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Ecumenical council

An ecumenical council, also called general council, is a meeting of bishops and other church authorities to consider and rule on questions of Christian doctrine, administration, discipline, and other matters in which those entitled to vote are convoked from the whole world (oikoumene) and which secures the approbation of the whole Church.

The word "ecumenical" derives from the Late Latin oecumenicus "general, universal", from Greek oikoumenikos "from the whole world", from he oikoumene ge "the inhabited world" (as known to the ancient Greeks); the Greeks and their neighbors, considered as developed human society (as opposed to barbarian lands); in later use "the Roman world" and in the Christian sense in ecclesiastical Greek, from oikoumenos, present passive participle of oikein ("inhabit"), from oikos ("house, habitation"). The first seven ecumenical councils, recognised by both the eastern and western denominations comprising Chalcedonian Christianity, were convoked by Roman Emperors, who also enforced the decisions of those councils within the state church of the Roman Empire.

Starting with the third ecumenical council, noteworthy schisms led to non-participation by some members of what had previously been considered a single Christian Church. Thus, some parts of Christianity did not attend later councils, or attended but did not accept the results. Bishops belonging to what became known as the Eastern Orthodox Church accept seven ecumenical councils, as described below. Bishops belonging to what became known as the Church of the East participated in the first two councils. Bishops belonging to what became known as Oriental Orthodoxy participated in the first four councils, but rejected the decisions of the fourth and did not attend any subsequent ecumenical councils.

Acceptance of councils as ecumenical and authoritative varies between different Christian denominations. Disputes over Christological and other questions have led certain branches to reject some councils that others accept.

Acceptance of councils by denomination
The Church of the East (accused by others of adhering to Nestorianism) accepts as ecumenical the first two councils. Oriental Orthodox Churches accept the first three.

Both the Eastern Orthodox Church and Catholic Church recognize as ecumenical the first seven councils, held from the 4th to the 9th centuries. While some Eastern Orthodox accept one later council as ecumenical (which was later repudiated by the Catholic Church), the Catholic Church continues to hold general councils of the bishops in full communion with the Pope, reckoning them as ecumenical. In all, the Catholic Church recognizes twenty-one councils as ecumenical.

The first four ecumenical councils are recognized by some Lutheran Churches, Anglican Communion and Reformed Churches—though they are "considered subordinate to Scripture". The Lutheran World Federation recognizes the first seven Ecumenical Councils as "exercises of apostolic authority" and recognizes their decisions as authoritative; while member churches are not required to accept all theological statements produced by the Federation, but only to subscribe to the most basic Lutheran historical confessional documents, most do follow this recommendation.

Infallibility of ecumenical councils
The doctrine of the infallibility of ecumenical councils states that solemn definitions of ecumenical councils, which concern faith or morals, and to which the whole Church must adhere, are infallible. Such decrees are often labeled as 'Canons' and they often have an attached anathema, a penalty of excommunication, against those who refuse to believe the teaching. The doctrine does not claim that every aspect of every ecumenical council is dogmatic, but that every aspect of an ecumenical council is free of errors or impeccable.

Both the Eastern Orthodox and the Catholic churches uphold versions of this doctrine. However, the Catholic Church holds that solemn definitions of ecumenical councils meet the conditions of infallibility only when approved by the Pope, while the Eastern Orthodox Church holds that an ecumenical council is itself infallible when pronouncing on a specific matter.

Protestant churches would generally view ecumenical councils as fallible human institutions that have no more than a derived authority to the extent that they correctly expound Scripture (as most would generally consider occurred with the first four councils in regard to their dogmatic decisions).

Council documents
Church councils were, from the beginning, bureaucratic exercises. Written documents were circulated, speeches made and responded to, votes taken, and final documents published and distributed. A large part of what is known about the beliefs of heresies comes from the documents quoted in councils in order to be refuted, or indeed only from the deductions based on the refutations.

Most councils dealt not only with doctrinal but also with disciplinary matters, which were decided in canons ("laws"). Study of the canons of church councils is the foundation of the development of canon law, especially the reconciling of seemingly contradictory canons or the determination of priority between them. Canons consist of doctrinal statements and disciplinary measures—most Church councils and local synods dealt with immediate disciplinary concerns as well as major difficulties of doctrine. Eastern Orthodoxy typically views the purely doctrinal canons as dogmatic and applicable to the entire church at all times, while the disciplinary canons apply to a particular time and place and may or may not be applicable in other situations.

Circumstances of the first ecumenical councils
Of the seven councils recognised in whole or in part by both the Catholic and the Eastern Orthodox Church as ecumenical, all were called by a Roman emperor. The emperor gave them legal status within the entire Roman Empire. All were held in the eastern part of the Roman Empire. The bishop of Rome (self-styled as "pope" since the end of the fourth century) did not attend, although he sent legates to some of them.

Church councils were traditional and the ecumenical councils were a continuation of earlier councils (also known as synods) held in the Empire before Christianity was made legal. These include the Council of Jerusalem (c. 50), the Council of Rome (155), the Second Council of Rome (193), the Council of Ephesus (193), the Council of Carthage (251), the Council of Iconium (258), the Council of Antioch (264), the Councils of Arabia (246–247), the Council of Elvira (306), the Council of Carthage (311), the Synod of Neo-Caesarea (c. 314), the Council of Ancyra (314) and the Council of Arles (314).

The first seven councils recognised in both East and West as ecumenical and several others to which such recognition is refused were called by the Byzantine emperors. In the first millennium, various theological and political differences such as Nestorianism or Dyophysitism caused parts of the Church to separate after councils such as those of Ephesus and Chalcedon, but councils recognised as ecumenical continued to be held.

The Council of Hieria of 754, held at the imperial palace of that name close to Chalcedon in Anatolia, was summoned by Byzantine Emperor Constantine V and was attended by 338 bishops, who regarded it as the seventh ecumenical council. The Second Council of Nicaea, which annulled that of Hieria, was itself annulled at the synod held in 815 in Constantinople under Emperor Leo V. This synod, presided over by Patriarch Theodotus I of Constantinople, declared the Council of Hieria to be the seventh ecumenical council, but, although the Council of Hieria was called by an emperor and confirmed by another, and although it was held in the East, it later ceased to be considered ecumenical.

