Internationally, rules for when to capitalize the names of objects in astronomy are currently set by the International Astronomical Union (IAU). In general, all planets, stars, and spacecraft should be capitalized as proper nouns - a grammatical category referring to the proper names of specific people (e.g. Nicolaus Copernicus), places (e.g. Minnesota), and things (e.g. USS Constitution).
- When to Capitalize -
In general, the IAU rules explain that capitals should be used whenever referring to a specific body or vessel. For example, all planets should be capitalized: Mercury, Earth, Jupiter, Saturn, and so on.
In addition, all human space probes should be capitalized: Cassini, New Horizons, Spirit, Opportunity, etc. Note that NASA and other space agencies sometimes bend this rule by incorporating all-capitalized names for spacecraft. The technical spelling for the NASA spacecraft currently en route to Mercury, for example, is MESSENGER - not Messenger - because it is an acronym standing for the full name of the spacecraft: "Mercury Surface, Space Environment, Geochemistry and Ranging."
Finally, all stars and other astronomical phenomena should be capitalized when they are being referred to as specific, named objects. Under this rule, stars should be capitalized (e.g. Alpha Centauri). Galaxies are capitalized, as well: for example, the Andromeda Galaxy. Note that "star," "galaxy," etc. are here capitalized only if they are part of the formal name of the object: for example, "star" is capitalized in the name Pistol Star (which is the full name of that star), but "galaxy" is not capitalized when referring to the Maffei 1 galaxy (since the full name of that galaxy is just "Maffei 1"). The same rule is applied when referring to spacecraft, such as the Hubble Space Telescope.
In some cases, the IAU suggests using capitals even when what is being referred to does not have a formal name, provided that it is a unique, specific location. For example, "the Galactic Centre" is capitalized when referring to the centre of the Milky Way Galaxy.
- When Not to Capitalize -
Capitalization is not necessary when referring to a class of objects in general, or when referring to an object without its full proper name. For example, referring to a planet (or planets), star (or stars), galaxy (or galaxies), black hole, etc., is done without using any capitals.
The same rule is true of human objects: "space telescopes" or "space probes," for example, are categories of objects which are not capitalized. Again, however, there are certain exceptions which have been introduced by various nations. For example, NASA always capitalizes the word "Space Shuttle," even though there are actually multiple Space Shuttles which have unique, individual names (e.g. Atlantis and Endeavour).