – Zavon Hines, Jamaican-English footballer
 * 1988  – Ok Taec-yeon, South Korean singer and actor
 * 1988  – Rick Porcello, American baseball player
 * 1988  – Hayley Williams, American singer-songwriter
 * 1989 – Ingrid Várgas Calvo, Peruvian tennis player
 * 1990 – Max Lindholm, Finnish figure skater
 * 1990  – Jonathan Marchessault, Canadian ice hockey player
 * 1990  – Milos Raonic, Canadian tennis player
 * 1990  – Zelina Vega, American wrestler
 * 1991 – Chloe Bridges, American actress
 * 1991  – Michael Morgan, Australian rugby league player
 * 1991  – Beth Potter, Scottish triathlete and long-distance runner
 * 1991  – Danny Wilson, Scottish footballer
 * 1992 – Joel Indermitte, Estonian footballer
 * 1992  – Maicel Uibo, Estonian decathlete
 * 1993 – Olivia Cooke, English actress
 * 1995 – Timothée Chalamet, French-American actor
 * 1995  – Nick Chubb, American football player
 * 1995  – Mark Lapidus, Estonian chess player
 * 1997 – Vachirawit Chivaaree, Thai actor and singer
 * 1997  – Ana Konjuh, Croatian tennis player
 * 1998 – Luka Garza, American basketball player
 * 1999 – Brock Purdy, American football player

Pre-1600

 * 683 – Gaozong of Tang, founding emperor of the Chinese Tang dynasty (b. 628)
 * 870 – Aeneas of Paris, Frankish bishop
 * 975 – Balderic, bishop of Utrecht (b. 897)
 * 1003 – Emma of Blois, French duchess and regent
 * 1005 – Nilus the Younger, Byzantine abbot (b. 910)
 * 1076 – Sviatoslav II, Grand Prince of Kiev (b. 1027)
 * 1087 – Bertha of Savoy, Holy Roman Empress (b. 1051)
 * 1381 – Edmund Mortimer, 3rd Earl of March, English politician (b. 1352)
 * 1518 – Mahmood Shah Bahmani II, sultan of the Bahmani Sultanate (b. c. 1470)
 * 1543 – George, margrave of Brandenburg-Ansbach (b. 1484)
 * 1548 – Francesco Spiera, Italian lawyer and jurist (b. 1502)

1601–1900

 * 1603 – Thomas Cartwright, English minister and theologian (b. 1535)
 * 1637 – Vincenzo Giustiniani, Italian banker (b. 1564)
 * 1641 – Francis van Aarssens, Dutch diplomat (b. 1572)
 * 1642 – Herman op den Graeff, Dutch bishop (b. 1585)
 * 1656 – Andrew White, English Jesuit missionary (b. 1579)
 * 1660 – Hervey Bagot, English politician (b. 1591)
 * 1663 – Christine of France, Duchess of Savoy (b. 1606)
 * 1672 – Jacques Rohault, French philosopher (b. 1618)
 * 1683 – Maria Francisca of Savoy, Queen consort of Portugal (b. 1646)
 * 1689 – Gervase Bryan, English clergyman (b. 1622)
 * 1693 – Henri de Villars, French prelate (b. 1621)
 * 1694 – Henrik Span, naval officer in the Dutch (b. 1634)
 * 1704 – Hans Albrecht von Barfus, Prussian field marshal and politician (b. 1635)
 * 1707 – Jean Mabillon, French monk and scholar (b. 1632)
 * 1707 – Robert Leke, 3rd Earl of Scarsdale, English earl, politician (b. 1654)
 * 1737 – William Bowyer, English printer (b. 1663)
 * 1743 – Hyacinthe Rigaud, French painter (b. 1659)
 * 1771 – Henri Pitot, French engineer, invented the Pitot tube (b. 1695)
 * 1776 – Johann Rall, Hessian colonel (b. c. 1726)
 * 1782 – Henry Home, Lord Kames, Scottish judge and philosopher (b. 1697)
 * 1800 – Hugh Blair, Scottish minister and author (b. 1718)
 * 1812 – Joanna Southcott, English religious leader (b. 1750)
 * 1834 – Charles Lamb, English essayist and poet (b. 1775)
 * 1836 – Stephen F. Austin, American soldier and politician (b. 1793)
 * 1858 – Alexandre Pierre François Boëly, French pianist and composer (b. 1785)
 * 1895 – Eivind Astrup, Norwegian explorer (b. 1871)
 * 1896 – John Brown, English businessman and politician (b. 1816)
 * 1900 – William Armstrong, 1st Baron Armstrong, English engineer and businessman, founded Armstrong Whitworth (b. 1810)

1901–present

 * 1914 – Charles Martin Hall, American chemist and engineer (b. 1863)
 * 1919 – Achilles Alferaki, Russian-Greek composer and politician, Governor of Taganrog (b. 1846)
 * 1923 – Gustave Eiffel, French architect and engineer, co-designed the Eiffel Tower (b. 1832)
 * 1924 – Agda Meyerson, Swedish nurse and healthcare activist (b. 1866)
 * 1936 – Mehmet Akif Ersoy, Turkish poet, academic, and politician (b. 1873)
 * 1938 – Calvin Bridges, American geneticist and academic (b. 1889)
 * 1938  – Osip Mandelstam, Polish-Russian poet and critic (b. 1891)
 * 1938  – Zona Gale, American novelist, short story writer, and playwright (b. 1874)
 * 1939 – Rinaldo Cuneo, American painter (b. 1877)
 * 1943 – Ants Kurvits, Estonian general and politician, 10th Estonian Minister of War (b. 1887)
 * 1950 – Max Beckmann, German-American painter and sculptor (b. 1884)
 * 1952 – Patrick Joseph Hartigan, Australian priest, author, and educator (b. 1878)
 * 1953 – Şükrü Saracoğlu, Turkish soldier and politician, 6th Prime Minister of Turkey (b. 1887)
 * 1953  – Julian Tuwim, Polish poet and author (b. 1894)
 * 1955 – Alfred Carpenter, English admiral, Victoria Cross recipient (b. 1881)
 * 1956 – Lambert McKenna, Irish priest and lexicographer (b. 1870)
 * 1965 – Edgar Ende, German painter (b. 1901)
 * 1972 – Lester B. Pearson, Canadian historian and politician, 14th Prime Minister of Canada, Nobel Prize laureate (b. 1897)
 * 1974 – Vladimir Fock, Russian physicist and mathematician (b. 1898)
 * 1974  – Amy Vanderbilt, American author (b. 1908)
 * 1978 – Chris Bell, American singer-songwriter and guitarist (b. 1951)
 * 1978  – Houari Boumediene, Algerian colonel and politician, 2nd President of Algeria (b. 1932)
 * 1978  – Bob Luman, American singer-songwriter and guitarist (b. 1937)
 * 1979 – Hafizullah Amin, Afghan educator and politician, 2nd General Secretary of the People's Democratic Party of Afghanistan (b. 1929)
 * 1981 – Hoagy Carmichael, American singer-songwriter, pianist, and actor (b. 1899)
 * 1982 – Jack Swigert, American pilot, astronaut, and politician (b. 1931)
 * 1985 – Jean Rondeau, French racing driver (b. 1946)
 * 1986 – George Dangerfield, English-American historian and journalist (b. 1904)
 * 1986  – Dumas Malone, American historian and author (b. 1892)
 * 1987 – Rewi Alley, New Zealand writer and political activist (b. 1897)
 * 1988 – Hal Ashby, American director and producer (b. 1929)
 * 1992 – Kay Boyle, American novelist, poet, and educator (b. 1902)
 * 1993 – Feliks Kibbermann, Estonian chess player and philologist (b. 1902)
 * 1993  – Evald Mikson, Estonian footballer (b. 1911)
 * 1993  – André Pilette, Belgian racing driver (b. 1918)
 * 1994 – Fanny Cradock, English author and critic (b. 1909)
 * 1994  – J. B. L. Reyes, Filipino lawyer and jurist (b. 1902)
 * 1995 – Shura Cherkassky, Ukrainian-American pianist (b. 1909)
 * 1995  – Genrikh Kasparyan, Armenian chess player and composer (b. 1910)
 * 1997 – Brendan Gill, American journalist and essayist (b. 1914)
 * 1997  – Billy Wright, Northern Irish loyalist leader (b. 1960)
 * 1999 – Michael McDowell, American author and screenwriter (b. 1950)
 * 2002 – George Roy Hill, American actor, director, producer, and screenwriter (b. 1921)
 * 2003 – Alan Bates, English actor (b. 1934)
 * 2003  – Iván Calderón, Puerto Rican-American baseball player (b. 1962)
 * 2004 – Hank Garland, American guitarist (b. 1930)
 * 2007 – Benazir Bhutto, Pakistani politician, Prime Minister of Pakistan (b. 1953)
 * 2007  – Jerzy Kawalerowicz, Polish director and screenwriter (b. 1922)
 * 2007  – Jaan Kross, Estonian author and poet (b. 1920)
 * 2008 – Delaney Bramlett, American singer-songwriter, guitarist, and producer (b. 1939)
 * 2008  – Robert Graham, Mexican-American sculptor (b. 1938)
 * 2009 – Isaac Schwartz, Ukrainian-Russian composer and educator (b. 1923)
 * 2011 – Catê, Brazilian footballer and manager (b. 1973)
 * 2011  – Michael Dummett, English soldier, philosopher, and academic (b. 1925)
 * 2011  – Helen Frankenthaler, American painter and educator (b. 1928)
 * 2011  – Johnny Wilson, Canadian-American ice hockey player and coach (b. 1929)
 * 2012 – Harry Carey, Jr., American actor, producer, and screenwriter (b. 1921)
 * 2012  – Lloyd Charmers, Jamaican singer, keyboard player, and producer (b. 1938)
 * 2012  – Tingye Li, Chinese-American physicist and engineer (b. 1931)
 * 2012  – Archie Roy, Scottish astronomer and academic (b. 1924)
 * 2012  – Norman Schwarzkopf, Jr., American general and engineer (b. 1934)
 * 2012  – Salt Walther, American race car driver (b. 1947)
 * 2013 – Richard Ambler, English-Scottish biologist and academic (b. 1933)
 * 2013  – Mohamad Chatah, Lebanese economist and politician, Lebanese Minister of Finance (b. 1951)
 * 2013  – Gianna D'Angelo, American soprano and educator (b. 1929)
 * 2013  – John Matheson, Canadian colonel, lawyer, and politician (b. 1917)
 * 2013  – Farooq Sheikh, Indian actor, philanthropist and a popular television presenter (b. 1948)
 * 2014 – Ben Ammi Ben-Israel, American-Israeli religious leader, founded the African Hebrew Israelites of Jerusalem (b. 1939)
 * 2014  – Ulises Estrella, Ecuadorian poet and academic (b. 1939)
 * 2014  – Ronald Li, Hong Kong accountant and businessman (b. 1929)
 * 2014  – Karel Poma, Belgian bacteriologist and politician (b. 1920)
 * 2015 – Stein Eriksen, Norwegian-American skier (b. 1927)
 * 2015  – Dave Henderson, American baseball player and sportscaster (b. 1958)
 * 2015  – Ellsworth Kelly, American painter and sculptor (b. 1923)
 * 2015  – Meadowlark Lemon, American basketball player and minister (b. 1932)
 * 2015  – Alfredo Pacheco, Salvadoran footballer (b. 1982)
 * 2015  – Stevie Wright, English-Australian singer-songwriter (b. 1947)
 * 2016 – Carrie Fisher, American actress, screenwriter, author, producer, and speaker (b. 1956)
 * 2016  – Ratnasiri Wickremanayake, Sri Lankan politician (b. 1933)
 * 2018 – Frank Blaichman, Polish resistance fighter (b. 1922)
 * 2019 – Maria Creveling, American League of Legends player (b. 1995)
 * 2023 – Lee Sun-kyun, South Korean actor (b. 1975)
 * 2023 – Gaston Glock, Austrian firearm engineer and founder of Glock (b. 1929)