Similarly, the Second Council of Ephesus of 449, also held in Anatolia, was called by the Byzantine Emperor Theodosius II and, though annulled by the Council of Chalcedon, was confirmed by Emperor Basiliscus, who annulled the Council of Chalcedon. This too ceased to be considered an ecumenical council.

Catholic views on those circumstances
The Catholic Church does not consider the validity of an ecumenical council's teaching to be in any way dependent on where it is held or on the granting or withholding of prior authorization or legal status by any state, in line with the attitude of the 5th-century bishops who "saw the definition of the church's faith and canons as supremely their affair, with or without the leave of the Emperor" and who "needed no one to remind them that Synodical process pre-dated the Christianisation of the royal court by several centuries".

The Catholic Church recognizes as ecumenical various councils held later than the First Council of Ephesus (after which churches out of communion with the Holy See because of the Nestorian Schism did not participate), later than the Council of Chalcedon (after which there was no participation by churches that rejected Dyophysitism), later than the Second Council of Nicaea (after which there was no participation by the Eastern Orthodox Church), and later than the Fifth Council of the Lateran (after which groups that adhered to Protestantism did not participate).

Of the twenty-one ecumenical councils recognised by the Catholic Church, some gained recognition as ecumenical only later. Thus the Eastern First Council of Constantinople became ecumenical only when its decrees were accepted in the West also.

First seven ecumenical councils
In the history of Christianity, the first seven ecumenical councils, from the First Council of Nicaea (325) to the Second Council of Nicaea (787), represent an attempt to reach an orthodox consensus and to unify Christendom.

All of the original seven ecumenical councils as recognized in whole or in part were called by an emperor of the Eastern Roman Empire and all were held in the Eastern Roman Empire, a recognition denied to other councils similarly called by an Eastern Roman emperor and held in his territory, in particular the Council of Serdica (343), the Second Council of Ephesus (449) and the Council of Hieria (754), which saw themselves as ecumenical or were intended as such.

This and all the following councils in this list are not recognised by all of the Church of the East. Though originally convened as an ecumenical council, this council is not recognised as ecumenical and is denounced as a Robber Council by the Chalcedonians (Catholics, Eastern Orthodox, Protestants).
 * 1) The First Council of Nicaea (325) repudiated Arianism, declared that Christ is "homoousios with the Father" (of the same substance as the Father), and adopted the original Nicene Creed; addressed the Quartodeciman controversy by fixing the date of Easter;  recognised authority of the sees of Rome, Alexandria and Antioch outside their own civil provinces and granted the see of Jerusalem a position of honour.
 * 2) The First Council of Constantinople (381) repudiated Arianism and Macedonianism, declared that Christ is "born of the Father before all time", revised the Nicene Creed in regard to the Holy Spirit and water baptism.
 * 3) The Council of Ephesus (431) repudiated Nestorianism, proclaimed the Virgin Mary as the Theotokos ("Birth-giver to God", "God-bearer", "Mother of God"), repudiated Pelagianism, and reaffirmed the Nicene Creed.
 * 1) The First Council of Constantinople (381) repudiated Arianism and Macedonianism, declared that Christ is "born of the Father before all time", revised the Nicene Creed in regard to the Holy Spirit and water baptism.
 * 2) The Council of Ephesus (431) repudiated Nestorianism, proclaimed the Virgin Mary as the Theotokos ("Birth-giver to God", "God-bearer", "Mother of God"), repudiated Pelagianism, and reaffirmed the Nicene Creed.
 * 1) The Council of Ephesus (431) repudiated Nestorianism, proclaimed the Virgin Mary as the Theotokos ("Birth-giver to God", "God-bearer", "Mother of God"), repudiated Pelagianism, and reaffirmed the Nicene Creed.
 * 1) * The Second Council of Ephesus (449) received Eutyches as orthodox based on his petition outlining his confession of faith. Deposed Theodoret of Cyrrhus and Ibas of Edessa. Condemned Ibas's Letter to "Maris the Persian" (possibly a misunderstood title, indicating as the receiver a certain Catholicus Dadyeshu, bishop of Ardashir/Ctesiphon between 421 and 456; this same letter later became one of the Three Chapters).
 * 1) The Council of Chalcedon (451) repudiated the Eutychian doctrine of monophysitism; adopted the Chalcedonian Creed, which described the hypostatic union of the two natures of Christ, human and divine; reinstated those deposed in 449 including Theodoret of Cyrus. Restored Ibas of Edessa to his see and declared him innocent upon reading his letter. Deposed Dioscorus of Alexandria; and elevated the bishoprics of Constantinople and Jerusalem to the status of patriarchates. This is also the last council explicitly recognised by the Anglican Communion. This council is rejected by Oriental Orthodox churches.
 * 2) * The Third Council of Ephesus (475) ratified an encyclical of Emperor Basiliscus which repudiated the Council of Chalcedon and particularly the Tome of Leo. This council is recognised only by Oriental Orthodox churches, but is not recognised by the Chalcedonians (Catholics, Eastern Orthodox, Protestants). All the following councils in this list are rejected or at least not explicitly affirmed by Oriental Orthodox churches.
 * 3) The Second Council of Constantinople (553) repudiated the Three Chapters as Nestorian, condemned Origenism, and decreed the theopaschite formula.
 * 4) The Third Council of Constantinople (680–681) repudiated monothelitism and monoenergism.
 * 5) * The Quinisext Council, also called Council in Trullo (692) addressed matters of discipline (in amendment to the 5th and 6th councils). The ecumenical status of this council was repudiated by the Western churches.
 * 6) The Second Council of Nicaea (787) restored the veneration of icons (condemned at the Council of Hieria, 754) and repudiated iconoclasm.
 * 1) The Third Council of Constantinople (680–681) repudiated monothelitism and monoenergism.
 * 2) * The Quinisext Council, also called Council in Trullo (692) addressed matters of discipline (in amendment to the 5th and 6th councils). The ecumenical status of this council was repudiated by the Western churches.
 * 3) The Second Council of Nicaea (787) restored the veneration of icons (condemned at the Council of Hieria, 754) and repudiated iconoclasm.
 * 1) The Second Council of Nicaea (787) restored the veneration of icons (condemned at the Council of Hieria, 754) and repudiated iconoclasm.

