Astronomical Naming Conventions - Names of Planets

Names of Planets

The brightest planets in the sky have been named from ancient times. The scientific names are taken from the names given by the Romans: Mercury, Venus, Mars, Jupiter, and Saturn. Our own planet is usually named the Earth, or the equivalent in the language being spoken (for instance, two astronomers speaking French would call it la Terre). However, it is only recently in human history that it has been thought of as a planet. The Earth, when viewed as a planet, is sometimes also called by its Latin name Terra.

At least two more bodies were discovered later, and called planets:

• Uranus, discovered by William Herschel in 1781
• Neptune, discovered by Johann Gottfried Galle in 1846 (based on predictions by Urbain Le Verrier and John Couch Adams)

These were given names from Greek or Roman myth, to match the ancient planet names—but only after some controversy. For example, Sir William Herschel discovered Uranus in 1781, and originally called it Georgium Sidus (George's Star) in honour of King George III of the United Kingdom. French astronomers began calling it Herschel before German Johann Bode proposed the name Uranus, after the Greek and Roman god. The name "Uranus" did not come into common usage until around 1850.

Starting in 1801, asteroids were discovered between Mars and Jupiter. The first few (Ceres, Pallas, Juno, Vesta) were initially considered minor planets and joined the ranks of the planets. As more and more were discovered, they were soon stripped of their planetary status. On the other hand, Pluto was considered to be a planet at the time of its discovery in 1930, as it was found far beyond any then-known asteroid's greatest distance from the Sun.

Following this pattern, several hypothetical bodies were given names:

• Vulcan, for a planet within the orbit of Mercury;
• Phaeton, for a planet between Mars and Jupiter which was the precursor of the asteroids;
• Themis, for a moon of Saturn;
• Persephone (and several other names), for a trans-Plutonian planet.

These names, derived from Classical mythology, are only considered standard in Western discussion of the planets. Astronomers in societies which have alternate traditional names for the planets may use those names in scientific discourse. The IAU does not disapprove astronomers discussing Jupiter in Arabic using the term المشتري Al-Mushtarīy or astronomers speaking in Mandarin Chinese discussing Neptune referring to the planet as 海王星 Hǎiwángxīng.

Some sixty years after the discovery of Pluto, a large number of large trans-Neptunian objects began to be discovered. Under the criteria of classifying these Kuiper belt objects (KBOs), it became dubious whether Pluto would have been called a planet had it been discovered in the 1990s. Its mass is now known to be much smaller than once thought and, with the discovery of Eris, it is simply one of the two largest known trans-Neptunian objects. In 2006, Pluto was therefore reclassified into a different class of astronomical bodies known as dwarf planets, along with Eris and others.