Exoplanet naming convention

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The exoplanet naming convention is an extension of the system used for naming multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the name is normally formed by taking the name of its parent star and adding a lowercase letter. A provisional IAU-sanctioned standard exists to accommodate the naming of planets that orbit two stars, which are known as circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.

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Exoplanet naming standard

The convention for naming exoplanets is an extension of the one used by the Washington Multiplicity Catalog (WMC) for multiple-star systems, and adopted by the International Astronomical Union. [1]

In the WMC naming system, the brightest member of a star system receives the letter "A". Distinct components not contained within "A" are labeled "B", "C", etc. Subcomponents are designated by one or more suffixes with the primary label, starting with lowercase letters for the second hierarchical level and then numbers for the third. [2] For example, if there is a triple star system in which two stars orbit each other closely with a third star in a more distant orbit, the two closely orbiting stars would be named Aa and Ab, whereas the distant star would be named B. For historical reasons, this standard is not always followed: for example Alpha Centauri A, B and C are not labelled Alpha Centauri Aa, Ab and B.

Following an extension of the above standard, an exoplanet's name is normally formed by taking the name of its parent star and adding a lowercase letter. The first planet discovered in a system is given the designation "b" (the parent star is considered to be "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.

The orbits of the planets 55 Cancri e, 55 Cancri b, 55 Cancri c and 55 Cancri f 55 Cancri Inner Orbits.svg
The orbits of the planets 55 Cancri e, 55 Cancri b, 55 Cancri c and 55 Cancri f

For instance, in the 55 Cancri system the first planet – 55 Cancri b  – was discovered in 1996; two additional farther planets were simultaneously discovered in 2002 with the nearest to the star being named 55 Cancri c and the other 55 Cancri d; a fourth planet was claimed (its existence was later disputed) in 2004 and named 55 Cancri e despite lying closer to the star than 55 Cancri b; and the most recently discovered planet, in 2007, was named 55 Cancri f despite lying between 55 Cancri c and 55 Cancri d. [3] As of April 2012 the highest letter in use is "j", for the unconfirmed planet HD 10180 j, [4] and, as of July 2020, with "i" being the highest letter for a confirmed planet (Kepler-90i). [5]

If a planet orbits one member of a binary star system, then an uppercase letter for the star will be followed by a lowercase letter for the planet. Examples are 16 Cygni Bb [6] and HD 178911 Bb. [7] Planets orbiting the primary or "A" star should have 'Ab' after the name of the system, as in HD 41004 Ab. [8] However, the "A" is sometimes omitted; for example the first planet discovered around the primary star of the Tau Boötis binary system is usually called simply Tau Boötis b. [9] The star designation is necessary when more than one star in the system has its own planetary system such as in case of WASP-94 A and WASP-94 B. [10]

If the parent star is a single star, then it may still be regarded as having an "A" designation, though the "A" is not normally written. The first exoplanet found to be orbiting such a star could then be regarded as a secondary subcomponent that should be given the suffix "Ab". For example, 51 Peg Aa is the host star in the system 51 Peg; and the first exoplanet is then 51 Peg Ab. Because most exoplanets are in single-star systems, the implicit "A" designation was simply dropped, leaving the exoplanet name with the lower-case letter only: 51 Peg b.

A few exoplanets have been given names that do not conform to the above standard. For example, the planets that orbit the pulsar PSR 1257 are often referred to with capital rather than lowercase letters. Also, the underlying name of the star system itself can follow several different systems. In fact, some stars (such as Kepler-11) have only received their names due to their inclusion in planet-search programs, previously only being referred to by their celestial coordinates.

Circumbinary planets and 2010 proposal

According to Hessman et al., the implicit system for exoplanet names "utterly failed with the discovery of circumbinary planets", and they state that it is unhelpful. [1] They note that the discoverers of the two planets around HW Virginis tried to circumvent the naming problem by calling them "HW Vir 3" and "HW Vir 4", i.e. the latter is the 4th object – stellar or planetary – discovered in the system. They also note that the discoverers of the two planets around NN Serpentis were confronted with multiple suggestions from various official sources and finally chose to use the designations "NN Ser c" and "NN Ser d".