Further councils recognised as ecumenical in the Catholic Church
As late as the 11th century, seven councils were recognised as ecumenical in the Catholic Church. Then, in the time of Pope Gregory VII (1073–1085), canonists who in the Investiture Controversy quoted the prohibition in canon 22 of the Council of Constantinople of 869–870 against laymen influencing the appointment of prelates elevated this council to the rank of ecumenical council. Only in the 16th century was recognition as ecumenical granted by Catholic scholars to the Councils of the Lateran, of Lyon and those that followed. The following is a list of further councils generally recognised as ecumenical by Catholic theologians:

 * 1) Fourth Council of Constantinople (Catholic) (869–870) deposed Patriarch Photios I of Constantinople as an usurper and reinstated his predecessor Saint Ignatius. Photius had already been declared deposed by the Pope, an act to which the See of Constantinople acquiesced at this council.
 * 2) First Council of the Lateran (1123) addressed investment of bishops and the Holy Roman Emperor's role therein.
 * 3) Second Council of the Lateran (1139) reaffirmed Lateran I and addressed clerical discipline (clerical celibacy, dress).
 * 4) Third Council of the Lateran (1179) restricted papal election to the cardinals, condemned simony, and introduced minimum ages for ordination (thirty for bishops).
 * 5) Fourth Council of the Lateran (1215) defined transubstantiation, addressed papal primacy and clerical discipline.
 * 6) First Council of Lyon (1245) proclaimed the deposition of Emperor Frederick II and instituted a levy to support the Holy Land.
 * 7) Second Council of Lyon (1274) attempted reunion with the Eastern churches, defined teaching on purgatory, approved Franciscan and Dominican orders, a tithe to support crusades, and conclave procedures.
 * 8) Council of Vienne (1311–1312) disbanded the Knights Templar.
 * 9) *Council of Pisa (1409) attempted to solve the Great Western Schism. The council is not numbered because it was not convened by a pope and its outcome was repudiated at Constance.
 * 10) Council of Constance (1414–1418) resolved the Great Western Schism and condemned John Hus. The Catholic Church declared invalid the first sessions of the Council of Constance, gathered under the authority of Antipope John XXIII, which included the famous decree Haec Sancta Synodus, which marked the high-water mark of the conciliar movement of reform. Decrees of the council later annulled by Pope Sixtus IV.
 * 11) *Council of Siena (1423–1424) addressed church reform. Not numbered as it was swiftly disbanded.
 * 12) Council of Basel, Ferrara and Florence (1431–1445) addressed church reform and reunion with the Eastern Churches but split into two parties. The fathers remaining at Basel became the apogee of conciliarism. The fathers at Florence achieved union with various Eastern Churches and temporarily with the Eastern Orthodox Church.
 * 13) Fifth Council of the Lateran (1512–1517) addressed church reform.
 * 14) Council of Trent (1545–1563, with interruptions) addressed church reform and repudiated Protestantism, defined the role and canon of Scripture and the seven sacraments, and strengthened clerical discipline and education. Considered the founding event of the Counter-Reformation. Temporarily attended by Lutheran delegates.
 * 15) First Council of the Vatican (1869–1870) defined the Pope's primacy in church governance and his infallibility, repudiated rationalism, materialism and atheism, addressed revelation, interpretation of scripture and the relationship of faith and reason.
 * 16) Second Council of the Vatican (1962–1965) addressed pastoral and disciplinary issues dealing with the Church and its relation to the modern world, including liturgy and ecumenism.

Further councils recognised as ecumenical by some Eastern Orthodox
Eastern Orthodox catechisms teach that there are seven ecumenical councils and there are feast days for seven ecumenical councils. Nonetheless, some Eastern Orthodox consider events like the Council of Constantinople of 879–880, that of Constantinople in 1341–1351 and that of Jerusalem in 1672 to be ecumenical:
 * 1) Council in Trullo (692) debates on ritual observance and clerical discipline in different parts of the Christian Church.
 * 2) Fourth Council of Constantinople (Eastern Orthodox) (879–880) restored Photius to the See of Constantinople. This happened after the death of Ignatius and with papal approval.
 * 3) Fifth Council of Constantinople (1341–1351) affirmed hesychastic theology according to Gregory Palamas and condemned Barlaam of Seminara.
 * 4) Synod of Iași (1642) reviewed and amended Peter Mogila's Expositio fidei (Statement of Faith, also known as the Orthodox Confession).
 * 5) Synod of Jerusalem (1672) defined Orthodoxy relative to Catholicism and Protestantism, defined the orthodox Biblical canon.
 * 6) Synod of Constantinople (1872) addressing with nationalism or phyletism in the unity of Orthodoxy.

It is unlikely that formal ecumenical recognition will be granted to these councils, despite the acknowledged orthodoxy of their decisions, so that seven are universally recognized among the Eastern Orthodox as ecumenical.

The 2016 Pan-Orthodox Council was sometimes referred to as a potential "Eighth Ecumenical Council" following debates on several issues facing Eastern Orthodoxy, however not all autocephalous churches were represented.

The Eastern Orthodox Church has held many binding and infallible councils since the 7th Ecumenical. Generally speaking, any council that is signed by all of the autocephalous patriarchs is considered infallible. This signing which leads to an infallible council in Orthodoxy can be performed presently at the council or at a later date.