Holidays and observances

 * Christian feast day:
 * Blessed Francesco Spoto
 * Blessed Sára Salkaházi
 * Fabiola
 * John the Apostle
 * Pope Maximus of Alexandria
 * Nicarete
 * Theodorus and Theophanes
 * December 27 (Eastern Orthodox liturgics)
 * Constitution Day (North Korea)
 * Emergency Rescuer's Day (Russia)
 * St. Stephen's Day (Eastern Orthodox Church; a public holiday in Romania)
 * The third of the Twelve Days of Christmas (Western Christianity)

title: December 27


text:
Double planet

In astronomy, a double planet (also binary planet) is a binary satellite system where both objects are planets, or planetary-mass objects, and whose joint barycenter is external to both planetary bodies.

Although up to a third of the star systems in the Milky Way are binary, double planets are expected to be much rarer given the typical planet to satellite mass ratio is around 1:10000, they are influenced heavily by the gravitational pull of the parent star and according to the giant-impact hypothesis are gravitationally stable only under particular circumstances.

The Solar System does not have an official double planet, however the Earth–Moon system is sometimes considered to be one. In promotional materials advertising the SMART-1 mission, the European Space Agency referred to the Earth–Moon system as a double planet.

Several dwarf planet candidates can be described as binary planets. At its 2006 General Assembly, the International Astronomical Union considered a proposal that Pluto and Charon be reclassified as a double planet, but the proposal was abandoned in favor of the current IAU definition of planet. Other trans-Neptunian systems with proportionally large planetary-mass satellites include Eris–Dysnomia, Orcus–Vanth and Varda–Ilmarë.

Binary asteroids with components of roughly equal mass are sometimes referred to as double minor planets. These include binary asteroids 69230 Hermes and 90 Antiope and binary Kuiper belt objects (KBOs) 79360 Sila–Nunam and.

Definition of "double planet"
There is debate as to what criteria should be used to distinguish a "double planet" from a "planet–moon system". The following are considerations.

Both bodies satisfy planet criterion
A definition proposed in the Astronomical Journal calls for both bodies to individually satisfy an orbit-clearing criterion in order to be called a double planet.

Mass ratios closer to 1
One important consideration for defining "double planets" is the ratio of the masses of the two bodies. A mass ratio of 1 would indicate bodies of equal mass, and bodies with mass ratios closer to 1 are more attractive to label as "doubles". Using this definition, the satellites of Mars, Jupiter, Saturn, Uranus, and Neptune can all easily be excluded; they all have masses less than 0.00025 ($1/undefined$) of the planets around which they revolve. Some dwarf planets, too, have satellites substantially less massive than the dwarf planets themselves.

The most notable exception is the Pluto–Charon system. The Charon-to-Pluto mass ratio of 0.122 (≈ $1/8$) is close enough to 1 that Pluto and Charon have frequently been described by many scientists as "double dwarf planets" ("double planets" prior to the 2006 definition of "planet"). The International Astronomical Union (IAU) earlier classified Charon as a satellite of Pluto, but had also explicitly expressed the willingness to reconsider the bodies as double dwarf planets in the future. However, a 2006 IAU report classified Charon–Pluto as a double planet. The Moon-to-Earth mass ratio of 0.01230 (≈ $1/undefined$) is also notably close to 1 when compared to all other satellite-to-planet ratios. Consequently, some scientists view the Earth–Moon system as a double planet as well, though this is a minority view. Eris's lone satellite, Dysnomia, has a radius somewhere around $1/4$ that of Eris; assuming similar densities (Dysnomia's compositional make-up may or may not differ substantially from Eris's), the mass ratio would be near $1/undefined$, a value intermediate to the Moon–Earth and Charon–Pluto ratios.

Center-of-mass position
Currently, the most commonly proposed definition for a double-planet system is one in which the barycenter, around which both bodies orbit, lies outside both bodies. Under this definition, Pluto and Charon are double dwarf planets, since they orbit a point clearly outside of Pluto, as visible in animations created from images of the New Horizons space probe in June 2015.

Under this definition, the Earth–Moon system is not currently a double planet; although the Moon is massive enough to cause the Earth to make a noticeable revolution around this center of mass, this point nevertheless lies well within Earth. However, the Moon currently migrates outward from Earth at a rate of approximately 1.5 in per year; in a few billion years, the Earth–Moon system's center of mass will lie outside Earth, which would make it a double-planet system.

The center of mass of the Jupiter–Sun system lies outside the surface of the Sun, though arguing that Jupiter and the Sun are a double star is not analogous to arguing Pluto–Charon is a double dwarf planet. Jupiter is too light to be a fusor; were it thirteen times heavier, it would achieve deuterium fusion and become a brown dwarf.

