Naming Exoplanets

Figure 1: An artist’s impression of an exoplanetary system as seen from the surface of an exomoon. Image credit: IAU/L. Calçada.

Quick Links

• Introduction
• Scientific Designations
• Naming Rules
• List of Exoplanet Names
• More Information
• Acknowledgements
• References

Introduction

The search for planets beyond the Solar System reaches back to the middle of the 20th century, but the first extrasolar planet (exoplanet for short) around a Sun-like star was only discovered in 1995 (Mayor & Queloz 1995), a breakthrough that was awarded the 2019 Nobel Prize in Physics. As such, exoplanet research is a rather young field in astronomy that has culminated in dedicated exoplanet ground-based surveys and space missions, leading to thousands of new exoplanet detections. On average, humanity now finds roughly the same order of magnitude of planets beyond the Solar System per week as have been known in the Solar System through unaided-eye observations since the beginning of astronomy.

It’s important to note that exoplanets discovered by the transit method (that is, we observe the exoplanet passing in front of and behind the disc of its host star during its orbit, causing a change in the light from that star that reaches the Earth) depend heavily on the relative position of the star, exoplanet, and Earth. While we’ve found many thousands of exoplanets (see the National Aeronautics and Space Association's (NASA) Exoplanet Catalog, the Extrasolar Planet Encyclopedia, and the Open Exoplanet Catalogue, among others) using this method, it is possible there are many thousands more that we cannot observe from our Earth-bound perspective. 

While some exoplanets have been dubbed with an informal nickname (e.g., “Osiris”, the first exoplanet known to transit its host star; Charbonneau et al. 2000), most exoplanets are usually referred to by their scientific designation. This formal reference often reflects the catalogue entry of their host star, such as the Henry Draper star catalogue (e.g., HD209458). The scientific designation of the exoplanet is the catalogue name of the host star followed by a letter (HD209458 b). In other cases, the designation of an exoplanet reflects the historically rooted naming convention of their host stars in a particular constellation, such as 51 Pegasi b, the first exoplanet discovered around a solar-type star. That said, no unified naming convention for exoplanets exists.

The IAU fully supports public involvement in naming astronomical objects, whether directly or through an independently organised vote, as in the IAU’s global competition, NameExoWorlds. This follows from a well-established tradition for naming Solar System objects.

The IAU does not believe that it has a monopoly on naming celestial objects — anyone can adopt names the way they choose. However, given the publicity and emotional investment associated with the discovery of exoplanets, worldwide recognition is important. For this reason, the IAU offers a unique way to facilitate the public naming process: a process that is distinct from the scientific designation. We will discuss each in turn.

Scientific Designations

The scientific nomenclature for the designations of exoplanets usually consists of two elements: (1) a proper noun or abbreviation, sometimes with associated numbers, followed by (2) a lowercase letter.

The first element can derive from several sources. One common source is the host star's widely recognised, common, or astronomical catalogue name. Additionally, exoplanets are often named after the scientific instrument or project that discovered the exoplanet.

For example, 51 Pegasi b is an exoplanet around the star 51 Pegasi in the constellation Pegasus. The star received this particular catalogue designation by English astronomer John Flamsteed in his star atlas (Flamsteed, 1725; read more about star catalogues here). Another common star catalogue used for exoplanet names is the GJ catalogue of German astronomers Wilhelm Gliese and Hartmut Jahreiß (Gliese & Jahreiß, 1991). An example is the exoplanet GJ 1214 b, which orbits a host star that is the 1214th entry in this star catalogue. Still other exoplanets are named after the Henry Draper (HD) and Hipparchos (HIP) catalogues, and some are named for their star’s common name, as in the case of Fomalhaut b. 

Some exoplanet designations often stem from the instrument that discovered them, such as the Kepler planets (named for the NASA Kepler space telescope; e.g., Kepler-186f) and the CoRoT planets (named for the French Space Agency's (CNES) and the European Space Agency's (ESA) spacecraft, COnvection ROtation and planetary Transits; e.g., CoRoT-7b). The number in each planet designation refers to the order of the extrasolar system's detection or identification in the instrument's data.

Exoplanets named after astronomical or planet-searching projects include the HAT planets (after the Hungarian Automated Telescope Network (HATNet)), and the WASP planets (after the SuperWASP project), both of which are ground-based efforts. Numerous other examples exist. The nation of Qatar funds an exoplanet-hunting project, the Qatar Exoplanet Survey (QES), and its planets follow the naming scheme of Qatar-1b, and so on. The MOA and OGLE exoplanets were discovered via a particular observation technique — gravitational microlensing — by the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) projects.

The second element, a lowercase letter, applies in almost all exoplanet nomenclature styles. The letter indicates the order of the planet's discovery around its host star. The first exoplanet discovered in an extrasolar system is designated b; the second, c; the third, d; and so on. The letter does not necessarily indicate the planet’s orbital placement around its host star, so exoplanet c can be closer to, or farther away from, the star that it co-orbits with exoplanet b.

