Flamsteed Astronomy Society

When is a planet not a planet?

by Dr Eddie Yeadon former Chairman of the Flamsteed — transcribed from f@nmm issue 5 Autumn 2006

The International Astronomical Union (IAU) has decided that Pluto isn’t a planet after all, and that we must call it a Dwarf Planet now.   The term ‘planet’ originally meant a wandering star which could be seen to move among the unchanging patterns of the fixed stars.   There were five of these visible to the naked eye — Mercury, Venus, Mars, Jupiter and Saturn.  The heliocentric Copernican system recognised Earth as a similar body, and that they all travelled in near-circular orbits about the Sun.  With improved telescopes, two more, Uranus and Neptune, were discovered and it was found that while the inner four were relatively small, dense rocky objects, the outer ones were enormous, low-density ‘gas giants’.  But they were all ‘planets’.  Telescopes also revealed other, much smaller, bodies orbiting the Sun between the orbits of Mars and Jupiter.  These were named ‘minor planets’ or ‘asteroids’ as they were recognisably different from the established planets.

In 1930 an object was discovered beyond the orbit of Neptune and it was named as the ninth planet, Pluto.  But it is not like the other planets, being even smaller than Mercury, with a companion ‘moon’, Charon, nearly as big as itself, and in an eccentric orbit which crosses through that of Neptune.  In many ways it fits better into the category of a ‘Kuiper Belt object’ — icy relics of the primordial Solar System which failed to condense into planets, in cold and very remote orbits.  Other Kuiper Belt objects have been discovered and the latest, found in January 2005 (officially 2003 UB313 but provisionally called Xena *) is larger than Pluto.   Was this to be a planet too?  How about Charon, or Ceres, the largest of the minor planets?  How many more objects, yet to be discovered in the Kuiper Belt, would qualify to be planets?


Pluto (L) and its ‘moon’ Charon taken from the Hubble Space Telescope 1994.  Pluto was 4.4 billion km (2.6 billion miles) from Earth — 30 times the distance from Earth to the Sun


courtesy Hubble STScI/NASA

A lively debate followed among astronomers about how these objects should be categorised and named.  The International Astronomical Union set up a committee to study the matter, and this reported to the 26th General Assembly of the IAU held in Prague in August 2006.  The resolutions passed at this meeting declared three categories :

Planets:  These must be in orbit around the Sun, be massive enough for their gravity to pull them into a nearly round shape, and have cleared the neighbourhood around their orbits of other objects by gravitational attraction.  This neatly covers all eight of the classical planets, but excludes Pluto.

Dwarf Planets: These would be round objects orbiting the Sun, that have not cleared the neighbourhood around their orbits, and would include  bodies like Ceres, as well as Pluto, Charon, and Xena*

Small Solar System Bodies:  This term would apply to all other objects orbiting the Sun, like comets and asteroids, but not to satellites (moons) of planets.


There is still some discord among astronomers, some of whom object to an object being classified by what is around it rather than what it is in itself.  But does it really matter what we call these things?  There are such great differences in the properties within each category (for example between the planets Mercury and Jupiter, or between the dwarf planets Ceres and Pluto) that a single class name cannot completely describe them.  The ancients saw stars that moved and stars that remain fixed, and they used names that expressed this difference.

We know too much about them now to fit them into such simple categories — they are all different, whatever labels we apply.

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* the object formerly known provisionally as ‘Xena’ is now officially called ‘Eris’ (Ed)

courtesy NASA, ESA & A. Feild (STScI)

courtesy NASA

Lunar & Planetary Laboratory