Tug-of-war value
Isaac Asimov suggested a distinction between planet–moon and double-planet structures based in part on what he called a "tug-of-war" value, which does not consider their relative sizes. This quantity is simply the ratio of the force exerted on the smaller body by the larger (primary) body to the force exerted on the smaller body by the Sun. This can be shown to equal

$$\text{tug-of-war value} = \frac{m_\mathrm{p}}{m_\mathrm{s}} \cdot \left( \frac{d_\mathrm{s}}{d_\mathrm{p}} \right)^2$$

where $m_{p}$ is the mass of the primary (the larger body), $m_{s}$ is the mass of the Sun, $d_{s}$ is the distance between the smaller body and the Sun, and $d_{p}$ is the distance between the smaller body and the primary. The tug-of-war value does not rely on the mass of the satellite (the smaller body).

This formula actually reflects the relation of the gravitational effects on the smaller body from the larger body and from the Sun. The tug-of-war figure for Saturn's moon Titan is 380, which means that Saturn's hold on Titan is 380 times as strong as the Sun's hold on Titan. Titan's tug-of-war value may be compared with that of Saturn's moon Phoebe, which has a tug-of-war value of just 3.5; that is, Saturn's hold on Phoebe is only 3.5 times as strong as the Sun's hold on Phoebe.

Asimov calculated tug-of-war values for several satellites of the planets. He showed that even the largest gas giant, Jupiter, had only a slightly better hold than the Sun on its outer captured satellites, some with tug-of-war values not much higher than one. In nearly every one of Asimov's calculations the tug-of-war value was found to be greater than one, so in those cases the Sun loses the tug-of-war with the planets. The one exception was Earth's Moon, where the Sun wins the tug-of-war with a value of 0.46, which means that Earth's hold on the Moon is less than half as strong as the Sun's. Asimov included this with his other arguments that Earth and the Moon should be considered a binary planet.

"We might look upon the Moon, then, as neither a true satellite of the Earth nor a captured one, but as a planet in its own right, moving about the Sun in careful step with the Earth. From within the Earth–Moon system, the simplest way of picturing the situation is to have the Moon revolve about the Earth; but if you were to draw a picture of the orbits of the Earth and Moon about the Sun exactly to scale, you would see that the Moon's orbit is everywhere concave toward the Sun. It is always 'falling toward' the Sun. All the other satellites, without exception, 'fall away' from the Sun through part of their orbits, caught as they are by the superior pull of their primary planets – but not the Moon." See the Path of Earth and Moon around Sun section in the "Orbit of the Moon" article for a more detailed explanation.

This definition of double planet depends on the pair's distance from the Sun. If the Earth–Moon system happened to orbit farther away from the Sun than it does now, then Earth would win the tug of war. For example, at the orbit of Mars, the Moon's tug-of-war value would be 1.05. Also, several tiny moons discovered since Asimov's proposal would qualify as double planets by this argument. Neptune's small outer moons Neso and Psamathe, for example, have tug-of-war values of 0.42 and 0.44, less than that of Earth's Moon. Yet their masses are tiny compared to Neptune's, with an estimated ratio of 1.5 ($1/undefined$) and 0.4 ($1/undefined$).

Formation of the system
A final consideration is the way in which the two bodies came to form a system. Both the Earth–Moon and Pluto–Charon systems are thought to have been formed as a result of giant impacts: one body was impacted by a second body, resulting in a debris disk, and through accretion, either two new bodies formed or one new body formed, with the larger body remaining (but changed). However, a giant impact is not a sufficient condition for two bodies being "double planets" because such impacts can also produce tiny satellites, such as the four small outer satellites of Pluto.

A now-abandoned hypothesis for the origin of the Moon was actually called the "double-planet hypothesis"; the idea was that the Earth and the Moon formed in the same region of the Solar System's proto-planetary disk, forming a system under gravitational interaction. This idea, too, is a problematic condition for defining two bodies as "double planets" because planets can "capture" moons through gravitational interaction. For example, the moons of Mars (Phobos and Deimos) are thought to be asteroids captured long ago by Mars. Such a definition would also deem Neptune–Triton a double planet, since Triton was a Kuiper belt body the same size and of similar composition to Pluto, later captured by Neptune.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Double planet


text:
Talk:Double planet

(unorganized)
Ok, so here's a proposed re-write:

A double planet is a set of two planets of comparable mass orbiting one another. There is no accepted definition on where to draw the line between a double planet and a system consisting of a planet and moon, so the term "double planet" is an informal one. One example is Pluo and its moon Charon

In most cases, a moon is of very small mass relative to its host planet. However, there are some examples of moon/planet mass ratios much closer than average: particularly, the Earth and its moon Luna (mass ratio 0.01230), and Pluto with its moon Charon (mass ratio 0.147). (Ignoring the debate over whether Pluto should be considered a planet, or simply a larger Kuiper belt object.)

Several approaches have been suggested for defining a double-planet. One method, suggested by Isaac Asimov, is to compute the force of gravitation exerted by a planet on its moon (Fp), and the force exerted by the Sun on that moon (Fs). The ratio Fp/Fs is the Tug Of War (TOW) value. A TOW value less than 1 indicates that the moon is more firmly gripped by the Sun than its planet. The TOW value for Earth and Luna is approximately 0.455. Two recently discovered satellites of Jupiter, and the outermost satellite of Neptune, also have TOW values less than one.

Charon was still undiscovered at the time Asimov proposed the TOW ratio. The TOW ratio for Charon is over 600, so by Asimov’s reckoning Charon would be a moon of Pluto.

However, astronomers prefer to regard Pluto and Charon as a binary system because (a) they are so close together and so close to the same size that the point they orbit about (the barycenter) is above Pluto's surface, and (b) they orbit with the same side facing each other. By the barycenter method, Pluto/Charon is a double planet, but Earth/Luna is not.

--Gary Puckering, 2004-03-15 — Preceding unsigned comment added by 205.210.232.62 (talk) 00:00, 2004 March 16 (UTC)

Some excellent work on this subject was published in 1975 by Isaac Asimov in "Time and Space and Other Things", Avon Books, New York. Asimov's approach is to consider the tug-of-war (TOW) ratio between the satellite and it's primary, and the satellite and the sun. One would expect a satellite to be more firmly grasped (gravitationally speaking) by it's primary than by the sun, and such is the case for all the satellites in our solar system, including Charon. But is not the case for our Moon. For a good accounting of Asimov's treatise on the web, see

Asimov suggests that the appropriate cutoff points for "moon" status are the TOW distance and the Roche Limit. A true "moon" must lie between these two points. It should be fairly obvious that the Roche Limit is a more reliable minimum distance than the surface of the primary -- regardless of the location of the barycenter -- since it cannot exist inside the limit without being destroyed by tidal forces.

As for the maximum distance, I'm with Asimov -- the TOW distance makes the most sense. And by this line of reasoning, Earth and Moon would be classified as a double-planet, whereas Charon would be classified as a satellite of Pluto -- a conclusion that is quite the opposite of that drawn by Bryan Erickson (below).

--Gary Puckering, 2004-03-15 — Preceding unsigned comment added by 205.210.232.62 (talk) 22:40, 2004 March 15 (UTC)

Not having actually read this article, I see that two instances of the phrase "I propose" occur and that it's signed. Wikipedia isn't the place to make proposals (i.e., to do original research). To do so is to ask the people here to act as peer reviewers and, worse, to pass off your proposals as the state-of-the-art research in the field. For all I know about you (Bryan Erickson), they might be, but I doubt it. But I feel strongly enough about this particular (proposed) rule that, on Nupedia, a very distinguished Classics editor quit when I refused to allow him to expound his latest theories on something or other. I'm not in the same position of authority on Wikipedia (because this is a wiki :-) ) but I do tend to get put out by this sort of thing nonetheless.

Beyond these concerns about the propriety of original research, there is also the very generally accepted rule about writing from the NeutralPointOfView.