Acceptance of the councils
Although some Protestants reject the concept of an ecumenical council establishing doctrine for the entire Christian faith, Catholics, Lutherans, Anglicans, Methodists, Eastern Orthodox and Oriental Orthodox all accept the authority of ecumenical councils in principle. Where they differ is in which councils they accept and what the conditions are for a council to be considered "ecumenical". The relationship of the Papacy to the validity of ecumenical councils is a ground of controversy between Catholicism and the Eastern Orthodox Churches. The Catholic Church holds that recognition by the Pope is an essential element in qualifying a council as ecumenical; Eastern Orthodox view approval by the Bishop of Rome (the Pope) as being roughly equivalent to that of other patriarchs. Some have held that a council is ecumenical only when all five patriarchs of the Pentarchy are represented at it. Others reject this theory in part because there were no patriarchs of Constantinople and Jerusalem at the time of the first ecumenical council.

Catholic Church
Both the Catholic and Eastern Orthodox churches recognize seven councils in the early centuries of the church, but Catholics also recognize fourteen councils in later times called or confirmed by the Pope. At the urging of German King Sigismund, who was to become Holy Roman Emperor in 1433, the Council of Constance was convoked in 1414 by Antipope John XXIII, one of three claimants to the papal throne, and was reconvened in 1415 by the Roman Pope Gregory XII. The Council of Florence is an example of a council accepted as ecumenical in spite of being rejected by the East, as the Councils of Ephesus and Chalcedon are accepted in spite of being rejected respectively by the Church of the East and Oriental Orthodoxy.

The Catholic Church teaches that an ecumenical council is a gathering of the College of Bishops (of which the Bishop of Rome is an essential part) to exercise in a solemn manner its supreme and full power over the whole Church. It holds that "there never is an ecumenical council which is not confirmed or at least recognized as such by Peter's successor". Its present canon law requires that an ecumenical council be convoked and presided over, either personally or through a delegate, by the Pope, who is also to decide the agenda; but the church makes no claim that all past ecumenical councils observed these present rules, declaring only that the Pope's confirmation or at least recognition has always been required, and saying that the version of the Nicene Creed adopted at the First Council of Constantinople (381) was accepted by the Church of Rome only seventy years later, in 451.

Eastern Orthodox Church
The Eastern Orthodox Church accepts seven ecumenical councils, with the disputed Council in Trullo—rejected by Catholics—being incorporated into, and considered as a continuation of, the Third Council of Constantinople.

To be considered ecumenical, Orthodox accept a council that meets the condition that it was accepted by the whole church. That it was called together legally is also an important factor. A case in point is the Third Ecumenical Council, where two groups met as duly called for by the emperor, each claiming to be the legitimate council. The Emperor had called for bishops to assemble in the city of Ephesus. Theodosius did not attend but sent his representative Candidian to preside. However, Cyril managed to open the council over Candidian's insistent demands that the bishops disperse until the delegation from Syria could arrive. Cyril was able to completely control the proceedings, completely neutralizing Candidian, who favored Cyril's antagonist, Nestorius. When the pro-Nestorius Antiochene delegation finally arrived, they decided to convene their own council, over which Candidian presided. The proceedings of both councils were reported to the emperor, who decided ultimately to depose Cyril, Memnon and Nestorius. Nonetheless, the Orthodox accept Cyril's group as being the legitimate council because it maintained the same teaching that the church has always taught.

Paraphrasing a rule by St Vincent of Lérins, Hasler states "...a teaching can only be defined if it has been held to be revealed at all times, everywhere, and by all believers."

Orthodox believe that councils could over-rule or even depose popes. At the Sixth Ecumenical Council, Pope Honorius and Patriarch Sergius were declared heretics. The council anathematized them and declared them tools of the devil and cast them out of the church.

It is their position that, since the Seventh Ecumenical Council, there has been no synod or council of the same scope. Local meetings of hierarchs have been called "pan-Orthodox", but these have invariably been simply meetings of local hierarchs of whatever Eastern Orthodox jurisdictions are party to a specific local matter. From this point of view, there has been no fully "pan-Orthodox" (Ecumenical) council since 787. The use of the term "pan-Orthodox" is confusing to those not within Eastern Orthodoxy, and it leads to mistaken impressions that these are ersatz ecumenical councils rather than purely local councils to which nearby Orthodox hierarchs, regardless of jurisdiction, are invited.

Others, including 20th-century theologians Metropolitan Hierotheos (Vlachos) of Naupactus, Fr. John S. Romanides, and Fr. George Metallinos (all of whom refer repeatedly to the "Eighth and Ninth Ecumenical Councils"), Fr. George Dragas, and the 1848 Encyclical of the Eastern Patriarchs (which refers explicitly to the "Eighth Ecumenical Council" and was signed by the patriarchs of Constantinople, Jerusalem, Antioch, and Alexandria as well as the Holy Synods of the first three), regard other synods beyond the Seventh Ecumenical Council as being ecumenical.

From the Eastern Orthodox perspective, a council is accepted as being ecumenical if it is accepted by the Eastern Orthodox church at large—clergy, monks and assembly of believers. Teachings from councils that purport to be ecumenical, but which lack this acceptance by the church at large, are, therefore, not considered ecumenical.

Oriental Orthodoxy
Oriental Orthodoxy accepts three ecumenical councils, the First Council of Nicaea, the First Council of Constantinople, and the Council of Ephesus. The formulation of the Chalcedonian Creed caused a schism in the Alexandrian and Syriac churches. Reconciliatory efforts between Oriental Orthodox with the Eastern Orthodox and the Catholic Church in the mid- and late 20th century have led to common Christological declarations. The Oriental and Eastern Churches have also been working toward reconciliation as a consequence of the ecumenical movement.

The Oriental Orthodox hold that the Dyophysite formula of two natures formulated at the Council of Chalcedon is inferior to the Miaphysite formula of "One Incarnate Nature of God the Word" (Byzantine Greek: Mia physis tou theou logou sarkousomene) and that the proceedings of Chalcedon themselves were motivated by imperial politics. The Alexandrian Church, the main Oriental Orthodox body, also felt unfairly underrepresented at the council following the deposition of their Pope, Dioscorus of Alexandria at the council.

