Alan Stern, the principal investigator of the New Horizons Mission to Pluto is calling for a new definition for planets in order to return Pluto to, what he and other see as its former glory, the status of a planet. The so called demotion of Pluto caused the release of strong emotions amongst the distant planet’s fans and the stunning success of the New Horizons mission added fuel to the flames in the on going debate. Many of those participating seem to be somewhat unaware of the fact that the definition of what is a planet has changed down the centuries and I thought I would write a brief guide to the changing fortunes of the term planet since its inception in antiquity.
It should be made clear that I shall only be talking about European astronomy and not any other traditions such as Chinese, Indian, Mayan astronomies etc. European astronomy/astrology has its roots in ancient Babylon. The Babylonian tradition was most concerned with the Moon and the Sun but the Babylonians were aware of the planets Mercury, Venus, Mars, Jupiter and Saturn, which they like other ancient cultures regarded as divinities. They tracked their orbits over very long periods of time and developed algorithms to determine their appearances and disappearances for omen astrological purposes. They don’t appear to have been interested in the mechanism of the planetary orbits. I’m anything but an expert on Babylonian astronomy/astrology and I don’t know if they had a collective name for them.
The direct inheritors of the Babylonian celestial interests were the ancient Greeks and they were very much interested in orbital mechanics and they also coined the term planet. For the Greeks all illuminated objects in the heavens were stars (aster, astron), as I explained in an earlier post. The stars as we know them were the fixed stars because they appeared to remain in place relative to each other whilst the sphere of the fixed stars rotated about the celestial axis once every twenty-four hours. It was of course the Earth that rotated about its axis and not the stars but the Greeks were not aware of that. The illusion that the stars, visible to the naked-eye, are all equidistant to the Earth is easy to experience. Just go out into the countryside were there is no light pollution and look up at the night sky on a clear night. You will see the ‘sphere of the fixed stars’, as experienced by the ancient Greeks. Comets, much rarer and apparently random, were hairy stars, the word comet derives from the Greek aster kometes, literally long-haired star. The five planets known to the Babylonians and the Moon and the Sun were all present on a regular basis but unlike the fixed stars they appeared to wander around the heavens and so they became asteres planetai that is wandering stars, from planasthai to wander. The Greeks had seven wanderers Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn. The Earth was of course stationary at the middle of the whole system and so was not a planet.
Subsequent European cultures and the Islamic Empire inherited the Greek model of the heavens with its seven wanderers and nothing of significance changed down the centuries until the Renaissance and the advent of Copernican heliocentrism in 1543. Copernicus’s new model was of course a major upheaval. The Sun became stationary and the Earth became a planet wandering through the heavens. The Moon acquired a strange new status, no longer orbiting the centre, now the Sun, but orbiting the Earth. Heliocentricity took more than one hundred years to become establish and Copernicus’ upheaval brought no immediate change of terminology.
The first change came in 1610 with the telescopic discovery of the four largest moons of Jupiter by Galileo and Simon Marius. Here we have four new celestial bodies orbiting a planet, as with the Moon, and not the centre of the cosmos. At first Galileo referred to them as stars or planets, leading Kepler, who was at first not clear what the four new objects were, to panic and fear that Giordano Bruno was right and that all stars had planets. This conflicted with Kepler’s own finite universe cosmology. He was greatly relieved to discover that the new planets were in reality moons and coined the term satellite from the Latin satillitem meaning attendant, companion, courtier, accomplice or assistant. Kepler was very fond of creating new scientific terminology. The term was not adopted immediately but by the end of the seventeenth century astronomers differentiated between planets and satellites, around the same time as heliocentricity became firmly established and the Sun finally ceased to be a planet and the Earth finally became one. Around the same time astronomers became convinced that the Sun was actually one of the ‘fixed’ stars.
We entered the eighteenth century with six planets, Mercury, Venus, Earth, Mars, Jupiter and Saturn and so it remained until the musician and amateur astronomer William Herschel shocked the world with the discovery of a seventh one, Uranus on 13 March 1781. The first new planet discovered in about four thousand years of planetary astronomy.
In the middle of the eighteenth century Johann Elert Bode published what is now know as the Titus-Bode law in which the distance of the planets from the sun seemed to fit an arithmetical series with a gap in the series between Mars and Jupiter. Herschel’s discovery of Uranus beyond Saturn fit the Titus-Bode series, which led the German astronomer Baron Franz Xaver von Zach to organise a systematic search for that ‘missing planet’ between Mars and Jupiter. In fact the discovery was made by the Italian astronomer Giuseppe Piazzi, who was not part of Zach’s search team but discovered Ceres on 1 January 1801, exactly, where it should be according to the Titus-Bode law and then there were eight. Interestingly Piazzi lost Ceres and Carl Friedrich Gauss developed a new method of determining planetary orbits, which allowed astronomers to find it again. Very soon other astronomers discovered Pallas, Juno and Vesta and there were now eleven planets. It was not long before it became clear that the four new celestial bodies were somehow different to the other planets and Herschel coined the term ἀστεροειδής, or asteroeidēs, meaning ‘star-like, star-shaped’, in English asteroid. These smaller wanderers were also known as minor planets or planetoids although it was first in the later nineteenth century, by which time several more asteroids had been discovered that these terms became established and the number of planets was once again reduced, not to seven but to eight!
It was eight because in the mean time both the English astronomer John Crouch Adams and the French astronomer Urbain Le Verrier had predicted the existence of an eighth planet based on gravitational anomalies in the orbit of Uranus and on 23 September 1846 the German observational astronomer discovered Neptune, the eighth planet, based on the predictions of Le Verrier.
In the late nineteenth century similar anomalies in the orbit of Neptune led Percival Lowell to predict the existence of a ninth planet and he set up his own observatory to search for it. In 1916 Lowell died without having found his predicted planet. However in 1929/30 the young Clyde Tombaugh discovered Pluto, the ninth planet.
As with Ceres and the asteroids Pluto’s planetary status was challenged by the discovery of other orbiting objects in the Kuiper belt outside of the orbit of Neptune from the 1990s onward. The discovery of Eris in 2005 led to a serious reconsideration of Pluto’s planetary status and famously in 2006 the International Astronomical Union introduced a new formal definition of the term planet, which removed Pluto’s planetary status and according to Pluto’s fans demoted it to the status of a dwarf planet. At the moment there are five recognised dwarf planets Pluto, Ceres (the largest asteroid), Haumea, Makemake and Eris.
As I said at the beginning the Pluto fan club has not given up the fight and are now proposing a new definition of the term planet, which would not only return Pluto to its planetary status but also apparently the Moon. I hope I have shown that the term planet has gone through quite a lot of changes over the last two and a half thousand years or so since the ancient Greeks first coined it and we can, I think, assume that it will go through quite a few more in the future in particular with respect to the thousands of exoplanets that astronomers are busy discovering.