So, how might we rewrite this article so as to remove idiosyncratic proposals? --Larry Sanger

Frankly, the whole thing is just an argument about the use of terminology--it belongs in a style guide for publications on Astronomy or something. I don't see that it has any purpose in an encyclopedia at all. An article entitled "double planet" should simply mention that the definition is necessarily vague, and then list a few and describe them. I want to learn something about double planets, not something about what astronomers argue about. --LDC — Preceding unsigned comment added by Lee Daniel Crocker (talk • contribs) 17:26, 2001 September 14 (UTC)

Following are the problematic paragraphs:

I propose two different quantitative analyses that could draw a sharp distinction between double planets and planet-moon systems. The first is the Center of Mass Method. If the center of mass of the two bodies lies under the surface of one body, then that body will be defined as a planet and the other body will be defined as a moon. This method has the following advantages: it's relatively easy to perform when the masses and volumes of the two planets are observable; it is logically sensible; and it provides a definite demarcation between the two classes of two-body systems in almost all cases. The disadvantages are: in a few hypothetical cases, the center of mass might lie almost exactly on the surface of the more massive body, so that the center of mass is below the surface when the maybe-moon happens to be passing over mountain ranges but above the surface when the maybe-moon is above a valley; this would require further definition of exactly where the surface is, is it sea level, or if the planet is among the great majority that don't have seas, is it above the "datum" or average radius; secondly, this method requires observation not only of the masses of the two bodies but also of their radii; in the near future, as observations of planets in other star systems improve, we will reach the point of measuring masses and tracing orbital paths of planets but remaining capable of imaging them only as individual pixels. This would be under the level of technology embodied in NASA's Terrestrial Planet Finder telescope. It will be a long time after that before our data gathering technology achieves the ability to make reliable measurements of the radii of these planets. Therefore, the Center of Mass Method for distinguishing double planet systems and planet-moon systems, while working fine for our own Solar System, would be face more difficulty for other star systems.

Which leads to the second analysis, the Lagrange Method. If the mass ratio between the two bodies is at least about 0.081, the 4th and 5th Lagrange points of the two-body system do not provide a stable equilibrium. If the ratio is less than that amount, Lagrange 4 and 5 do provide stable equilibrium. This provides a distinction that could be used to define the cutoff between double planet systems and planet-moon systems. It has the advantage that it requires only determination of the masses of the two bodies, which could be accomplished for extra-solar planets at a much lower level of technology; we already measure the minimum mass of extrasolar bodies, and with observations of their orbital paths, even with only single-pixel resolution, as with the proposed NASA Terrestrial Planet Finder orbiting telescope, we will be able to define their masses pretty well. This method also provides a definite cutoff between the two categories. However, it has the drawback of providing a less logically compelling definition. Therefore, I propose that the Center of Mass Method be adapted as the official analysis. It could still be applied, albeit with less precision, to extrasolar planets under the Terrestrial Planet Finder level of technology, if we use a model of a planet's density based on its mass and the semimajor axis - average distance - from its host star. This density model would provide a good prediction of the planet's radius, which could be used together with measured mass to determine whether the center of mass is beneath the surface of the larger mass or is in space between the two bodies.

Coming back to our solar system, under the preferred Center of Mass method, Pluto and Charon are clearly double planets, but Earth and its Moon are not; the center of mass is almost 2000 kilometers, or about 30%, of the way to the center of the Earth.

Must the terms be mutually exclusive? I'd been fine with "planet-moon" being a broader term (one larger than the other), and "double planet" being a more specific term (barycenter) CobraA1 09:02, 7 April 2007 (UTC)

Double planets
Don't forget that the moon is slowly moving out in its orbit...in six or seven billion years, when the sun becomes a red giant, the center of mass of the Earth/Moon system should be outside of the Earth's surface, due to tidal evolution.

Tom Mazanec — Preceding unsigned comment added by 199.183.107.97 (talk) 04:39, 2003 September 21 (UTC)

Agreed
Tom hopes the facts speak for themselves. They don't.

I was disappointed that the new planet definition sidestepped the double planet question. The arguments to try and keep the moon a satellite are simply because of social inertia. If a body has the sun as it's primary it is a planet. If it has a planet as it's primary then it is a satellite.

I think copericus would agree.

Sit outside when you can see multiple planets. The moon travels in the planetary arc just like any other planet. The longer you look the more obvious it becomes. It's quite an emotional feeling. The moon orbits the sun. It doesn't orbit the equator like a good long term satellite should. It doesn't have a weird captured satellite orbit either.

I did the math in high school in the 60s and have discussed whether the moon is a satellite or planet with about a dozen astronomers and the truth is none of them cared. The common reply is "it's just a three body problem anyway." Orbits are fasinating to them but none felt any need to try to start explaining to the general population why they should turn their concept of the solar system around. Why get in an argument about what orbits what when to that astromer who sees the bigger picture everything dances around everything else. It is my opinion that the definition avoided clarifing double planet in order to avoid social turmoil.

That said, to me it is clear that this is an issue for wikipedia because the issue concerns general people more than astronomers.

To try and justify the moon as a satellite when it is not captured by the earth and is drifting away on it's own path around it's primary is really untenable no matter where the center of mass is.

I picture a cartoon of a man, a dog and an animal control officer. The dog has a leash around it's neck but the other end is lying on the ground, albeit between his legs. The man is telling the officer than the dog is under his control because it is on a leash. — Preceding unsigned comment added by Strider22 (talk • contribs) 04:09, 2006 September 12 (UTC)

- re: Two recently discovered satellites of Jupiter, and the outermost satellite of Neptune, also have TOW values less than one.

Hmmm. As the attraction from the planet decreases with time and distance, will these satellites remain in a stable orbit of the sun?

--Strider22 04:09, 12 September 2006 (UTC)

A couple of notes on today's major edit of the Double Planet entry:
A couple of notes on today's major edit of the Double Planet entry:

1. Mr. Puckering's proposed definition forces a conclusion that is quite the opposite of that found in the professional literature by the scientists in the field as sampled in the references provided, since he concedes that his definition would exclude Pluto and Charon from double planet status while including tiny moons of Jupiter and Neptune as new double "planets", along with Earth's moon. He might also pay attention to the fact that Asimov was not defining limits for a double planet at all, but for a moon. The Roche Limit has no effect on the question of defining whether a moon qualifies as a double planet, rather than of how close a moon (or double planet) can get to the host planet before shredding apart. A double planet can get along just fine with its barycenter well under the Roche limit just as a moon get get along just fine with its planet-relative barycenter under the planet's surface.

 * Comment by Strider22: As you say the Azimov proposal defines a moon. If the object exists beyond the TOW limit then it is not a moon. Asimov asks "What is our moon doing way out there?". He has concluded that the moon does not have earth as it's primary but rather has the sun as it's primary. If the moon is not a satellite, travels around it's primary along with another planet, what is it? — Preceding unsigned comment added by Strider22 (talk • contribs) 00:21, 2006 October 27 (UTC)

2. The first, relatively minor edit corrected a factual error, consiting of a definition so broad it applied to all moons rather than double planet-systems as opposed to planet-moon systems. It said that:

"Hypothetically, the moon/planet mass ratios between two bodies could vary anywhere between zero through one, where two planets of exactly the same mass orbit each other..."

But a mass ratio approaching zero would include every conceivable planet-moon system, eliminating any possible difference between a double planet system and a planet-moon system. It would define literally thousands of orbiting debris objects to be in "double planet" systems with the Earth.

I'd like to point out that Asimov's satellite definition is problematic for another reason. Not only would Charon be a satellite of Pluto by this definition, but Pluto would be a satellite of Charon.129.6.54.5 (talk) 20:48, 13 March 2009 (UTC)

 * The "TOW definition" or "Asimov definition" is intended to define what counts as a satellite, a dividing line between a "double-planet system" and a "planet-moon system". If we use that definition, then Luna and these thousands of orbiting debris are not satellites. "not a satellite, ... what is it?" -- If everyone we to switch to this definition, then what are those "tiny moons of Jupiter and Neptune" and "orbiting debris objects"? I presume they would be considered asteroids (not planets), similar to the Trojan asteroids and 3753 Cruithne. My only question is: would Luna be promoted all the way to planet, or would be considered a "dwarf planet" like the much smaller Ceres (dwarf planet)? --68.0.124.33 (talk) 22:00, 17 March 2008 (UTC)
 * If one assumes IAU clearing-neighbourhood requirement + Asimov definition for satellites, then Luna would be promoted all the way to planet indeed. Double sharp (talk) 07:35, 14 August 2021 (UTC)

Corrected mismatch of terms.
One sentence in the article read, "In particular, the Earth-Moon and Pluto-Charon systems are the only examples in our present Solar System where the mass of a moon is larger than one fortieth of one percent of the volume of the host planet (i.e. mass ratio of 0.00025 or less)." (italics added)

I changed this to read, "In particular, the Earth-Moon and Pluto-Charon systems are the only examples in our present Solar System where the mass of a moon is larger than one fortieth of one percent of the mass of the host planet (i.e. mass ratio of 0.00025 or less)," so that the terms would match up. Linguofreak 05:09, 19 March 2006 (UTC)

Proposed deletion of "two planets colliding with each other"
I propose to delete the following:

 * "The second usage for the term double planet, in the context of the theory of the origin of the Moon (i.e. Earth's Moon), is a set of two planets of comparable mass that collide with each other - i.e. with at least transiently overlapping orbits. A double planet in this sense occurred in the very early solar system, consisting of the proto-Earth and a second, Mars-sized planet that collided with it at an oblique angle, in the consensus hypothesis of the formation of the Earth-Moon system. The second body was not a proto-Moon because most of its mass was incorporated into the Earth, while the Moon formed from a small fraction of debris kicked up from the Earth by the collision. These double planet precursor bodies to the Earth-Moon system had roughly comparable mass - i.e. a mass ratio in the neighborhood of 10:1. This happens to be similar to the mass ratio of Pluto-Charon."

