The discovery of exoplanets has transformed our understanding of the universe. Thousands of planets now orbit stars beyond our solar system. You might wonder how are exoplanets named. The answer lies in a precise system managed by the International Astronomical Union (IAU). This system ensures every known exoplanet receives a unique, unambiguous label.
How are exoplanets named: the IAU host star and lowercase letter convention
The IAU, the same body that defines planets in our solar system, also governs exoplanet nomenclature. The core rule is straightforward: an exoplanet is named after its host star, followed by a lowercase letter. The first planet discovered around a star receives the letter b. Subsequent planets receive c, d, e, and so on, in order of discovery.
For example, the star 51 Pegasi hosts the planet 51 Pegasi b. This planet, discovered in 1995 by Michel Mayor and Didier Queloz (for which they won the Nobel Prize in Physics in 2019), was the first exoplanet found orbiting a Sun-like star. The star designation comes first, and the letter identifies the planet within that system. To fully understand how are exoplanets named, you must recognise that the system avoids the letter a because that is reserved for the star itself. If a system has multiple planets, the letters are assigned chronologically by discovery date, not by orbital distance. This means a planet closer to its star might receive a later letter than a more distant one discovered earlier.
Understanding how are exoplanets named starts with the star
The naming process begins with the star. A star must have a confirmed, accepted designation before any planet can be named. This is crucial because exoplanets are rarely observed directly. Astronomers infer their existence through methods like the transit method (monitoring dips in starlight) or the radial velocity method (detecting tiny wobbles in the star's motion).
The host star's name can come from several sources. Bright stars visible to the naked eye may have historical or Arabic names (e.g., Pollux). Many have catalog designations from surveys. Once the star is named, the lettered suffix is appended.
This hierarchical system prevents confusion. If two different surveys independently discovered planets around the same star, the IAU would assign the letters based on the first confirmed discovery.
How Are Exoplanets Named: Kepler, TOI, HD, WASP, and Others
Different telescopes and surveys produce different star designations. Understanding these prefixes is key to decoding how are exoplanets named.
How are exoplanets named: Kepler
NASA's Kepler mission (2009-2018) found thousands of exoplanets by monitoring a single patch of sky. Kepler stars are labeled with a K followed by a number (e.g., Kepler-186). The first discovered planet around that star is Kepler-186b. The system includes Kepler-186c, Kepler-186d, Kepler-186e, and Kepler-186f. Kepler-186f is notable as the first Earth-size planet discovered in a habitable zone.
How Are Exoplanets Named? Understanding the TOI System
NASA's Transiting Exoplanet Survey Satellite (TESS), launched in 2018, scans the entire sky. TESS designations begin with TOI, short for TESS Objects of Interest. A planet like TOI-700 d orbits the star TOI-700. TESS has identified many planets around bright, nearby stars, making them ideal for follow-up study.
HD (Henry Draper Catalog)
The Henry Draper Catalog is a historical catalog of stellar spectra. Stars are labeled HD followed by a number (e.g., HD 209458). The planet HD 209458 b, discovered in 1999, was the first exoplanet found to transit its star. This detection allowed astronomers to measure the planet's atmosphere.
WASP (Wide Angle Search for Planets)
The WASP consortium uses ground-based telescopes to find transiting planets. WASP stars are designated WASP followed by a number (e.g., WASP-1). The planet WASP-1b was among the first discovered by the project. The WASP prefix often appears in the names of hot Jupiters (gas giants in very close orbits).
Other Common Prefixes
- GJ (Gliese-Jahreiss) or LHS (Luyten Half-Second) for nearby red dwarf stars.
- OGLE (Optical Gravitational Lensing Experiment) for planets found via microlensing.
- CoRoT (Convection, Rotation and planetary Transits) for planets from the European CoRoT mission.
Why Some Planets Get Proper Names
Most exoplanets have only catalog designations. However, the IAU also runs a process for naming selected exoplanets and their host stars with proper names. These names are often drawn from literature, mythology, or cultural figures.
The NameExoWorlds Campaign
Since 2019, the IAU has organized the NameExoWorlds campaign. Each campaign invites the public to propose and vote on names for specific exoplanet systems. Participating countries can submit names through accredited organizations.
Examples of proper names from these campaigns include:
- Dimidium (Latin for "half") is the proper name for 51 Pegasi b. The name reflects the planet's mass, which is about half that of Jupiter.
- The star Helvetios is the proper name for 51 Pegasi, assigned through the NameExoWorlds campaign. Its planet 51 Pegasi b received the proper name Dimidium.
Other notable proper names:
- Poltergeist and Phobetor for the planets PSR B1257+12 b and c (around a pulsar).
- Lalande for the planet formerly known as GJ 436 b was assigned through a NameExoWorlds process, though the exact status of some of these names may require verification with IAU records.
The Distinction Between Catalog and Proper Names
Proper names are not exclusive. They coexist with the scientific designations. Astronomers still use 51 Pegasi b in research papers. The proper name is for public outreach and cultural significance. The IAU ensures proper names are not confused with catalog labels.
How Citizen Science Naming Campaigns Work
Citizen science naming campaigns are organized by the IAU and its partners. These campaigns follow a structured process:
- Selection of Systems: The IAU selects a list of exoplanet systems, typically those discovered by a particular mission (e.g., Kepler or TESS) or in a given region of the sky.
- Public Proposal Phase: Individuals or groups, often through national outreach organizations (like astronomy clubs or universities), submit name proposals. Proposals must include a justification, relevance, and cultural significance.
- Vetting by IAU: The IAU's Working Group on Exoplanet Nomenclature reviews all submissions for:
- Compliance with naming rules (no offensive names, no trademarks, no commercial names).
- Uniqueness (no duplication with existing names in astronomy).
- Cultural appropriateness.
- Public Voting: Approved names are placed on a public ballot. Anyone can vote for their preferred name.
- Final Approval: The IAU's Executive Committee confirms the winning names.
The process is designed to be inclusive. For example, the NameExoWorlds 2022 campaign allowed many countries to propose names for their own exoplanet systems.
Why the System Matters
The naming system prevents chaos. Without it, multiple teams could assign different names to the same object. The IAU's rules ensure each exoplanet is uniquely identifiable. The lettered suffix also provides information: a planet designated "e" tells you it was the fifth planet discovered in that system. For deeper context, explore our guide to Stars and Planets.
For public understanding, the system makes the Milky Way feel a little more structured. Instead of arbitrary nicknames, we have a logical, universal language.
Sources & References
- International Astronomical Union. "Naming of Exoplanets." https://www.iau.org/public/themes/naming_exoplanets/
- NASA Exoplanet Exploration. "Exoplanet Naming Convention." https://exoplanets.nasa.gov/faq/35/what-are-exoplanets-named/
- European Space Agency. "How Exoplanets Get Their Names." https://www.esa.int/Science_Exploration/Space_Science/How_exoplanets_get_their_names
- International Astronomical Union. "NameExoWorlds Campaign." https://www.nameexoworlds.info/
Further reading: NASA Exoplanet Exploration, and Astronomical naming conventions on Wikipedia.