How Exoplanets Get Named

Exoplanets are named in a systematic way that combines catalog designations from telescope surveys with a lowercase letter to show the sequence of planetary discoveries. While there is no single unified naming convention, most planets get a designation pairing the host star’s catalog name with that lowercase position. Public naming campaigns are separate from the scientific designation.

How the catalog name works

The scientific designation of an exoplanet typically merges the name of the star it orbits with a lowercase letter. This starts with b, rather than a, even though it is the first discovered planet in that system. No unified naming convention exists, and the exact format can vary, but this is the most common pattern. Take TrES-1b: here, TrES- is the catalog prefix for the Transatlantic Exoplanet Survey, and the 1b shows this is the first (and so far only) planet found around the star TrES-1.

Where names like Kepler, HD, and TOI come from

The Kepler, HD, and TOI labels we see in catalysts are surveys and catalogs, not individualized names. Kepler comes from the astronomy mission of the same name; HD stands for Henry Draper, a 19th-century astronomer; less well-known catalogs supply names like TOI for TESS Objects of Interest. These are the leading acronym prefixes that a scientific designation must begin with, followed by a sequential number and optionally a lowercase planet letter.

Why the IAU matters

The International Astronomical Union (IAU) has rules on naming astronomical objects beyond the Solar System. These require a leading catalog acronym, like Kepler or HD, plus a sequence value, usually a number. The IAU also offers what it calls People’s Choice public naming competitions, typically once a year; the 2022 edition offered a name selection for every IAU member state,. That isn’t the scientific label, though. The star designation Kepler-1652 remains for the scientists.

These public names are thematic; for example, all countries have to propose one planetary name and one host star name, both sharing the same naming theme, with common-sense restrictions against names that are hard to pronounce or already in use. But the formal designation for scientists working on that star and planet in their research stays the catalog and sequence naming system.

TRAPPIST-1 as the running example

Let’s pull this together in practice. The TRAPPIST-1 system has been in the news because it has seven known planets. The designation TRAPPIST- comes from the Transiting Planets and PlanetesImals Small Telescope survey that discovered the star TRAPPIST-1.

Under this pattern, the first planet would be designated b, where b would denote a planet but a would be reserved for the star; the seventh and most recently discovered planet is h..

What readers should remember

So what’s a memorably named exoplanet in a JWST survey? First and foremost, know where to find the catalog name, and always trust the b-lettering explorer name over an unofficial popular one, regardless of the star, planet, or orbit probability of planetary mass, habitability index, as indicated by emissions in time of observation (.. Second, know the IAU doesn’t break its rules to assign names, popularity contests and public nomenclature asides. For everyday exoplanet names, think catalog and sequence value, two parts at a minimum, catalog prefix and sequentially listed planet order, plus whatever prefix shows the exoplanet-star relation as parents.