planet, dwarf planet

Our word planet ultimately comes from the Greek πλάνητ-, πλάνης (planet-, planes) meaning wanderer, a reference to the motion of the planets relative to the stars. The word came into English via Old French planete and the Latin planeta. While the etymology of the word has never been in doubt, exactly what constitutes a planet has constantly changed over the centuries and is still hotly debated today.

The Latin and Greek words were known to the Anglo-Saxons, although they did not fully incorporate them into Old English. Ælfric, in his De temporibus anni (Regarding the seasons of the year), a text on astronomy written in 992, records:

Seo sunne, & se mona, & æfensteorra, & dægsteorra, & oðre ðry steorran ne sind na fæste on ðam firmamentum, ac habbað heora agenne gang on sundron. Þa seofon sind gehatene Septem planete.

(The sun, the moon, the evening star, the morning star, and three other stars are not fixed in the firmament, but have their own path apart. The seven are called Septem planete.)

Byrhtferth’s Enchiridion (Handbook), another book on astronomy, written c. 1011, contains the following:

Þa steorran þe man hæt planete on Lyden and on Grecisc apo tes planes (hoc est ab errore) oðre hwile hig beoð on eastende þære heofone swa sunne byð dæghwamlice.

(The stars which people called planete in Latin and in Greek apo tes planes (that is, from wandering) are sometimes at the eastern end of the heaven, as the sun is very day.)

It isn’t until the Middle English period that planet starts appearing in English works as a naturalized word, by this date borrowed from Norman French. The South English Legendary, a collection of saint’s lives written c. 1300 and surviving in many versions, records the word. The version of the life of St. Michael the Archangel, contained in Oxford, Bodleian Library, Laud Misc. 108 contains the following:

þat planete i-cleoped is [...] Saturnus is al a-boue and Iupiter sethþe next, þanne Mars bi-neoþen him, and sethþe þe sonne is, Venus sethþe, þe clere steorre, Mercurius þanne [...] þe Mone is next þe grounde.

(That planet is called [...] Saturn is highest and Jupiter follows next, then Mars beneath him, and then follows the son, Venus, the bright star, follows, then Mercury [...] the moon is next to the ground.)

These early uses of the word referred to seven celestial bodies: the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn. These are the planets of ancient astronomy and astrology. (The ancients and medievals did not distinguish between the two). This definition held until the Copernican revolution.

The heliocentric solar system promulgated by Copernicus and Galileo necessitated a redefinition of planet. The sun and moon were excluded from the list, and Earth was added. The first to formally consider the Earth as a planet was John Wilkins, who would go on to become the Bishop of Chester and one of founders of the Royal Society, who in 1640 published a tract titled, “A Discourse concerning a New Planet. Tending to prove, That ‘tis probable our Earth is one of the Planets.”

Over the succeeding centuries the list of planets expanded and contracted. Uranus and Neptune were added upon their discovery. The asteroid Ceres was added and then removed. The planet Vulcan (not the one from Star Trek), believed to orbit the sun closer than Mercury, was considered a planet for a while, until it was shown to not exist. Pluto, the first of the Kuiper Belt objects to be discovered, was added to the list upon its discovery in 1930, and for the rest of the twentieth century the list consisted of nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

In 2005, the discovery of additional Kuiper Belt objects, Eris, Makemake, and Haumea, prompted a problem. Were these to be considered planets too? Eris is roughly the same size as Pluto, and therefore should be considered in the same class. (The New Horizons probe has given us a good measurement of Pluto’s size, but that of Eris is continually being revised up and down as new observations come in. Some days it’s bigger than Pluto; on others it’s smaller.) And there would likely be hundreds of Kuiper Belt objects discovered in coming years, with quite a few being the same size as Pluto and Eris. The list of planets could grow to be unmanageable.

In 2006 the International Astronomical Union (IAU) promulgated a new definition. To be considered a planet an object had to:

  • orbit the sun
  • achieve hydrostatic equilibrium (i.e., have a round shape)
  • have cleared its neighborhood of other objects.

Pluto and Eris failed this last criterion. The list of planets was officially capped at eight.

The definition succeeded in keeping the list manageable, but it’s a scientifically useless classification. First, it arbitrarily limits the planets to our own solar system. There are no other planets in the universe—planets outside the solar system are officially dubbed exoplanets, a term that dates to 1996. Hydrostatic equilibrium depends on both mass and composition, so a more massive rocky object might fail the criterion, while a smaller gaseous body might meet it. And whether an object can “clear its neighborhood” depends not only on size, but also on distance from the sun. Mercury, Venus, Earth, and Mars would not have sufficient mass to clear their neighborhood if they orbited in the Kuiper Belt. And planets can shift their orbits over their lifespans, so it is possible for an object to be a planet in one epoch and not one in another. Ganymede, the largest moon of Jupiter and of the solar system, has far more in common with Mercury than Jupiter does, yet Mercury and Jupiter are in the same category and Ganymede is not. (Ganymede has a larger diameter than Mercury, although Mercury is twice as massive.)

Those solar objects that fail to make the planetary cut have been dubbed dwarf planets. Currently there are about a dozen objects that meet the official criteria for this category, including Pluto, Eris, and the asteroid Ceres. But dwarf planet, like its larger cousin, is not a new term and what constitutes a member of the category has also shifted over the years. Dwarf planet was first used in 1839 in reference to the asteroids Ceres, Vesta, Pallas, and Juno. Over the years various objects have been considered dwarf planets, including Earth, Venus, and Mars. (Which makes sense when you compare them to a behemoth like Jupiter.)

The IAU definition dominates the discussion currently, but it will assuredly be edged aside eventually because it makes no sense, either scientifically or lexicographically. Planets have always been those things that people call planets, and the list of planets has expanded and contracted with the fashion of the age. The logic is circular, but that’s how language works. Perhaps the best method to determine whether or not a celestial object should be dubbed a planet is the Star Trek method—if it would like a planet if seen from the viewscreen of the starship Enterprise, then it is a planet.

Ælfric, De temporibus anni, Heinrich Henel, ed., EETS O. S. 213, London: Oxford University Press, 1942.

Byrhtferth, Enchiridion, Peter S. Baker and Michael Lapidge, eds., EETS S. S. 15, Oxford: Oxford University Press, 1995.

Middle English Dictionary, University of Michigan, 2001, s. v. planet(e (n.(1)), s. v. exoplanet.

Oxford English Dictionary Online, third edition, s. v. dwarf planet, n. (June 2009) planet, n. (June 2006)

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