The proposal of Hessman et al. starts with the following two rules:

Rule 1. The formal name of an exoplanet is obtained by appending the appropriate suffixes to the formal name of the host star or stellar system. The upper hierarchy is defined by uppercase letters, followed by lower-case letters, followed by numbers, etc. The naming order within a hierarchical level is for the order of discovery only. (This rule corresponds to the present provisional WMC naming convention.)
Rule 2. Whenever the leading capital letter designation is missing, this is interpreted as being an informal form with an implicit "A" unless otherwise explicitly stated. (This rule corresponds to the present exoplanet community usage for planets around single stars.)

They note that under these two proposed rules all of the present names for 99% of the planets around single stars are preserved as informal forms of the IAU sanctioned provisional standard. They would rename Tau Boötis b formally as Tau Boötis Ab, retaining the prior form as an informal usage (using Rule 2, above).

To deal with the difficulties relating to circumbinary planets, the proposal contains two further rules:

Rule 3. As an alternative to the nomenclature standard in Rule 1, a hierarchical relationship can be expressed by concatenating the names of the higher order system and placing them in parentheses, after which the suffix for a lower order system is added.
Rule 4. When in doubt (i.e. if a different name has not been clearly set in the literature), the hierarchy expressed by the nomenclature should correspond to dynamically distinct (sub)systems in order of their dynamical relevance. The choice of hierarchical levels should be made to emphasize dynamical relationships, if known.

They submit that the new form using parentheses is the best for known circumbinary planets and has the desirable effect of giving these planets identical sublevel hierarchical labels and stellar component names that conform to the usage for binary stars. They say that it requires the complete renaming of only two exoplanetary systems: The planets around HW Virginis would be renamed HW Vir (AB) b & (AB) c, whereas those around NN Serpentis would be renamed NN Ser (AB) b & (AB) c. In addition the previously known single circumbinary planets around PSR B1620-26 and DP Leonis can almost retain their names (PSR B1620-26 b and DP Leonis b) as unofficial informal forms of the "(AB)b" designation where the "(AB)" is left out.

The discoverers of the circumbinary planet around Kepler-16 followed the naming scheme proposed by Hessman et al. when naming the body Kepler-16 (AB)-b, or simply Kepler-16b when there is no ambiguity. [11]

Proper names

The proper names of the four innermost planets orbiting 55 Cancri 55 Cancri Inner Orbits (proper names).svg
The proper names of the four innermost planets orbiting 55 Cancri

Most exoplanets have catalog names that are explained in the preceding sections, but in July 2014 the IAU launched NameExoWorlds, a process for giving proper names to exoplanets. [12] The process involved public nomination and voting for the new names, [13] the results of which were announced in December 2015. [14] The planets so named were AEgir [lower-alpha 1] , Amateru, Arion, Arkas, Brahe, Dagon, Dimidium, Draugr, Dulcinea, Fortitudo, Galileo, Harriot, Hypatia, Janssen, Lipperhey, Majriti, Meztli, Orbitar, Phobetor, Poltergeist, Quijote, Rocinante, Saffar, Samh, Smertrios, Sancho, Spe, Tadmor, Taphao Kaew, Taphao Thong and Thestias.

The decision to give the planets new names followed the private company Uwingu's exoplanet naming contest, which the IAU harshly criticized. [15] Previously a few planets had received unofficial names: notably Osiris (HD 209458 b), Bellerophon (51 Pegasi b), [16] and Methuselah (PSR B1620-26 b).

In 2019 the IAU gave every country in the world the chance to name an exoplanet and its host star via national competitions. [17] The results were announced in December 2019. [18]

Other naming systems

Another nomenclature, often seen in science fiction, uses Roman numerals in the order of planets' positions from the star. (This was inspired by an old system for naming moons of the outer planets, such as "Jupiter IV" for Callisto.)