I have never heard anyone calling the proto-Earth and impactor (in the context of the giant impact hypothesis) a "double planet". When were these two a double planet? At the instant of impact? The intersection of two orbits doesn't make a double planet. Moreover, this section is poorly written and confusing. Lunokhod 20:04, 4 December 2006 (UTC)

Angular momentum as a criterion for defining a double planet
Does anyone here know if anyone has attempted to use the relative angular momentum of a satellite to define whether or not it is a double-planet or planet-satellite system? It seems to me that the only two candidates in our Solar System for being called double-planet systems are Earth-Moon and Pluto-Charon, and both of these satellites contain an abnormally high angular momentum. Indeed, it appears that the commonality between these two systems is that they formed by giant impacts. Lunokhod 09:55, 8 December 2006 (UTC)

barometric centre
i think the definition of a double planet system as having a barometric centre outside the surface of the bigger element is unscientific, since the barometric centre of every system will move out of the radius of the bigger element as a factor of time. so this definition says, it's a double planet when it is old enough.. well if some pair of celestial bodys becomes a double planet at one point in their life or existence, i must either think it had been a pair before, or it didn't couple just after that. since that is ridiculous i think it might be an error in the article?24.132.171.225 (talk) 08:26, 7 February 2010 (UTC)

Dubious
I read that entire BLOG reference and have found: While this claim is not damaging, it appears to be false and unverifiable. Therefore it should be either clarified and verified, or removed. — Paine ( Ellsworth's   Climax )  05:34, 31 May 2010 (UTC)
 * At no time is the author's name mentioned, so how is it that it has been ascribed to Alan Stern?
 * At no time does the author, whoever he or she might be, refer to the Pluto-Charon system as a "double planet". The author cites and quotes material that refers to the Pluto-Charon system as a double planet, but the author of the blog does not do so.

The dubious claim has been removed and brought here, in case it needs to be discussed... "" — Paine ( Ellsworth's   Climax )  06:53, 30 August 2010 (UTC)

Recent lede edits
The purpose of my recent edits was to get away from the "special" case of a planet with a moon that might be a planet instead of a natural satellite. I feel that the first part of the lede is now more "general".

To the editor that changed "requirement" to "concept", Kheider... We might get opposition to that change from those who feel that when an "official" definition of "double planet" is formed, the requirement (or concept, if you will) of a barycenter above both surfaces shall be one of the first things included in that definition. However, there is no opposition to your edit from me, as I consider that distinction to be dubious. I figure to wait to see if any other editors can come up with a reliable source, perhaps look for one myself when there is time, and then rm the "concept" sentence if no verification is forthcoming.

Also, if I'm not mistaken, the last big meeting of the IAU, when such things can be decided, was in 2009, hence the use of that date in the lede. I am not against "bringing it up to date", though.

On a separate issue, I designed the "citation needed" wrap template to include a Tooltip (hover mouse over text), and it looks like there is also underlining of the questioned text if an editor uses the Firefox or Chrome web browser (the underlining does not appear in Internet Explorer). In case you're interested in this issue, it is presently being discussed here and in other sections of that Talk page. – PIE  (  C LIMAX   )  05:32, 30 August 2010 (UTC)

 * Since there is no official definition of a double planet, we should not claim a barycenter "requirement". I also suspect that the IAU will never waste time on such a minor issue (minor from a scientific standpoint.)  The whole barycenter concept is primarily a result of leftovers from the 2006 IAU draft proposal. Professional astronomers do have have more fruitful projects to focus on. -- Kheider (talk) 06:43, 30 August 2010 (UTC)

 * It is agreed that such a definition is presently a mundane, minor issue; however, there will soon come a time when we are able to detect exosatellites and binary exoplanets. It might be good to have a working definition already in place at that time.  Also, it might keep us from calling a spade a rake, i.e., a "planet" a "satellite", as in the case of Pluto's Charon, and perhaps even Earth's Moon. –  PIE  (  C LIMAX   )  07:11, 30 August 2010 (UTC)

 * The working definition is that the more massive body will be the planet and the secondary will be the moon. If we can not tell which body is more massive only then will it be a candidate double planet. As long as moons larger than Pluto are still called satellites (and not planets), there will always be grey areas.  Your statement above, "it might keep us from calling a spade a rake, i.e., a "planet" a "satellite", as in the case of Pluto's Charon" almost suggests that you are biased towards Charon being a true planet. We need to avoid inadvertently enforcing such suggestions in the lede. -- Kheider (talk) 08:39, 30 August 2010 (UTC)

More than a year has elapsed since I placed a Citation needed template after the following sentence from the lede, so I've moved it here for now in case someone can find a reliable source for it: One unofficial definition is that the objects orbit a common center of gravity, the barycenter, that is above both their surfaces.– PIE  (  C LIMAX   )  05:33, 15 January 2012 (UTC)
 * One reason this may be difficult to source is that the definition is time-sensitive. In the case of Earth's Moon, for example, the Moon gets farther from Earth every moment, so every moment the Earth-Moon barycenter gets a little closer to the surface of the Earth.  There will come a time when that barycenter is outside the surface of Earth.  When that time comes, the Earth-Moon system will, under the above definition, cease being a planet-satellite and become a double planet.  So the above definition is not very effective to describe a double planet. –  PIE  (  C LIMAX   )  17:15, 16 January 2012 (UTC)
 * Note: It was pointed out on another talk page that it will take a very long time, perhaps long after our Sun goes red giant, before the Moon's orbital radius grows to the point that the Earth-Moon barycenter is outside the surface of the Earth, so the time sensitivity is, it is conceded, not all that sensitive. – PIE  (  C LIMAX   )  05:25, 24 January 2012 (UTC)

Since the Citatation needed templates were both over a year old, I am pulling the following quote from the first section: A commonly accepted cutoff point by the vast majority of scientists for deciding between a planet–satellite or double-planet system is based on the location of the barycenter of the two objects (that is, the center of gravity). If the barycenter is not located under the surface of either body, then one may refer to the system as a double-planet system. In this case, both bodies orbit about a point in the free space between them. By this definition, Pluto and Charon would be seen as a double-dwarf-planet system, whereas the Earth and Moon would remain defined as a planet–satellite system. Due to the increasing distance from Earth of the Moon's orbital path (because of tidal forces, the Moon currently drifts away from Earth about 3.74 cm or 1.5 in per year ), the barycenter that now lies under the surface of the Earth will someday move outside the surface. So by this definition, the Earth–Moon system might be seen as a double planet billions of years from now. In 2006 the International Astronomical Union briefly considered a formal definition of double planet which would have included Pluto and Charon, but this definition was not ratified.

