Following on from Monday's Judgement Day for Pluto the International Astronomical Union (IAU) may have come up with a solution which downgrades Pluto but still recognises it as a planet. The following is based on a report on the BBC website.
The IAU proposal recognises eight classical planets, three planets belonging to a new category called "plutons" (Pluto, its satellite Charon and UB313, aka Xena) and the largest asteroid Ceres. Pluto thus remains a planet, but becomes the basis for a new category. Astronomers gathered at the IAU General Assembly in Prague will vote on the plan next Thursday.
Dr Andrew Coates of the Mullard Space Science Laboratory in Dorking said he thought the plan was "a good compromise". He explained: "It keeps the idea of eight classical planets, while Pluto is allowed to retain its status. But other objects are allowed in, which I suppose makes life more interesting. Something had to be done about the definition.... It does change the textbooks somewhat, but it also demonstrates that this is a vibrant area of research..... The surprise is Ceres, because most people thought of it as an asteroid". (Ceres is the largest object in the asteroid belt between Mars and Jupiter, and like a planet is spherical in shape)
The basis for this re-evaluation is a new scientific definition of a planet which uses gravity as the determining factor. According to this definition, two conditions must be satisfied for an object to qualify as a planet: The object must be in orbit around a star, but must not itself be a star; It must have enough mass for the body's own gravity to pull it into a nearly spherical shape
More objects are likely to be announced as planets in the future. The IAU has a "watchlist" of at least a dozen other potential candidates that could become planets once more is known about their sizes and orbits. These include the distant objects Sedna, Orcus, Quaoar and 2003 EL61 and the asteroids Vesta, Pallas and Hygiea.