Notes

  1. ↑ There is some question on whether the name should be spelled Ægir (with an æ ligature), but the official press release from the IAU says AEgir. Until a superseding document from the IAU changes the typography, the current spelling is correct.

Related Research Articles

PSR B1257+12 Millisecond pulsar in the constellation Virgo

PSR B1257+12, previously designated PSR 1257+12, alternatively designated PSR J1300+1240, is a millisecond pulsar located 2,300 light-years from the Sun in the constellation of Virgo, rotating at about 161 times per second. It is also named Lich, after a powerful, fictional undead creature of the same name.

55 Cancri Binary star with at least five exoplanets 41 light-years away

55 Cancri is a binary star system located 41 light-years away from the Sun in the zodiac constellation of Cancer. It has the Bayer designation Rho1 Cancri (ρ1 Cancri); 55 Cancri is the Flamsteed designation. The system consists of a K-type star and a smaller red dwarf.

Tau Boötis b Extrasolar planet that is 51 light years away

Tau Boötis b, or more precisely Tau Boötis Ab, is an extrasolar planet approximately 51 light-years away. The planet and its host star is one of the planetary systems selected by the International Astronomical Union as part of NameExoWorlds, their public process for giving proper names to exoplanets and their host star. The process involved public nomination and voting for the new names, and the IAU planned to announce the new names in mid-December 2015. However, the IAU annulled the vote as the winning name was judged not to conform with the IAU rules for naming exoplanets.

55 Cancri e Extrasolar planet in the constellation Cancer

55 Cancri e is an exoplanet in the orbit of its Sun-like host star 55 Cancri A. The mass of the exoplanet is about 8.63 Earth masses and its diameter is about twice that of the Earth, thus classifying it as the first super-Earth discovered around a main sequence star, predating Gliese 876 d by a year. It takes less than 18 hours to complete an orbit and is the innermost-known planet in its planetary system. 55 Cancri e was discovered on 30 August 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star. In October 2012, it was announced that 55 Cancri e could be a carbon planet.

55 Cancri b Extrasolar planet in the constellation Cancer

55 Cancri b, occasionally designated 55 Cancri Ab, also named Galileo, is an exoplanet orbiting the Sun-like star 55 Cancri A every 14.65 days. It is the second planet in order of distance from its star, and is an example of a hot Jupiter, or possibly rather "warm Jupiter".

55 Cancri c Extrasolar planet in the constellation Cancer

55 Cancri c, formally named Brahe, is an extrasolar planet in an eccentric orbit around the Sun-like star 55 Cancri A, making one revolution every 44.34 days. It is the third known planet in order of distance from its star. 55 Cancri c was discovered on June 13, 2002, and has a mass roughly half of Saturn.

55 Cancri d Extrasolar planet in the constellation Cancer

55 Cancri d, formally named Lipperhey, is an extrasolar planet in a long-period orbit around the Sun-like star 55 Cancri A. Located at a similar distance from its star as Jupiter is from our Sun, it is the fifth and outermost known planet in its planetary system. 55 Cancri d was discovered on June 13, 2002.

Upsilon Andromedae b Extrasolar planet in the Andromeda constellation

Upsilon Andromedae b, formally named Saffar, is an extrasolar planet approximately 44 light-years away from the Sun in the constellation of Andromeda. The planet orbits the solar analog star, Upsilon Andromedae A, approximately every five days. Discovered in June 1996 by Geoffrey Marcy and R. Paul Butler, it was one of the first hot Jupiters to be discovered. It is also one of the first non-resolved planets to be detected directly. Upsilon Andromedae b is the innermost-known planet in its planetary system.

Lists of exoplanets Wikipedia list article of lists

This is a list of exoplanets. As of 1 October 2021, there are 4,843 confirmed exoplanets in 3,579 planetary systems, with 797 systems having more than one planet. Most of these were discovered by the Kepler space telescope. There are an additional 2,366 potential exoplanets from Kepler's first mission yet to be confirmed, as well as 889 from its "Second Light" mission and 1,385 from the Transiting Exoplanet Survey Satellite (TESS) mission.