Church of the East
The Church of the East accepts two ecumenical councils, the First Council of Nicaea and the First Council of Constantinople, as well as a series of their own national councils, starting with the Council of Seleucia-Ctesiphon in 410 AD. It was the formulation of Mary as the Theotokos which caused a schism with the Church of the East, now divided between the Assyrian Church of the East and the Ancient Church of the East, while the Chaldean Catholic Church entered into full communion with Rome in the 16th century. Meetings between Pope John Paul II and the Assyrian Patriarch Mar Dinkha IV led to a common Christological declaration on 11 November 1994 that "the humanity to which the Blessed Virgin Mary gave birth always was that of the Son of God himself". Both sides recognised the legitimacy and rightness, as expressions of the same faith, of the Assyrian Church's liturgical invocation of Mary as "the Mother of Christ our God and Saviour" and the Catholic Church's use of "the Mother of God" and also as "the Mother of Christ".

Lutheran Churches
The Lutheran World Federation, in ecumenical dialogues with the Ecumenical Patriarch of Constantinople, has affirmed all of the first seven councils as ecumenical and authoritative. It teaches: "Both Orthodox and Lutherans affirm that apostolic authority was exercised in the ecumenical councils of the Church in which the bishops, through illumination and glorification brought about by the Holy Spirit, exercised responsibility. Ecumenical councils are a special gift of God to the Church and are an authoritative inheritance through the ages. Through ecumenical councils the Holy Spirit has led the Church to preserve and transmit the faith once delivered to the saints. They handed on the prophetic and apostolic truth, formulated it against heresies of their time and safeguarded the unity of the churches."

Anglican Communion
Article XXI of the Thirty-nine Articles of Religion of Anglicanism teaches: "General Councils ... when they be gathered together, forasmuch as they be an assembly of men, whereof all be not governed with the Spirit and word of God, they may err and sometime have erred, even in things pertaining to God. Wherefore things ordained by them as necessary to salvation have neither strength nor authority, unless it may be declared that they be taken out of Holy Scripture."

The 19th Canon of 1571 asserted the authority of the Councils in this manner: "Let preachers take care that they never teach anything ... except what is agreeable to the doctrine of the Old and New Testament, and what the Catholic Fathers and ancient Bishops have collected from the same doctrine." This remains the Church of England's teaching on the subject. A modern version of this appeal to catholic consensus is found in the Canon Law of the Church of England and also in the liturgy published in Common Worship:

"The Church of England is part of the One, Holy, Catholic, and Apostolic Church, worshipping the one true God, Father, Son, and Holy Spirit. It professes the faith uniquely revealed in the Holy Scriptures and set forth in the catholic creeds, which faith the Church is called upon to proclaim afresh in each generation. Led by the Holy Spirit, it has borne witness to Christian truth in its historic formularies, the Thirty-nine Articles of Religion, The Book of Common Prayer and the Ordering of Bishops, Priests and Deacons.

I, AB, do so affirm, and accordingly declare my belief in the faith which is revealed in the Holy Scriptures and set forth in the catholic creeds and to which the historic formularies of the Church of England bear witness; and in public prayer and administration of the sacraments, I will use only the forms of service which are authorized or allowed by Canon."

The 1559 Act of Supremacy made a distinction between the decisions of the first four ecumenical councils, which were to be used as sufficient proof that something was heresy, as opposed to those of later councils, which could only be used to that purpose if "the same was declared heresy by the express and plain words of the ... canonical Scriptures". As such, the Anglican tradition accepts the first four ecumenical councils, though they "considered subordinate to Scripture".

While the Councils are part of the "historic formularies" of Anglican tradition, it is difficult to locate an explicit reference in Anglicanism to the unconditional acceptance of all Seven Ecumenical Councils. There is little evidence of dogmatic or canonical acceptance beyond the statements of individual Anglican theologians and bishops. Anglican cleric of Anglo-Catholic churchmanship Bishop Chandler Holder Jones, SSC, explains:

"We indeed and absolutely believe all Seven Councils are truly ecumenical and Catholic—on the basis of the received Tradition of the ancient Undivided Church of East and West. The Anglican formularies address only particular critical theological and disciplinary concerns of the sixteenth and seventeenth centuries, and that certainly by design. Behind them, however, stands the universal authority of the Holy and Apostolic Tradition, which did not have to be rehashed or redebated by Anglican Catholics."

He quotes William Tighe, Associate Professor of History at Muhlenberg College in Allentown, Pennsylvania, (another member of the Anglo-Catholic wing of Anglicanism): "...despite the fact that advocates of all sides to the 16th-century religious conflict, Catholic, Lutheran and Reformed alike, were given to claiming that their particular doctrinal stances and, in some cases, distinctive practices, were in accord with those of the Early Church Fathers, or at least with those of high standing (such as St. Augustine), none [but Anglicanism] were willing to require, or even permit, their confessional stances to be judged by, or subordinated to, a hypothetical 'patristic consensus' of the first four or five centuries of Christianity. But Anglicanism most certainly did, and does so to this day."

Methodist Churches
Methodist theologian Charles W. Brockwell Jr wrote that the first "four ecumenical councils produced and clarified the Niceno-Constantinopolitan Symbol (Nicene Creed), the most important document in Christian history after the Bible itself."

The Manual of the Church of the Nazarene, part of the Wesleyan-Holiness movement within Methodism, states "Our denomination receives the creeds of the first five Christian centuries as expressions of its own faith," including the Christological doctrines formulated during the first four Ecumenical Councils.

Other Protestant denominations
Some, including some fundamentalist Christians, condemn the ecumenical councils for other reasons. Independency or congregationalist polity among Protestants may involve the rejection of any governmental structure or binding authority above local congregations; conformity to the decisions of these councils is therefore considered purely voluntary and the councils are to be considered binding only insofar as those doctrines are derived from the Scriptures. Many of these churches reject the idea that anyone other than the authors of Scripture can directly lead other Christians by original divine authority; after the New Testament, they assert, the doors of revelation were closed and councils can only give advice or guidance, but have no authority. They consider new doctrines not derived from the sealed canon of Scripture to be both impossible and unnecessary whether proposed by church councils or by more recent prophets. Catholic and Orthodox objections to this position point to the fact that the Canon of Scripture itself was fixed by these councils. They conclude that this would lead to a logical inconsistency of a non-authoritative body fixing a supposedly authoritative source.

