Astronomers to vote on potential new planets

August 16, 2006

The International Astronomical Union (IAU) has offered up a new definition of the word "planet" that could potentially increase our solar system's nine planets to at least twelve. According to the proposed definition, "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet." Roughly speaking, the former includes objects over 5 x 1020 kilograms (1/12,000th of Earth's mass) and 800 kilometers in diameter, but all borderline cases would require confirmation by observation of their shape.

According to this new definition, Ceres (an asteroid in the asteroid belt, between Mars and Jupiter), 2003UB313 (an astronomical object beyond Pluto which has previously been called the tenth planet), and Pluto's moon Charon may be dubbed planets. A dozen other candidates, like Sedna, Orcus, Quaoar, 2003 EL61 etc., along with the asteroids Vesta, Pallas and Hygiea are awaiting evaluation by the IAU.

The draft resolution also introduces the term "pluton", which refers to a growing subcategory of planets that have orbits around the sun that take at least 200 years to complete - effectively this will mean planets that orbit beyond Neptune. Plutons differ from classical planets: their orbit is highly tilted, eccentric and not circular, which suggests they have a different origin, the main reason why astronomers are interested in them.

The word "planet" comes from the ancient Greek word for "wanderer", because it was known in ancient times that certain lights in the sky moved in relation to other stars. However, since then no formal definition of the word planet was agreed upon. With the advent of powerful telescopes on the ground and in space, knowledge about heavenly bodies became more complex, stressing the need for unambiguous definitions. The issue came to a head in 2005 with the discovery of the trans-Neptunian object 2003UB313 (unofficially termed Xena by its discoverer, Michael E. Brown). 2003UB313 is a body larger than the smallest accepted planet, Pluto.

The suggested definition is not universally popular: some astronomers would like to draw the line at Neptune, and wouldn't classify icy dwarfs like Pluto as a planet, but as a trans-Neptunian object (similar to the plutons in the draft resolution) part of the Kuiper belt. Robin Catchpole, from the Institute of Astronomy, Cambridge, said in an interview: "The public are very clear about what they understand by "planets". Those are the big, dominant bodies in the Solar System that we're all familiar with, the eight - or nine if you include Pluto. I think including more is going to add confusion to the public, but not really be particularly useful for astronomers."

Pluto is now being considered to be moved off the list of classical planets, but as the prototype of the new plutons category. The ninth planet (as it is termed, but maybe not for long), discovered in 1930, is a curiosity among planets for more than one reason: because Charon is so big in comparison, both are considered twin planets by some.

The current proposal is the result of two years work of the Planet Definition Committee of the IAU, which is responsible for the naming of astronomical objects. The 26th IAU General Assembly in Prague plans to vote on the proposal on Thursday, August 24 of 2006. When asked if he was confident the proposal would get the necessary two thirds of the votes, Professor Owen Gingerich, Chair of the IAU Planet Definition Committee, replied: "I'm sure it will be controversial to those with a stake in some other solution, but I hope we will get an overwhelming endorsement."