The lowercase lettering style stems from the IAU-established rules for naming binary and multiple star systems (see the IAU Theme on Star Names). A primary star, brighter and often larger than its secondary or tertiary companion stars, is designated by a capitalised A. Its companions are labelled B, C, and so on. Sirius, the brightest star in the sky, demonstrates this nomenclature: it is a double star system composed of the bright star we see in Canis Major, Sirius A, and its dim companion, Sirius B. The first exoplanet tentatively identified around the second brightest star in the triple star system, Alpha Centauri, is called Alpha Centauri Bb. If an exoplanet orbits both of the stars in a binary system, its designation can be, for example, Kepler-34(AB)b.

Naming Rules

The public names proposed in naming campaigns (e.g., NameExoWorlds) should follow the naming rules and restrictions adopted for minor bodies of the Solar System (see the IAU Theme on the Naming of Astronomical Objects) and the guidelines laid out by the IAU Minor Planet Center (MPC).

In particular, proposed names should be:

• 16 characters or less in length
• Preferably one word
• Pronounceable (in some language)
• Non-offensive
• Not too similar to an existing name of an astronomical object. You can check if a name has already been assigned to an astronomical object using this link for galactic and extragalactic names, this link for a more general catalogue, and this link for all IAU-recognised names for celestial objects.

Proposed names should not be:

• Names of pets
• Names of a purely or principally commercial nature
• Names of individuals, places or events known for political, military or religious activities
• Names of living individuals
• The same name as its host star

The process must also be respectful of intellectual property. That is, the naming body must demonstrate that the proposed name is free for public use (i.e., not subject to copyright royalties, as might be the case for names in works of fiction, like books, plays, movies, etc.).

If a scientific designation for the object(s) already exists, the public name will not replace it. Rather, the IAU will recognise the public name as appropriate for public use. The IAU will publicise this and credit the organisation or individual that proposed it. The public name may be used internationally in addition to, or instead of, the scientific designation, permanently and without restrictions.

List of Exoplanet Names

(*) Note: These names are modified based on the original proposals, to be consistent with the IAU rules.

Note 1: The original name proposed, Veritas, is that of an asteroid important for the study of the solar system.

Note 2: The name originally proposed, Amaterasu, is already used for an asteroid.

Note 3: Note the typographical difference between AEgir and Aegir, the Norwegian transliteration. The same name, with the spelling Aegir, has been attributed to one of Saturn's satellites, discovered in 2004.

Note 4: The original proposed name, Leda, is already attributed to an asteroid and one of Jupiter's satellites. The name Althaea, sister to Leda, is also attributed to an asteroid.

Note 5: The original spelling of Lippershey was corrected to Lipperhey on 15 January 2016. The commonly seen spelling Lippershey (with an "s") results from a typographical error dating back to 1831 and should be avoided.

More Information

• Minor Planet Center Name Guidelines
• IAU Theme on Star Names
• IAU Theme on Naming Astronomical Objects
• NameExoWorlds
• NASA’s Exoplanet Catalog
• The Extrasolar Planet Encyclopedia
• The Open Exoplanet Catalogue
• Can One Buy the Right to Name a Planet?
• NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars
• Launch of the 2015 NameExoWorlds Contest
• Final Results of the 2015 NameExoWorlds Contest
• Launch of the 2019 NameExoWorlds Contest
• Final Results of the 2019 NameExoWorlds Contest
• Launch of the 2022 NameExoWorlds Contest

Acknowledgements

We would like to acknowledge the contributions of Commission F2 Exoplanets and the Solar System Organising Committee Members René Heller and Eric Mamajek, the IAU Director of Communications, Lars Lindberg Christensen and OAO Deputy Director Kelly Blumenthal.

References

• Charbonneau, D., Brown, T. M., Latham, D. W. & Mayor, M. (2000). Detection of Planetary Transits Across a Sun-like Star. The Astrophysical Journal, 529(1), pp. L45-L48. https://doi.org/10.1086/312457 
• Flamsteed, J. (1725). Stellarium inerrantium catalogus britannicus. Historia cœlestis britannica, tribus voluminibus contenta. Volumen tertium. London: (n.p.).
• Gliese, W. & Jahreiß, H. (1991). Preliminary Version of the Third Catalogue of Nearby Stars. On: The Astronomical Data Center CD-ROM: Selected Astronomical Catalogs, Vol. I; L.E. Brotzmann, S.E. Gesser (Eds.), NASA/Astronomical Data Center, Goddard Space Flight Center, Greenbelt, MD
• Mayor, M. & Queloz, D. (1995). A Jupiter-mass companion to a solar-type star. Nature, 378(6555), pp. 355-359. https://doi.org/10.1038/378355a0

Read in other languages:

• Naming of Exoplanets (English)
• Comment sont nommées les exoplanètes? (French)
• Penamaan Eksoplanet (Indonesian)
• Denominação de exoplanetas (European Portuguese)
• Denominação de exoplanetas (Brazilian Portuguese)
• Bautizando Exoplanetas (Spanish)
• Ötegezegenlerin İsimlendirilmesi (Turkish)