Nontrinitarian groups
Ecumenical councils are not recognised by nontrinitarian churches such as the Church of Jesus Christ of Latter-day Saints (and other denominations within the Latter Day Saint movement), Christadelphians, Jehovah's Witnesses, Church of God (Seventh-Day), their descendants and Unitarians. They view the ecumenical councils as misguided human attempts to establish doctrine, and as attempts to define dogmas by debate rather than by revelation.

title: Ecumenical council


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Exoplanet

An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.

There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.

The least massive exoplanet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive exoplanet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter. However, according to some definitions of a planet (based on the nuclear fusion of deuterium ), it is too massive to be a planet and might be a brown dwarf instead. Known orbital times for exoplanets vary from less than an hour (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it.

Almost all planets detected so far are within the Milky Way. However, there is evidence that extragalactic planets, exoplanets located in other galaxies, may exist. The nearest exoplanets are located 4.2 light-years (1.3 parsecs) from Earth and orbit Proxima Centauri, the closest star to the Sun.

The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone (sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for life as we know it, to exist on the surface. However, the study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.

Rogue planets are those that do not orbit any star. Such objects are considered a separate category of planets, especially if they are gas giants, often counted as sub-brown dwarfs. The rogue planets in the Milky Way possibly number in the billions or more.

IAU
The official definition of the term planet used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets. The IAU Working Group on Extrasolar Planets issued a position statement containing a working definition of "planet" in 2001 and which was modified in 2003. An exoplanet was defined by the following criteria:

"* Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are 'planets' (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
 * Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are 'brown dwarfs', no matter how they formed or where they are located.
 * Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not 'planets', but are 'sub-brown dwarfs' (or whatever name is most appropriate)."

This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018. The official working definition of an exoplanet is now as follows:

"* Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+$\sqrt{621}$)) are 'planets' (no matter how they formed).
 * The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System."

Alternatives
The IAU's working definition is not always used. One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of their formation. It is widely thought that giant planets form through core accretion, which may sometimes produce planets with masses above the deuterium fusion threshold; massive planets of that sort may have already been observed. Brown dwarfs form like stars from the direct gravitational collapse of clouds of gas, and this formation mechanism also produces objects that are below the limit and can be as low as. Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of AU and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have a composition more similar to their host star than accretion-formed planets, which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent the low-mass end of a brown dwarf formation. One study suggests that objects above formed through gravitational instability and should not be thought of as planets.

Also, the 13-Jupiter-mass cutoff does not have a precise physical significance. Deuterium fusion can occur in some objects with a mass below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object. As of 2011, the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around in the observed mass spectrum reinforces the choice to forget this mass limit". As of 2016, this limit was increased to 60 Jupiter masses based on a study of mass–density relationships. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity." The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses. Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by Coulomb pressure or electron degeneracy pressure with the dividing line at around 5 Jupiter masses.

Nomenclature
The convention for naming exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter. Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.

History of detection
For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they were real in fact, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.

The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized by the gravity of the star, the resulting dust then falling onto the star.

The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992 from the Arecibo Observatory, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating, "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion."

On 21 March 2022, the 5000th exoplanet beyond the Solar System was confirmed.

On 11 January 2023, NASA scientists reported the detection of LHS 475 b, an Earth-like exoplanet – and the first exoplanet discovered by the James Webb Space Telescope.

Early speculations
In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that fixed stars are similar to the Sun and are likewise accompanied by planets.

In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."

In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason that planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.

Discredited claims
Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855, William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars. However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.

During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star. Astronomers now generally regard all early reports of detection as erroneous.

In 1991, Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it.

Confirmed discoveries
As of, a total of confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s. The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia. Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.

On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12. This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press. These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits. As pulsars are aggressive stars, it was considered unlikely at the time that a planet may be able to be formed in their orbit.

In the early 1990s, a group of astronomers led by Donald Backer, who were studying what they thought was a binary pulsar (PSR B1620−26 b), determined that a third object was needed to explain the observed Doppler shifts. Within a few years, the gravitational effects of the planet on the orbit of the pulsar and white dwarf had been measured, giving an estimate of the mass of the third object that was too small for it to be a star. The conclusion that the third object was a planet was announced by Stephen Thorsett and his collaborators in 1993.

On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi. This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.

Initially, the most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets. Kepler-16 contains the first discovered planet that orbits a binary main-sequence star system.

On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity". Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they were more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.

On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.

On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo. This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called small planet radius gap. The gap, sometimes called the Fulton gap, is the observation that it is unusual to find exoplanets with sizes between 1.5 and 2 times the radius of the Earth.

In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.

Candidate discoveries
As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed, several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.

In September 2020, astronomers reported evidence, for the first time, of an extragalactic planet, M51-ULS-1b, detected by eclipsing a bright X-ray source (XRS), in the Whirlpool Galaxy (M51a).

Also in September 2020, astronomers using microlensing techniques reported the detection, for the first time, of an Earth-mass rogue planet unbounded by any star, and free floating in the Milky Way galaxy.

Direct imaging
Planets are extremely faint compared to their parent stars. For example, a Sun-like star is about a billion times brighter than the reflected light from any exoplanet orbiting it. It is difficult to detect such a faint light source, and furthermore, the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star to reduce the glare while leaving the light from the planet detectable; doing so is a major technical challenge which requires extreme optothermal stability. All exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their parent stars.

Specially designed direct-imaging instruments such as Gemini Planet Imager, VLT-SPHERE, and SCExAO will image dozens of gas giants, but the vast majority of known extrasolar planets have only been detected through indirect methods.

Indirect methods

 * Transit method
 * Dopspec-inline.gif a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. Because the transit method requires that the planet's orbit intersect a line-of-sight between the host star and Earth, the probability that an exoplanet in a randomly oriented orbit will be observed to transit the star is somewhat small. The Kepler telescope used this method.

 * Radial velocity or Doppler method
 * As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity—that is, the speed with which it moves towards or away from Earth—can be detected from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be observed, of 1 m/s or even somewhat less.