If the definition of a double- or binary-star system is used as a comparison, and it depended only on the location of the barycenter, then any revolving body with a barycenter beneath a star's surface would be a planet, and any body with a barycenter lying outside the surface of the star would be another star. In the Solar System, all of the major planets would be planets under this definition except one. The Sun–Jupiter barycenter is the only center of mass that lies outside the surface of the Sun. Therefore, since Jupiter is not a star, the difficulty faced by astronomers to derive a reality-based definition of double planet begins to become clear.– PIE  (  C LIMAX   )  17:13, 20 January 2012 (UTC)

Tug-of-War formula revisited
Could someone please add Asimov's formula to the article? Unfortunately, there seems to be no online resource on this (and the link given above on this discussion page has been removed, as being typical for web resources nowadays). If there is no formula available, or at least an explanation which physical quantity the "tug of war" value refers to (mutual force? orbital period?), this section does not really make sense.--SiriusB (talk) 12:28, 1 October 2010 (UTC)

 * There seems to be no addition to this issue so far. However, from trial and error one finds that the formula is probably
 * $$\text{tug-of-war index}=\left(\frac{a_\text{planet}}{a_\text{satellite}}\right)^2\times \frac{m_\text{planet}}{m_\text{star}}$$

In fact, this is just the ratio of the gravitational accelerations of the host star (here: the Sun) and the host planet on the satellite. I do not have Asimov's book, but I guess that the formula is given there in some form and adding it to the article won't be original research. One should note that this is not equivalent to the strength of the bond (and thus the stability of the orbit) which is better described using the ratio of the satellite orbit to the Hill radius of the planet.--SiriusB (talk) 08:03, 20 August 2011 (UTC)

 * Definitely not OR. I revised the formula just a bit to bring it in line with Asimov's presentation of the formula in his article "Just Mooning Around".  Also found the book in .pdf format on the web and added this to the ref. citation.  Made a few other upgrades to that section. –  PIE  (  C LIMAX   )  19:56, 20 January 2012 (UTC)

I want to know if the tiny moons of Neptune are also, as Asimov says of our moon, always concave towards the sun. Im having trouble picturing it in my head .... but they have such slow orbits that I *think* they would not always be concave towards the sun. And that means Asimov still has a point. — Soap — 06:23, 15 March 2020 (UTC)
 * Actually, the slower the orbit, the better. :) If Neso's orbit were circular (but still took 26.67 years to go around), then yes, it would be always concave towards the Sun. (See for the general derivation.) But it's quite far from being circular in reality, so for the real orbit, I couldn't say. Double sharp (talk) 07:32, 14 August 2021 (UTC)

New Link
Just added a link to the page about the Moon's path - it shows a bit more clearly how the Sun's effect on the Moon is greater than the Earth's. NJHartley (talk) 11:50, 8 October 2011 (UTC)

Mass Gap Between Planets and Moons
I think we are on the right track in considering the gravitational influences of the host planet and star. If you notice there is a significant "mass gap" between the largest moon and the smallest true planet. There is only one real moon-sized object inside the orbit of Neptune that hasn't been captured by a planet (Ceres). Irrespective of the method of capture, there seems to be a maximum mass for a moon, relative to the Sun and its orbiting planetary masses. Otherwise the only object in the Solar system large enough to capture it is the Sun itself. That maximum mass seems to lie somewhere between Ganymede (0.0248 Earth masses) and Mercury (0.0553 Earth masses). Under that definition, Moon is a moon, destined to be captured by some larger object, be it Earth or some other planet. I think it would also be fair to say that every planetary mass must have some maximum moon size that it can retain. Given the lack of any known planet-moon systems with mass ratios lower than 1/9, coupled with the relative instability of the Earth-Moon system over time, a "double-planet" configuration is at best only temporary if there are larger masses nearby to disturb it. — Preceding unsigned comment added by Lizardite (talk • contribs) 00:07, 17 November 2011 (UTC)

Aslaksen
I have added the Helmer Aslaksen page in as a reference citation. His page is used in the Orbit of the Moon article, and it clearly shows the scalloped shape of the Earth's and the Moon's Solar orbits. The only difference is that he describes the Earth-Moon motion around the Sun as "convex", while Asimov uses the term "concave" to describe the exact same motion. The difference is only the perspective of the observer. Aslaksen's POV is from outside the orbits, as if from the orbit of Mars, and Asimov's POV is from inside the orbits, as if from the orbit of Venus. Aslaksen's page also shows that there are no loops in the Moon's Solar orbit, as are seen in the Solar orbits of most satellites in our Solar system. – PIE  (  C LIMAX   )  05:34, 24 January 2012 (UTC)

Satellites? Dwarfs?
The double-body structure could consist of a primary planet and a satellite, a full-sized larger planet and a full-sized smaller planet, a larger dwarf planet and a smaller dwarf planet, or more generally, a primary (larger) body and a smaller body. The term "smaller body" covers all possibilities. The extensive usage of "dwarf" and "(dwarf)" detracts from the fact that this article is about Double planets. It is not only about double-dwarf planets nor is it only about planets and satellites. – Paine ( Climax !) 21:39, 30 January 2013 (UTC)
 * Good point. Do note that there are three bodies involved in the tug-of-war, also the central star/Sun. I must also reiterate that I hate it when people undo also the parts that they don't disagree with... --JorisvS (talk) 21:58, 30 January 2013 (UTC)
 * Not for anything, and just out of curiosity, how do you know for sure when people don't disagree, or at least are neutral about some changes? For example, "figured" vs. "calculated" – It's not that I disagreed or didn't disagree, it's that I accepted either as viable, although "calculated" does sound more encyclopedic. – Paine ( Climax !)  09:54, 31 January 2013 (UTC)
 * Of course, I don't for sure, but sometimes something is so straightforwardly better that I have a hard time imagining why someone would oppose the change. --JorisvS (talk) 10:06, 31 January 2013 (UTC)

Is there s.t. missing from Asimov's definition? It seems incredible that if we had a rubble pile in place of the Moon, it would still count as a double planet.

Added a couple other defs. Had these been removed? The barycenter def used by the IAU is far more common than TOW, yet TOW is the only example we used.

I suspect there are others. I wonder if anyone has considered like objects. A double terrestrial planemo would count as a double planet (assuming the primary qualifies), which would include Earth, but a terrestrial (or icy) planemo satellite of a gas giant (unlike) would not, sparing all four of the Solar giants. Also, Stern does consider "satellite planets" to be planets; don't know if he considers Jupiter to be a multiple planet, though. — kwami (talk) 23:55, 31 January 2013 (UTC)
 * Given what it is, I think the TOW definition is actually complete. But yeah, it's a preposterous definition if you think about it...
 * I'd also like to say that you're doing a great job expanding this article! --JorisvS (talk) 00:04, 1 February 2013 (UTC)

 * As can be seen by how long it is taking the IAU to assemble an official definition of "double planet", it will take more than one parameter, such as the TOW, to do the job. Asimov actually came up with much more than the TOW, which I have outlined here on this part of my website (that's a compilation of several UseNet posts made to the newsgroup, alt.astronomy).  Like all the parameters, the barycenter definition is not an official definition.  The Earth-Moon barycenter lies only a thousand miles beneath Earth's surface, which is only 1/8 the diameter of Earth.  You added to the article that the barycenter will lie outside the surface of Earth in a few hundred million years.  The figure I've read was more like a few billion years and actually longer than it will take for the Sun to go red giant.  Do you have a reliable source for the figure you used?  It would be good if you could add more reliable sources for your added material.  I agree that your expansions to the article are welcome. – Paine ( Climax !)  09:19, 1 February 2013 (UTC)

 * A cited figure would be better. The point, however, is that all of these definitions are arbitrary in an unintuitive way, when the concept of a double planet is simply two bodies of similar size. — kwami (talk) 10:20, 1 February 2013 (UTC)
 * Of similar size..or of similar mass? --JorisvS (talk) 10:33, 1 February 2013 (UTC)
 * Either. Actually, one of the defs is not arbitrary: co-accretion. Just turns out not to be relevant. — kwami (talk) 12:29, 1 February 2013 (UTC)
 * Well, one can come up with hypothetical situations in which the planemos' masses are similar but their sizes rather dissimilar or vice versa. --JorisvS (talk) 16:45, 1 February 2013 (UTC)

Barycenter
Over time this article has slowly moved towards Barycentric coordinates (astronomy) being removed. This is at odds with Moon, which gives http://www.iau.org/public_press/news/release/iau0601/q_answers/ as a source. I suspect that this should be mentioned, but I am not sure. If it is merely because of sourcing, what type of sources are needed? John Vandenberg (chat) 06:38, 7 January 2014 (UTC)

Double planet image updated (The Earth-Moon size comparison)
Better version of Moon Earth Comparison.png — Preceding unsigned comment added by Jcpag2012 (talk • contribs) 11:08, 14 February 2015 (UTC)

Terminology
I propose that this article be renamed binary planet and that double planet not be treated as a synonym. Multiple scientific sources use double planet to mean a system with two planets. Scientific sources pretty nearly always use binary planet to mean two gravitationally bound bodies. SpinningSpark 21:57, 25 December 2015 (UTC)
 * The term double-planet system is often used to refer to a system of two planets orbiting the same host star. This article refers to two planetary bodies orbiting one another, which is a different concept.  However, I agree that binary planet and double planet should not be treated as synonyms.  It may be better to write an article devoted to binary planets and to specify under which condition a binary planet can be considered a double planet.  (The IAU has yet to define planets - and binary planets - generally).  This structure would parallel the situation for binary asteroids, where a fraction of the binary asteroids are considered double asteroids.  (Again, with no official IAU classification at this time). JeanLucMargot (talk) 00:48, 26 December 2015 (UTC)