55 Cancri f Extrasolar planet in the constellation Cancer

55 Cancri f, also designated Rho1 Cancri f and formally named Harriot, is an exoplanet approximately 41 light-years away from Earth in the constellation of Cancer. 55 Cancri f is the fourth known planet from the star 55 Cancri and the first planet to have been given the designation of "f".

Circumbinary planet Planet that orbits two stars instead of one

A circumbinary planet is a planet that orbits two stars instead of one. Planets in stable orbits around one of the two stars in a binary are known. New studies showed that there is a strong hint that the planet and stars originate from a single disk.

51 Pegasi b Exoplanet in the constellation of Pegasus

51 Pegasi b, unofficially dubbed Bellerophon, later formally named Dimidium, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus. It was the first exoplanet to be discovered orbiting a main-sequence star, the Sun-like 51 Pegasi, and marked a breakthrough in astronomical research. It is the prototype for a class of planets called hot Jupiters.

Kepler-16b Extrasolar planet

Kepler-16b is an extrasolar planet. It is a Saturn-mass planet consisting of half gas and half rock and ice, and it orbits a binary star, Kepler-16, with a period of 229 days. "[It] is the first confirmed, unambiguous example of a circumbinary planet – a planet orbiting not one, but two stars," said Josh Carter of the Harvard-Smithsonian Center for Astrophysics, one of the discovery team.

Kepler-47 is a binary star system of constellation Cygnus with three exoplanets in orbit around the pair of stars located about 1055 parsecs away from Earth. The first two planets announced are designated Kepler-47b, and Kepler-47c. Kepler-47 is the first circumbinary multi-planet system discovered by the Kepler mission. The outermost of the planets is a gas giant orbiting within the habitable zone of the stars. Because most stars are binary, the discovery that multi-planet systems can form in such a system has impacted previous theories of planetary formation.

Kepler-69 Star in the constellation Cygnus

Kepler-69 is a G-type main-sequence star similar to the Sun in the constellation Cygnus, located about 2,430 ly (750 pc) from Earth. On April 18, 2013 it was announced that the star has two planets. Although initial estimates indicated that the terrestrial planet Kepler-69c might be within the star's habitable zone, further analysis showed that the planet very likely is interior to the habitable zone and is far more analogous to Venus than to Earth and thus completely inhospitable.

Kepler-47b Exoplanet in the Kepler 47 system

Kepler-47b is an exoplanet orbiting the binary star system Kepler-47, the innermost of three such planets discovered by NASA's Kepler spacecraft. The system, also involving two other exoplanets, is located about 3,400 light-years away.

Kepler-90 Star in the constellation Draco, orbited by eight planets

Kepler-90, also designated 2MASS J18574403+4918185, is a G-type main sequence star located about 2,840 light-years (870 pc) from Earth in the constellation of Draco. It is notable for possessing a planetary system that has the same number of observed planets as the Solar System.

Kepler-61 is a K-type main-sequence star approximately 1,100 light years from Earth in the constellation Cygnus. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission used to detect planets that may be transiting their stars. On April 24, 2013 it was announced that the star has an extrasolar planet orbiting in the inner edge of the habitable zone, named Kepler-61b.

Kepler-1229 is a red dwarf star located about 870 light-years (270 pc) away from the Earth in the constellation of Cygnus. It is known to host a super-Earth exoplanet within its habitable zone, Kepler-1229b, which was discovered in 2016.