 * Transit timing variation (TTV)
 * When multiple planets are present, each one slightly perturbs the others' orbits. Small variations in the times of transit for one planet can thus indicate the presence of another planet, which itself may or may not transit. For example, variations in the transits of the planet Kepler-19b suggest the existence of a second planet in the system, the non-transiting Kepler-19c.

 * Transit duration variation (TDV)
 * When a planet orbits multiple stars or if the planet has moons, its transit time can significantly vary per transit. Although no new planets or moons have been discovered with this method, it is used to successfully confirm many transiting circumbinary planets.

 * Gravitational microlensing
 * Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in magnification as it varies over time. Unlike most other methods which have a detection bias towards planets with small (or for resolved imaging, large) orbits, the microlensing method is most sensitive to detecting planets around 1–10 AU away from Sun-like stars.

 * Astrometry
 * Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because the motion is so small, however, this method was not very productive until the 2020s. It has produced only a few confirmed discoveries, though it has been successfully used to investigate the properties of planets found in other ways.

 * Pulsar timing
 * A pulsar (the small, ultradense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. The first confirmed discovery of an extrasolar planet was made using this method. But as of 2011, it has not been very productive; five planets have been detected in this way, around three different pulsars.

 * Variable star timing (pulsation frequency)
 * Like pulsars, there are some other types of stars which exhibit periodic activity. Deviations from periodicity can sometimes be caused by a planet orbiting it. As of 2013, a few planets have been discovered with this method.

 * Reflection/emission modulations
 * When a planet orbits very close to a star, it catches a considerable amount of starlight. As the planet orbits the star, the amount of light changes due to planets having phases from Earth's viewpoint or planets glowing more from one side than the other due to temperature differences.

 * Relativistic beaming
 * Relativistic beaming measures the observed flux from the star due to its motion. The brightness of the star changes as the planet moves closer or further away from its host star.
 * Ellipsoidal variations
 * Massive planets close to their host stars can slightly deform the shape of the star. This causes the brightness of the star to slightly deviate depending on how it is rotated relative to Earth.

 * Polarimetry
 * With the polarimetry method, a polarized light reflected off the planet is separated from unpolarized light emitted from the star. No new planets have been discovered with this method, although a few already discovered planets have been detected with this method.

 * Circumstellar disks
 * Disks of space dust surround many stars, thought to originate from collisions among asteroids and comets. The dust can be detected because it absorbs starlight and re-emits it as infrared radiation. Features on the disks may suggest the presence of planets, though this is not considered a definitive detection method.

Formation and evolution
Planets may form within a few to tens (or more) of millions of years of their star forming. The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming to planetary systems of over 10 Gyr old. When planets form in a gaseous protoplanetary disk, they accrete hydrogen/helium envelopes. These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space. This means that even terrestrial planets may start off with large radii if they form early enough. An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn, which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.

Planet-hosting stars
There is at least one planet on average per star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone.

Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method. Despite this, several tens of planets around red dwarfs have been discovered by the Kepler telescope, which uses the transit method to detect smaller planets.

Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star hosts a giant planet, similar to the size of Jupiter. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.

Some planets orbit one member of a binary star system, and several circumbinary planets have been discovered which orbit both members of a binary star. A few planets in triple star systems are known and one in the quadruple system Kepler-64.

Color and brightness
In 2013, the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue. Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color, and Kappa Andromedae b, which if seen up close would appear reddish in color. Helium planets are expected to be white or grey in appearance.

The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with a high albedo that is far from the star.

The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres, but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.

For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.

There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.

Temperatures of gas giants reduce over time and with distance from their stars. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures, ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.

Magnetic field
In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one-tenth as strong as Jupiter's.

The magnetic fields of exoplanets may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR. The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.

Earth's magnetic field results from its flowing liquid metallic core, but on massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures, which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.

Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up (Joule heating) causing it to expand. The more magnetically active a star is, the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.

In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic hydrogen form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.

Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system. The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.

In 2019, the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.

Plate tectonics
In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths with one team saying that plate tectonics would be episodic or stagnant and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.

If super-Earths have more than 80 times as much water as Earth, then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.

Volcanism
Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.

Rings
The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.

The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.

The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.

Moons
In December 2013 a candidate exomoon of a rogue planet was announced. On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.

Atmospheres

Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.

As of February 2014, more than fifty transiting and five directly imaged exoplanet atmospheres have been observed, resulting in detection of molecular spectral features; observation of day–night temperature gradients; and constraints on vertical atmospheric structure. Also, an atmosphere has been detected on the non-transiting hot Jupiter Tau Boötis b.

In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere. The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.

Comet-like tails
KIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet. The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.

In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 9 e6mi long.

Insolation pattern
Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot, which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball, with the hotspot being the pupil. Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres. Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.

Surface composition
Surface features can be distinguished from atmospheric features by comparing emission and reflection spectroscopy with transmission spectroscopy. Mid-infrared spectroscopy of exoplanets may detect rocky surfaces, and near-infrared may identify magma oceans or high-temperature lavas, hydrated silicate surfaces and water ice, giving an unambiguous method to distinguish between rocky and gaseous exoplanets.

Surface temperature
Measuring the intensity of the light it receives from its parent star can estimate the temperature of an exoplanet. For example, the planet OGLE-2005-BLG-390Lb is estimated to have a surface temperature of roughly −220 °C (50 K). However, such estimates may be substantially in error because they depend on the planet's usually unknown albedo, and because factors such as the greenhouse effect may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b has been estimated to have an average temperature of 1,205 K (932 °C) on its dayside and 973 K (700 °C) on its nightside.

Habitability
As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System. At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes). For example, molecular oxygen in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means. Furthermore, a potentially habitable planet must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.

Habitable zone
The habitable zone around a star is the region where the temperature is just right to allow liquid water to exist on the surface of a planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star, so that the habitable zone can be at different distances for different stars. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out. Rocky planets with a thick hydrogen atmosphere could maintain surface water much further out than the Earth–Sun distance. Planets with larger mass have wider habitable zones because gravity reduces the water cloud column depth which reduces the greenhouse effect of water vapor, thus moving the inner edge of the habitable zone closer to the star.