Habitability of red dwarf systems
In considerations of the Habitability of red dwarf systems, a double planet is treated as a pair that have eventually become tidally locked to each other, rather than having the larger planet locked to the sun. This is also a reasonable definition, perhaps more so than one which depends on the current distance between the two bodies, and on the density of the more massive one. It is also a definition with a useful meaning. (It also classifies the earth/moon pair is a double planet, as it too is predicted to become tidally locked, albeit on a longer time scale than the expected lifetime of the earth.) — Preceding unsigned comment added by 67.100.124.87 (talk) 17:08, 4 May 2017 (UTC)

Kepler 1625b
Just saw an article about a Neptune-sized exomoon orbiting Kepler 1625b. Would this constitute a double planet system? There is no question they satisfy the "Both bodies satisfy planet criterion" criterion and I have been trying to crunch the numbers on the available data to see if any of them work. It has a mass ratio of ~1/200, less than even the Moon-to-Earth mass ratio and a barycenter of ~15,000 km that still lies within the planetary radius. I haven't worked out the tug of war value due to some confusion I'm having with the equation, but I am wondering what anybody else thinks on the matter. Sir Trenzalore (talk) 23:56, 3 October 2018 (UTC)

"Both bodies satisfy planet criterion"
Could we have a statement on exactly what this criterion would result in?

In a New Scientist article on this, Margot is quoted as noting that the Earth–Moon system would be a double planet under this definition. Indeed, plugging in the numbers, the Moon, orbiting alone at 1 AU, would have planetary discriminant Π = 44.6. That's quite respectable and comparable to Mars' actual planetary discriminant.

But Titan, orbiting alone in Saturn's orbit, would have a planetary discriminant Π = 1.40. The Galilean moons would have 1.90 (Io), 1.02 (Europa), 3.15 (Ganymede), 2.30 (Callisto). Does this make Saturn–Titan a double planet and Jupiter–Io–Europa–Ganymede–Callisto a quintuple planet? Or are these values too borderline? Would Titan actually have cleared its orbit if it were sitting alone in Saturn's place? Double sharp (talk) 07:21, 14 August 2021 (UTC)
 * Pinging you about this question, since your "in principle" differentiation between two-body and double planets in is cited. Double sharp (talk) 15:07, 20 December 2022 (UTC)

 * Thank you. In my 2015 Astronomical Journal article "A Quantitative Criterion for Defining Planets", I do not define double planets.  I do define planets as bodies that orbit one or more stars or stellar remnants, which prevents satellites such as the Moon and Titan from becoming planets.

 * In Section 7.6 on satellites, I make an oblique reference to double planets in the following statement:

 * "The terminology could in principle differentiate between two-body planets (where the sum of the masses exceeds Mclear, but the individual component masses do not) and double planets (where the individual masses both exceed Mclear)."

 * So if the IAU were to define satellites, and if the IAU were to exclude the Moon from being a satellite, and if the IAU were to define double planets, and if the IAU were to use my proposed criterion to distinguish double planets, then indeed the Moon would have sufficient mass so that the "Earth-Moon system" would qualify as a "double planet". But that is a lot of "ifs", and it is not at all clear that the IAU will go in that direction. Regrettably, these caveats were not conveyed in the New Scientist article. JeanLucMargot (talk) 17:08, 19 March 2023 (UTC)

Solar system
I know the solar system can't be a double star by any definition, since Jupiter doesn't have the mass to be even a brown dwarf, but could it be a double planetary-mass object (though with the Sun containing around 99.8% of the system's mass) since the barycenter is outside the sun's surface? 174.103.211.189 (talk) 03:59, 13 December 2023 (UTC)
 * The Sun is not a planetary-mass object. Fortunately for those of us who aren't native to hydrothermal vents, it is more massive than that. :) Double sharp (talk) 14:44, 3 June 2024 (UTC)
 * But both are gravitationally rounded, right? That's y I said planetary-mass object, rather than planet. But yes, the Sun is massive compared to Jupiter (1 000×). That said, I personally wouldn't consider them double planetary mass objects, but I would consider it reasonable to do so under the barycenter definition. 2600:1008:B1A4:2EC9:404E:5A84:8918:4D85 (talk) 23:52, 7 June 2024 (UTC)

title: Talk:Double planet


text:
Denaturation (biochemistry)

In biochemistry, denaturation is a process in which proteins or nucleic acids lose folded structure present in their native state due to various factors, including application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent (e.g., alcohol or chloroform), agitation and radiation, or heat. If proteins in a living cell are denatured, this results in disruption of cell activity and possibly cell death. Protein denaturation is also a consequence of cell death. Denatured proteins can exhibit a wide range of characteristics, from conformational change and loss of solubility or dissociation of cofactors to aggregation due to the exposure of hydrophobic groups. The loss of solubility as a result of denaturation is called coagulation. Denatured proteins lose their 3D structure, and therefore, cannot function.

Proper protein folding is key to whether a globular or membrane protein can do its job correctly; it must be folded into the native shape to function. However, hydrogen bonds and cofactor-protein binding, which play a crucial role in folding, are rather weak, and thus, easily affected by heat, acidity, varying salt concentrations, chelating agents, and other stressors which can denature the protein. This is one reason why cellular homeostasis is physiologically necessary in most life forms.

Common examples

When food is cooked, some of its proteins become denatured. This is why boiled eggs become hard and cooked meat becomes firm.

A classic example of denaturing in proteins comes from egg whites, which are typically largely egg albumins in water. Fresh from the eggs, egg whites are transparent and liquid. Cooking the thermally unstable whites turns them opaque, forming an interconnected solid mass. The same transformation can be effected with a denaturing chemical. Pouring egg whites into a beaker of acetone will also turn egg whites translucent and solid. The skin that forms on curdled milk is another common example of denatured protein. The cold appetizer known as ceviche is prepared by chemically "cooking" raw fish and shellfish in an acidic citrus marinade, without heat.

Protein denaturation
Denatured proteins can exhibit a wide range of characteristics, from loss of solubility to protein aggregation.

[[File:Levels of structural organization of a protein.svg|thumb| Functional proteins have four levels of structural organization:1. Primary structure: the linear structure of amino acids in the polypeptide chain

2. Secondary structure: hydrogen bonds between peptide group chains in an alpha helix or beta sheet

3. Tertiary structure: three-dimensional structure of alpha helixes and beta helixes folded

4. Quaternary structure: three-dimensional structure of multiple polypeptides and how they fit together]]

[[File:Process of Denaturation.svg|thumb|Process of denaturation:1. Functional protein showing a quaternary structure

2. When heat is applied it alters the intramolecular bonds of the protein

3. Unfolding of the polypeptides (amino acids)]]

Background
Proteins or polypeptides are polymers of amino acids. A protein is created by ribosomes that "read" RNA that is encoded by codons in the gene and assemble the requisite amino acid combination from the genetic instruction, in a process known as translation. The newly created protein strand then undergoes posttranslational modification, in which additional atoms or molecules are added, for example copper, zinc, or iron. Once this post-translational modification process has been completed, the protein begins to fold (sometimes spontaneously and sometimes with enzymatic assistance), curling up on itself so that hydrophobic elements of the protein are buried deep inside the structure and hydrophilic elements end up on the outside. The final shape of a protein determines how it interacts with its environment.

Protein folding consists of a balance between a substantial amount of weak intra-molecular interactions within a protein (Hydrophobic, electrostatic, and Van Der Waals Interactions) and protein-solvent interactions. As a result, this process is heavily reliant on environmental state that the protein resides in. These environmental conditions include, and are not limited to, temperature, salinity, pressure, and the solvents that happen to be involved. Consequently, any exposure to extreme stresses (e.g. heat or radiation, high inorganic salt concentrations, strong acids and bases) can disrupt a protein's interaction and inevitably lead to denaturation.

When a protein is denatured, secondary and tertiary structures are altered but the peptide bonds of the primary structure between the amino acids are left intact. Since all structural levels of the protein determine its function, the protein can no longer perform its function once it has been denatured. This is in contrast to intrinsically unstructured proteins, which are unfolded in their native state, but still functionally active and tend to fold upon binding to their biological target.

