Astronomical Names

Astronomy is an old science. Its nomenclature is often derived from tradition and history (especially Greek/Roman mythology) rather than from what might today seem more sensible. The names of the objects in the solar system are particularly so. The International Astronomical Union (IAU) is officially in charge of assigning astronomical names and it is very sensitive to astronomical tradition.


Planets in our solar system get a name as soon as possible. There aren’t likely to be any new ones until we see them around another star. Who knows what convention will be used then.


Satellites of the planets are assigned a provisional designation (indicating year of discovery) by the Central Bureau for Astronomical Telegrams (CBAT) upon satisfactory demonstration of the existence of a new object. For example, when Voyager 2 found a bunch of new moons in its 1989 Neptune encounter, they were named S/1989 N 1, S/1989 N 2, etc. When the orbit of the new object is accurate enough to allow the prediction of future positions with reasonable precision, the object is assigned a Roman numeral (e.g. Neptune VIII), and the object may receive a name (e.g. Proteus). (The name is suggested by the discoverer(s), but following tradition is strongly encouraged; so if you want to name a planet or satellite, get out your books of mythology. Note also that the moons of Uranus are a special case — they get literary instead of mythological names.) This process is currently underway for the new Uranian moons discovered recently.


Asteroids are at first given provisional numbers by the Minor Planet Center (MPC) indicating the year and month of discovery. The provisional designation is the year, followed by a letter that indicates the half-month during which the discovery occurred, followed by a letter that indicates sequence within the half-month. So 1982 DB was reported in the last half of February 1982, and it’s the second asteroid discovered during that period. When their orbits are understood well enough that their future position can be predicted well, they are given a permanent number and name. “243 Ida” is the 243rd asteroid to be numbered (not necessarily the 243rd discovered). The name is again chosen by the discoverer, but there’s much more latitude; asteroids can be named after a living person, or almost anything else (e.g. “2309 Mr. Spock”; several have been named for popular musicians). Final decisions are made by an IAU committee. For more information see “How Are Minor Planets Named?“.


In the past, Comets were first given a provisional designation, consisting of the year and a lowercase letter indicating the order of discovery in the year (e.g., 1994a was the first comet discovered or recovered in 1994, 1994b the second, etc.). The name was also assigned at an early stage. Up to three (preferably independent) discoverers may have been attached to the comet. Some time later, the comets that had passed perihelion in a given year were assigned Roman numeral designations indicating the order of perihelion passage within the year. The Roman numeral designations for 1993 and 1994 are given in the Jan. 1995 batch of Minor Planet Circulars (MPCs).

Note that whole comet designation system was revamped starting in the beginning of 1995. The main points of the new scheme are:

  1. the provisional designation system now closely matches the designation system for minor planets. The first comet discovered in the first half of 1995 Jan. is designated 1995 A1, the second 1995 A2, etc.
  2. long-period comets and one-apparition periodic comets receive only a provisional designation — there is no equivalent of the Roman numeral designation.
  3. upon recovery at a second apparition (or following through aphelion) periodic comets receive a sequential number. E.g., P/Halley is 1P.
  4. routine recoveries of periodic comets do not receive provisional designations.
  5. the nature of the comet orbit is indicated by a prefix: P/ for periodic comets, C/ for long-periodic comets, D/ for defunct comets (e.g., 1993e) and X/ for uncertain comets. Additionally, A/ is used to indicate that the object is a minor planet.
  6. comets continue to be named in general terms for their discoverers ensuring fairness and simplicity.
  7. Provisional comet designations are assigned by the CBAT. Permanent comet numbers are assigned by the Minor Planet Center.

The new scheme was backdated, so old comets received new-style designations. There is an interactive converter between old- and new-style comet designations.

Some examples of new comet designations:

  C/1995 Q2 (Hartley-Drinkwater)
  P/1994 P1-A (Machholz 2)     Fragment A of a split comet
  P/1996 A1 (Jedicke)          New periodic comet
  125P                         Routine observation of periodic comet

Here is a copy of the official IAU resolution.

Surface Features

Landscape features on planets, satellites, and asteroids follow complicated conventions set by the IAU Nomenclature Committee. Among these are the restriction that a planetary feature may not bear the name of a living person or of a political or religious figure from the last 200 years. A good explanation of this may be found in *Planetary Mapping*, edited by Greeley & Batson Cambridge U.Press, 1990. (Gee, it’s handy having Phil Stooke’s Planetary Map FAQ lying around on my disk!)

[ Adapted from usenet postings by Bill Higgins and Gareth Williams but any errors are mine. ]

Gazetteer of Planetary Nomenclature

This site contains detailed information about all names of topographic and albedo features on planets and satellites (and some planetary ring and ring-gap systems) that the International Astronomical Union (IAU) has named and approved.