Planetary rotation rate is one of the major factors determining the circulation of the atmosphere and hence the pattern of clouds: slowly rotating planets create thick clouds that reflect more and so can be habitable much closer to their star. Earth with its current atmosphere would be habitable in Venus's orbit, if it had Venus's slow rotation. If Venus lost its water ocean due to a runaway greenhouse effect, it is likely to have had a higher rotation rate in the past. Alternatively, Venus never had an ocean because water vapor was lost to space during its formation and could have had its slow rotation throughout its history.

Tidally locked planets (a.k.a. "eyeball" planets ) can be habitable closer to their star than previously thought due to the effect of clouds: at high stellar flux, strong convection produces thick water clouds near the substellar point that greatly increase the planetary albedo and reduce surface temperatures.

Planets in the habitable zones of stars with low metallicity are more habitable for complex life on land than high metallicity stars because the stellar spectrum of high metallicity stars is less likely to cause the formation of ozone thus enabling more ultraviolet rays to reach the planet's surface.

Habitable zones have usually been defined in terms of surface temperature, however over half of Earth's biomass is from subsurface microbes, and the temperature increases with depth, so the subsurface can be conducive for microbial life when the surface is frozen and if this is considered, the habitable zone extends much further from the star, even rogue planets could have liquid water at sufficient depths underground. In an earlier era of the universe the temperature of the cosmic microwave background would have allowed any rocky planets that existed to have liquid water on their surface regardless of their distance from a star. Jupiter-like planets might not be habitable, but they could have habitable moons.

Ice ages and snowball states
The outer edge of the habitable zone is where planets are completely frozen, but planets well inside the habitable zone can periodically become frozen. If orbital fluctuations or other causes produce cooling, then this creates more ice, but ice reflects sunlight causing even more cooling, creating a feedback loop until the planet is completely or nearly completely frozen. When the surface is frozen, this stops carbon dioxide weathering, resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a greenhouse effect which thaws the planet again. Planets with a large axial tilt are less likely to enter snowball states and can retain liquid water further from their star. Large fluctuations of axial tilt can have even more of a warming effect than a fixed large tilt. Paradoxically, planets orbiting cooler stars, such as red dwarfs, are less likely to enter snowball states because the infrared radiation emitted by cooler stars is mostly at wavelengths that are absorbed by ice which heats it up.

Tidal heating
If a planet has an eccentric orbit, then tidal heating can provide another source of energy besides stellar radiation. This means that eccentric planets in the radiative habitable zone can be too hot for liquid water. Tides also circularize orbits over time, so there could be planets in the habitable zone with circular orbits that have no water because they used to have eccentric orbits. Eccentric planets further out than the habitable zone would still have frozen surfaces, but the tidal heating could create a subsurface ocean similar to Europa's. In some planetary systems, such as in the Upsilon Andromedae system, the eccentricity of orbits is maintained or even periodically varied by perturbations from other planets in the system. Tidal heating can cause outgassing from the mantle, contributing to the formation and replenishment of an atmosphere.

Potentially habitable planets
A review in 2015 identified exoplanets Kepler-62f, Kepler-186f and Kepler-442b as the best candidates for being potentially habitable. These are at a distance of 1200, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is in similar size to Earth with its 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its red dwarf star.

When looking at the nearest terrestrial exoplanet candidates, Proxima Centauri b is about 4.2 light-years away. Its equilibrium temperature is estimated to be -39 C.

Earth-size planets

 * In November 2013, it was estimated that 22±8% of Sun-like stars in the Milky Way galaxy may have an Earth-sized planet in the habitable zone. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earths, rising to 40 billion if red dwarfs are included.
 * Kepler-186f, a 1.2-Earth-radius planet in the habitable zone of a red dwarf, was reported in April 2014.
 * Proxima Centauri b, a planet in the habitable zone of Proxima Centauri, the nearest known star to the solar system with an estimated minimum mass of 1.27 times the mass of the Earth.
 * In February 2013, researchers speculated that up to 6% of small red dwarfs may have Earth-size planets. This suggests that the closest one to the Solar System could be 13 light-years away. The estimated distance increases to 21 light-years when a 95% confidence interval is used. In March 2013, a revised estimate gave an occurrence rate of 50% for Earth-size planets in the habitable zone of red dwarfs.
 * At 1.63 times Earth's radius Kepler-452b is the first discovered near-Earth-size planet in the "habitable zone" around a G2-type Sun-like star (July 2015).

Planetary system
Exoplanets are often members of planetary systems of multiple planets around a star. The planets interact with each other gravitationally and sometimes form resonant systems where the orbital periods of the planets are in integer ratios. The Kepler-223 system contains four planets in an 8:6:4:3 orbital resonance.

Some hot Jupiters orbit their stars in the opposite direction to their stars' rotation. One proposed explanation is that hot Jupiters tend to form in dense clusters, where perturbations are more common and gravitational capture of planets by neighboring stars is possible.

Search projects

 * CoRoT – Space telescope that found the first transiting rocky planet.
 * Kepler – Mission to look for large numbers of exoplanets using the transit method.
 * TESS – To search for new exoplanets; rotating so by the end of its two-year mission it will have observed stars from all over the sky. It is expected to find at least 3,000 new exoplanets.
 * HARPS – High-precision echelle planet-finding spectrograph installed on the ESO's 3.6m telescope at La Silla Observatory in Chile.
 * ESPRESSO – A rocky planet-finding, and stable spectroscopic observing, spectrograph mounted on ESO's 4 by 8.2m VLT telescope, sited on the levelled summit of Cerro Paranal in the Atacama Desert of northern Chile.
 * ANDES – The ArmazoNes High Dispersion Echelle Spectrograph, a planet finding and planet characterisation spectrograph, is expected to be fitted onto ESO's ELT 39.3m telescope. ANDES was formally known as HIRES, which itself was created after a merger of the consortia behind the earlier CODEX (optical high-resolution) and SIMPLE (near-infrared high-resolution) spectrograph concepts.

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title: Exoplanet