How denaturation occurs at levels of protein structure

 * In quaternary structure denaturation, protein sub-units are dissociated and/or the spatial arrangement of protein subunits is disrupted.
 * Tertiary structure denaturation involves the disruption of:
 * Covalent interactions between amino acid side-chains (such as disulfide bridges between cysteine groups)
 * Non-covalent dipole-dipole interactions between polar amino acid side-chains (and the surrounding solvent)
 * Van der Waals (induced dipole) interactions between nonpolar amino acid side-chains.
 * In secondary structure denaturation, proteins lose all regular repeating patterns such as alpha-helices and beta-pleated sheets, and adopt a random coil configuration.
 * Primary structure, such as the sequence of amino acids held together by covalent peptide bonds, is not disrupted by denaturation.

Loss of function
Most biological substrates lose their biological function when denatured. For example, enzymes lose their activity, because the substrates can no longer bind to the active site, and because amino acid residues involved in stabilizing substrates' transition states are no longer positioned to be able to do so. The denaturing process and the associated loss of activity can be measured using techniques such as dual-polarization interferometry, CD, QCM-D and MP-SPR.

Loss of activity due to heavy metals and metalloids
By targeting proteins, heavy metals have been known to disrupt the function and activity carried out by proteins. Heavy metals fall into categories consisting of transition metals as well as a select amount of metalloid. These metals, when interacting with native, folded proteins, tend to play a role in obstructing their biological activity. This interference can be carried out in a different number of ways. These heavy metals can form a complex with the functional side chain groups present in a protein or form bonds to free thiols. Heavy metals also play a role in oxidizing amino acid side chains present in protein. Along with this, when interacting with metalloproteins, heavy metals can dislocate and replace key metal ions. As a result, heavy metals can interfere with folded proteins, which can strongly deter protein stability and activity.

Reversibility and irreversibility
In many cases, denaturation is reversible (the proteins can regain their native state when the denaturing influence is removed). This process can be called renaturation. This understanding has led to the notion that all the information needed for proteins to assume their native state was encoded in the primary structure of the protein, and hence in the DNA that codes for the protein, the so-called "Anfinsen's thermodynamic hypothesis".

Denaturation can also be irreversible. This irreversibility is typically a kinetic, not thermodynamic irreversibility, as a folded protein generally has lower free energy than when it is unfolded. Through kinetic irreversibility, the fact that the protein is stuck in a local minimum can stop it from ever refolding after it has been irreversibly denatured.

Protein denaturation due to pH
Denaturation can also be caused by changes in the pH which can affect the chemistry of the amino acids and their residues. The ionizable groups in amino acids are able to become ionized when changes in pH occur. A pH change to more acidic or more basic conditions can induce unfolding. Acid-induced unfolding often occurs between pH 2 and 5, base-induced unfolding usually requires pH 10 or higher.

Nucleic acid denaturation
Nucleic acids (including RNA and DNA) are nucleotide polymers synthesized by polymerase enzymes during either transcription or DNA replication. Following 5'-3' synthesis of the backbone, individual nitrogenous bases are capable of interacting with one another via hydrogen bonding, thus allowing for the formation of higher-order structures. Nucleic acid denaturation occurs when hydrogen bonding between nucleotides is disrupted, and results in the separation of previously annealed strands. For example, denaturation of DNA due to high temperatures results in the disruption of base pairs and the separation of the double stranded helix into two single strands. Nucleic acid strands are capable of re-annealling when "normal" conditions are restored, but if restoration occurs too quickly, the nucleic acid strands may re-anneal imperfectly resulting in the improper pairing of bases.

Biologically-induced denaturation
The non-covalent interactions between antiparallel strands in DNA can be broken in order to "open" the double helix when biologically important mechanisms such as DNA replication, transcription, DNA repair or protein binding are set to occur. The area of partially separated DNA is known as the denaturation bubble, which can be more specifically defined as the opening of a DNA double helix through the coordinated separation of base pairs.

The first model that attempted to describe the thermodynamics of the denaturation bubble was introduced in 1966 and called the Poland-Scheraga Model. This model describes the denaturation of DNA strands as a function of temperature. As the temperature increases, the hydrogen bonds between the base pairs are increasingly disturbed and "denatured loops" begin to form. However, the Poland-Scheraga Model is now considered elementary because it fails to account for the confounding implications of DNA sequence, chemical composition, stiffness and torsion.

Recent thermodynamic studies have inferred that the lifetime of a singular denaturation bubble ranges from 1 microsecond to 1 millisecond. This information is based on established timescales of DNA replication and transcription. Currently, biophysical and biochemical research studies are being performed to more fully elucidate the thermodynamic details of the denaturation bubble.

Denaturation due to chemical agents
With polymerase chain reaction (PCR) being among the most popular contexts in which DNA denaturation is desired, heating is the most frequent method of denaturation. Other than denaturation by heat, nucleic acids can undergo the denaturation process through various chemical agents such as formamide, guanidine, sodium salicylate, dimethyl sulfoxide (DMSO), propylene glycol, and urea. These chemical denaturing agents lower the melting temperature (Tm) by competing for hydrogen bond donors and acceptors with pre-existing nitrogenous base pairs. Some agents are even able to induce denaturation at room temperature. For example, alkaline agents (e.g. NaOH) have been shown to denature DNA by changing pH and removing hydrogen-bond contributing protons. These denaturants have been employed to make Denaturing Gradient Gel Electrophoresis gel (DGGE), which promotes denaturation of nucleic acids in order to eliminate the influence of nucleic acid shape on their electrophoretic mobility.

Chemical denaturation as an alternative
The optical activity (absorption and scattering of light) and hydrodynamic properties (translational diffusion, sedimentation coefficients, and rotational correlation times) of formamide denatured nucleic acids are similar to those of heat-denatured nucleic acids. Therefore, depending on the desired effect, chemically denaturing DNA can provide a gentler procedure for denaturing nucleic acids than denaturation induced by heat. Studies comparing different denaturation methods such as heating, beads mill of different bead sizes, probe sonication, and chemical denaturation show that chemical denaturation can provide quicker denaturation compared to the other physical denaturation methods described. Particularly in cases where rapid renaturation is desired, chemical denaturation agents can provide an ideal alternative to heating. For example, DNA strands denatured with alkaline agents such as NaOH renature as soon as phosphate buffer is added.

Denaturation due to air
Small, electronegative molecules such as nitrogen and oxygen, which are the primary gases in air, significantly impact the ability of surrounding molecules to participate in hydrogen bonding. These molecules compete with surrounding hydrogen bond acceptors for hydrogen bond donors, therefore acting as "hydrogen bond breakers" and weakening interactions between surrounding molecules in the environment. Antiparellel strands in DNA double helices are non-covalently bound by hydrogen bonding between base pairs; nitrogen and oxygen therefore maintain the potential to weaken the integrity of DNA when exposed to air. As a result, DNA strands exposed to air require less force to separate and exemplify lower melting temperatures.

Applications
Many laboratory techniques rely on the ability of nucleic acid strands to separate. By understanding the properties of nucleic acid denaturation, the following methods were created:
 * PCR
 * Southern blot
 * Northern blot
 * DNA sequencing

Acids
Acidic protein denaturants include:
 * Acetic acid
 * Trichloroacetic acid 12% in water
 * Sulfosalicylic acid

Bases
Bases work similarly to acids in denaturation. They include:
 * Sodium bicarbonate

Solvents
Most organic solvents are denaturing, including:
 * Ethanol

Cross-linking reagents
Cross-linking agents for proteins include:
 * Formaldehyde
 * Glutaraldehyde

Chaotropic agents
Chaotropic agents include:
 * Urea 6–8 mol/L
 * Guanidinium chloride 6 mol/L
 * Lithium perchlorate 4.5 mol/L
 * Sodium dodecyl sulfate

Disulfide bond reducers
Agents that break disulfide bonds by reduction include:
 * 2-Mercaptoethanol
 * Dithiothreitol
 * TCEP (tris(2-carboxyethyl)phosphine)

Chemically reactive agents
Agents such as hydrogen peroxide, elemental chlorine, hypochlorous acid (chlorine water), bromine, bromine water, iodine, nitric and oxidising acids, and ozone react with sensitive moieties such as sulfide/thiol, activated aromatic rings (phenylalanine) in effect damage the protein and render it useless.

Other

 * Mechanical agitation
 * Picric acid
 * Radiation
 * Temperature

Chemical
Acidic nucleic acid denaturants include:
 * Acetic acid
 * HCl
 * Nitric acid

Basic nucleic acid denaturants include:
 * NaOH

Other nucleic acid denaturants include:
 * DMSO
 * Formamide
 * Guanidine
 * Sodium salicylate
 * Propylene glycol
 * Urea

Physical

 * Thermal denaturation
 * Beads mill
 * Probe sonication
 * Radiation

title: Denaturation (biochemistry)