Posted by: dacalu | 3 March 2016

The Naming of Planets

This week at CTI I was asked to present on how we name the heavens in astronomy.  If you’ve ever wondered how Venus and Io and the Milky Way and 51 Peg b got their names, look no further.

Borrowing shamelessly from T.S. Eliot,

The Naming of stars is a difficult matter,
It isn’t just one of your holiday games;
You may think at first I’m as mad as a hatter
When I tell you, a star must have TWO DIFFERENT NAMES.

First, it is important to distinguish between official names and public names. The International Astronomical Union regulates official designations for astronomical objects or “celestial bodies,” but many of them have common names as well. The IAU attempts some regulation of those as well, for clear communication, but if you really want to call the second star to the right “Bob,” you are free to do so.  Such common names vary from place to place and country to country.

Getting to details:

The names of the planets (in our Solar System) are traditional. Seven planets appear in Plato’s Timaeus corresponding to the major gods of the Greek pantheon (4th c. BCE).[1] With the Copernican move to a heliocentric cosmos, Earth becomes a planet per se, while the Sun and Moon lose their places (16th c. CE). Uranus was discovered in 1781 by William Herschel, who named it Georgium Sidus, in honor of his patron, George III of Great Britain. This was not popular outside Britain, and over the next 70 years was replaced with Uranus, after the Greek god who fathered Saturn as Saturn fathered Jupiter. Neptune was discovered in 1846 by Urbain Le Verrier and a number of names were suggested, but one of La Verrier’s suggestions, Neptune, was accepted within a few years. Pluto was discovered in 1930 by Clyde Tombaugh and completed the set, as Neptune and Pluto were Jupiter’s brothers with whom he overthrew the Titans in mythology. Most of the planets go by the Roman names. The exceptions are Earth (Germanic) and Uranus (Greek).

The Solar System is comprised of eight planets and two belts all orbiting the Sun in a plane (“the plane of the elliptic”). Pluto, at 17 degrees off plane, was already anomalous and when a larger object was found in its region, the IAU downgraded it from “planet” to
dwarf planet.” The term planet now designates an object orbiting a star, large enough to form a sphere under gravity, that has cleared the neighborhood of other objects. Dwarf planets are spherical, but had not cleared the area.

The term minor planet refers to other objects orbiting a star. Most minor planets are asteroids or comets. Asteroids are rocky (more carbon and silicon, less water, hydrogen, and helium) minor planets, mostly orbiting in the asteroid belt between Mars and Jupiter. They are thought to be the remains of a planet or a planet that never formed and include the dwarf planet Ceres. Asteroids sharing Jupiter’s orbit are called Trojans and are named for Trojan heroes. Asteroids orbiting from Jupiter to Neptune are named for Centaurs. Other asteroids are open to naming, with minor rules and an exclusion of military or political figures within the past 100 years. Comets are volatile (more water etc.) minor planets whose orbits take them beyond Neptune. They receive official codes (e.g., Comet Hale-Bopp was originally designated C/1995 O1). Some exceptions are made, as with Hale-Bopp and Halley. Most comets orbit in a belt (the Kuiper Belt) near the plane of the ecliptic beyond Neptune, and as such are designated Trans-Neptunian Objects (TNOs). They include the dwarf planets Pluto, Eris, and Makemake. Those that receive names, are named for underworld deities or creation deities, depending on their orbital dynamics. The object that resulted in downgrading Pluto was named for Eris, the goddess of strife.

Moons and Planetary features are named by convention (see USGS and NASA pages) :

Mercury – named for the messenger god because of it’s swift orbit

No moons. Features named for artists and explorers

Venus – brightest star, named for the goddess of beauty

No moons. Features named for women: historical, fictional, or mythical

Moon – satellite of Earth.

Many classical feature names

Features named for scientists and mathematicians

Mars – named for the god of war, possibly due to red color

Phobos and Deimos named for Mars’ attendants, fear and dread (horror and terror); Phobos’ features named for characters in Gulliver’s Travels

Many classical feature names

Features named for towns

Jupiter – named for the Roman king of the gods

3 rings and 67 moons named for Jupiter’s “lovers and favorites” (1975) and daughters (2004) (Galileo wanted to name them for the Medici, but they are now known as the Galilean moons, after Galileo. Theses first four were named by Simon Marius in 1610, but called Jupiter I, II… until the 20th c. when Marius’ names were revived.)

Saturn – named for Jupiter’s father, slowest orbit among the classical 7

7 rings and 62 moons named for Titans (proposed by Herschel in the 18th c.) and other giants

Uranus – named for Saturn’s father

13 rings and 27 moons named for characters from Shakespeare’s plays and Pope’s “Rape of the Lock”

Neptune – named for the god of the sea

6 rings and 14 moons named for water gods and spirits


Stars often have names based on classical usage, including Flamsteed’s star atlas (early 18th c.) or Gliese’s index (1957). Extrasolar planets can be named for their star or their discovering instrument (e.g., Kepler) along with a lowercase letter, if there are multiple planets in the system. [Upper case letters are used for binary stars…] A few extrasolar planets have also been given semi-official popular names by public contest and committee approval. Galaxies, nebulae and other large features have designations. Historically these started with Messier’s Catalogue (1771). The Milky Way is a feature of the sky named by the Greeks for the appearance of milk spilled across the sky. Thomas Wright proposed this as a structure of stars in 1750 and Immanuel Kant first explained it as a galaxy in the modern sense in 1755.


[1] 38b-40b. Plato says he is describing the nature (physis) of the visible and generated gods (40c). These include Gaia (Earth), the greatest. Above Gaia are Selene (the Moon), Helios (the Sun), Hesperus (Venus), and Hermes (Mercury) in ascending order. The final three would have been Ares (Mars), Zeus (Jupiter), and Cronus (Saturn). There are probably older references, but this is the earliest I have on hand.



  1. […] of descent and environmental niche, are more commonly named than genus and species. Beyond biology, celestial bodies, geological time periods, and minerals likewise have their own nomenclature, code, and committees. […]

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