References

  1. 1 2 Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv: 1012.0707 [astro-ph.SR].
  2. ↑ Hartkopf, William I.; Mason, Brian D. "Addressing confusion in double star nomenclature: The Washington Multiplicity Catalog". United States Naval Observatory. Archived from the original on 17 May 2011. Retrieved 12 September 2008.
  3. ↑ Schneider, J. (2011). "Notes for star 55 Cnc". Extrasolar Planets Encyclopaedia. Archived from the original on 5 May 2012. Retrieved 26 September 2011.
  4. ↑ Schneider, J. "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopedia . Archived from the original on 2012-07-05. Retrieved 2016-07-28.
  5. ↑ "Planetary Systems Composite Data". NASA Exoplanet Archive. NASA Exoplanet Science Institute. Archived from the original on 7 October 2020. Retrieved 8 August 2020.
  6. ↑ Schneider, J. (2011). "Notes for Planet 16 Cyg B b". Extrasolar Planets Encyclopaedia. Archived from the original on 13 November 2007. Retrieved 26 September 2011.
  7. ↑ Schneider, J. (2011). "Notes for Planet HD 178911 B b". Extrasolar Planets Encyclopaedia. Archived from the original on 9 November 2011. Retrieved 26 September 2011.
  8. ↑ Schneider, J. (2011). "Notes for Planet HD 41004 A b". Extrasolar Planets Encyclopaedia. Archived from the original on 13 November 2007. Retrieved 26 September 2011.
  9. ↑ Schneider, J. (2011). "Notes for Planet Tau Boo b". Extrasolar Planets Encyclopaedia. Archived from the original on 9 November 2011. Retrieved 26 September 2011.
  10. ↑ Neveu-Vanmalle, M.; Queloz, D.; Anderson, D. R.; Charbonnel, C.; Collier Cameron, A.; Delrez, L.; Gillon, M.; Hellier, C.; Jehin, E.; Lendl, M.; Maxted, P. F. L.; Pepe, F.; Pollacco, D.; Ségransan, D.; Smalley, B.; Smith, A. M. S.; Southworth, J.; Triaud, A. H. M. J.; Udry, S.; West, R. G. (2014). "WASP-94 a and B planets: Hot-Jupiter cousins in a twin-star system". Astronomy & Astrophysics. 572: A49. arXiv: 1409.7566 . Bibcode:2014A&A...572A..49N. doi:10.1051/0004-6361/201424744. S2CID   53470970.
  11. ↑ Doyle, L. R.; Carter, J. A.; Fabrycky, D. C.; Slawson, R. W.; Howell, S. B.; Winn, J. N.; Orosz, J. A.; Prša, A.; Welsh, W. F.; Quinn, S. N.; Latham, D.; Torres, G.; Buchhave, L. A.; Marcy, G. W.; Fortney, J. J.; Shporer, A.; Ford, E. B.; Lissauer, J. J.; Ragozzine, D.; Rucker, M.; Batalha, N.; Jenkins, J. M.; Borucki, W. J.; Koch, D.; Middour, C. K.; Hall, J. R.; McCauliff, S.; Fanelli, M. N.; Quintana, E. V.; Holman, M. J.; et al. (2011). "Kepler-16: A Transiting Circumbinary Planet". Science. 333 (6049): 1602–6. arXiv: 1109.3432 . Bibcode:2011Sci...333.1602D. doi:10.1126/science.1210923. PMID   21921192. S2CID   206536332.
  12. ↑ "NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars" (Press release). IAU.org. 9 July 2014. Archived from the original on 4 September 2017. Retrieved 28 July 2016.
  13. ↑ "NameExoWorlds". IAU.org. July 2014. Archived from the original on 15 August 2015. Retrieved 9 July 2014.
  14. ↑ "Final Results of NameExoWorlds Public Vote Released" (Press release). IAU.org. 15 December 2015. Archived from the original on 2 December 2017. Retrieved 28 July 2016.
  15. ↑ Stromberg, Joseph (10 July 2014). "We've found hundreds of new planets. And now they're going to get cool names". Vox. Archived from the original on 13 July 2014. Retrieved 10 July 2014.
  16. ↑ Robert Sanders (17 January 1996). "Discovery of two new planets -- the second and third within the last three months -- proves they aren't rare in our galaxy". The University of California at Berkeley. Archived from the original on 11 January 2018. Retrieved 28 July 2016.
  17. ↑ "Name an Exoplanet". Archived from the original on 2019-06-16. Retrieved 2019-08-26.
  18. ↑ "Archived copy". Archived from the original on 2019-12-19. Retrieved 2019-12-18.CS1 maint: archived copy as title (link)
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