Stars and exoplanets: where do their names come from?

JPL | Videos | Q and Alien: What’s in an Exoplanet Name?, also at ▶ Q&Alien: What’s in an Exoplanet Name? – YouTube

Why do exoplanets have weird-looking names like these? Names like:

Kepler-7b, HD 189733b, GJ 1214b, Gliese 581g, Kapteyn b, Gliese 667 Cc, …

Why not names like Wu Tang Clan or Ghostface or Alderaan or Gallifrey? For starters, astronomers have discovered a *lot* of exoplanets, and giving them individual names would be a lot of trouble. Also, their names have a certain descriptive value:
(star) + (planet letter)

The star is typically (catalog) + (number):
Kepler-7 is the 7th star that the Kepler team discovered to have planets.
HD 189733 is the 189733’th star in the Henry Draper star catalog

The first planet named is small-letter b, because “a” is reserved for its star. They are named in order of discovery, and if several are discovered at the same time, they are then named in order of distance. Stars, however, have capital letters: A, B, C, … in order of brightness or discovery.

Using ease of observation as a proxy for discovery order, the Solar System would have this naming:

  • A: Sun
  • b: Earth
  • bb: Moon
  • c: Venus
  • d: Jupiter
  • e: Mars
  • f: Saturn
  • g: Mercury

No exomoons have been discovered yet, but I’ve extended the planet nomenclature to moons.

Turning to star names, they have a variety of sources.

The brightest stars have individual names like Sirius (Greek seirios: “scorcher”). Many of them have names related to what part of the constellation or asterism that they are in, like Denebola (Arabic dhanab al asad: “tail of the lion”). There are some other stars with “Deneb” in their names, those names referring to the tail of something.

But that system has its limits, and in 1609, Johann Bayer decided on a simpler system: (number as Greek letter) of (constellation) where the stars are listed in order of apparent brightness. The of-constellation part is done using the Latin genitive or of-case.

Sirius in it is Alpha Canis Majoris or α CMa, where “Canis Majoris” is the genitive of “Canis Major”: “Big Dog”. In English, it would be Alpha of the Big Dog.

In 1712, Edmond Halley and Isaac Newton published John Flamsteed’s catalog of stars without his permission. In that catalog, JF had listed stars as (number) of (constellation), where the number increases from west to east, in increasing “right ascension” (a sort of celestial longitude).

Sirius in it is 9 Canis Majoris, #9 of the Big Dog.

Variable stars have a rather complicated system of names per constellation.

  • Stars with Bayer names do not get new names.
  • Do R, S, …, Z
  • Do RR, RS, … RZ, then SS, ST, … SZ, up to ZZ
  • Do AA, AB, …, AZ, BB, BC, …, BZ, all the way up to QZ, omitting J in both positions
  • Do V335, V336, …

More recent star cataloguers have not bothered with constellation locations. Thus, Sirius is:

  • HD 48915 — Henry Draper Catalog #48915
  • HR 2491 — Harvard Revised or Yale Bright Star Catalog #2491
  • BD −16°1591 — Bonner Durchmunsterung (German: “Bonn Survey”) star #1591 in declination (a sort of celestial latitude) zone -16d to -17d.
  • GJ 244 A/B — Gliese-Jahreiss catalog, or Gliese Catalog of Nearby Stars #244 with stars A and B
  • GCTP 1577.00 A/B — the General Catalog of Trigonometric Parallaxes
  • HIP 32349 — Hipparcos astrometric-satellite catalog #32349
  • ADS 5423 — Aitken’s Double-Star Catalog #5423
  • LTT 2638 — Luyten Two-Tenths catalog #2638

Some notable stars still get individual names, like Barnard’s Star and Kapteyn’s Star, after some astronomers who discovered notable features of them. On the opposite end, some celestial objects do not have constellation-related names or catalog numbers but coordinate names, like pulsars. For instance, PSR 1913+16 is the pulsar at right ascension 19h13m declination 16d, where right ascension and declination are a sort of celestial longitude and latitude.

I won’t get into Solar-System nomenclature here, because that has plenty of complexity of its own.

I conclude with Naming of exoplanets | IAU by the International Astronomical Union. That page gives some guidelines for proposed names and name contests.


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