Under the Microscope: A popular question at table quizzes is - how many planets are in our solar system? Until very recently, the correct answer was nine, but on July 29th, 2005, the discovery of a 10th planet beyond Pluto was announced.
The new planet was photographed using the Samuel Oschin telescope at the Californian Palomer Observatory and the discovery was part of a Nasa-funded survey being carried out by astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale University).
Although first seen in 2003, the new planet is so far away that its motion was not detected until January 2005. The new planet currently bears the decidedly inelegant temporary name 2003UB313. A permanent, more elegant name has been proposed but won't become official until approved by the International Astronomical Union (IAU).
The new planet is the most distant in orbit around our sun. Until now, Pluto was accorded that distinction, but 2003UB313 is over three times more distant from the sun than Pluto - almost 10 billion miles away. Pluto orbits the sun every 250 years; 2003UB313 orbits the sun every 500 years.
The new planet is up to twice the size of Pluto and both are members of the Kuiper belt, a swarm of icy bodies that orbit around the sun beyond Neptune.
Both Pluto and 2003UB313 have quite eccentric orbits (ie deviate considerably from circular orbits) around the sun. Pluto moves between 30 to 50 times the sun-earth distance over the course of its orbit; 2003UB313 moves between 38 to 97 times the sun-Earth distance over the course of its orbit.
Because the new planet is so distant it moves very slowly when viewed by telescope from Earth. In fact it moves so slowly that when first photographed in 2003 the computers didn't notice the movement and the existence of the new planet was only confirmed on January 5th 2005 when the data was re-analysed. But the public announcement wasn't made until late July 2005, so why the delay?
Well, Brown and his colleagues were making further observations in order to confirm things and to provide a fuller picture before going pubic. However, in the end the discovery was hastily announced on July 29th after Brown learned that his "secure" website containing details of the location of his discovery had been hacked.
The major planets in our solar system in order of their proximity to the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto and, now, 2003UB313. Mercury, Venus, Earth and Mars are smallish rocky planets. Jupiter, Saturn, Uranus and Neptune are large compared to Earth and are composed mostly of gas and ice. Study to-date of the new planet 2003UB313 indicates that its composition is similar to that of Pluto - ie a surface covered with solid frozen methane and an interior likely to be composed of rock and ice. The two planets are not identical in composition however as indicated by the fact that Pluto's surface is red in colour whereas the new planet is grey.
The first eight planets in our solar system, from Mercury out to Neptune, all orbit the sun in almost circular orbits and essentially in the same plane. Both Pluto and 2003UB313 have very eccentric orbits and their orbits are also tilted with respect to the inner eight planets. 2003UB313 is tilted 45 degrees away from the plane of the orbit of the inner planets.
The new discovery re-ignites the debate over what constitutes a true planet. Some astronomers consider that Pluto should not be considered a true planet, and scientifically it seems clear that Pluto is not in the same category as the other planets. There is no universally accepted definition of planethood, but, except for Pluto and now 2003UB313, the other planets share common distinctions. They orbit the sun, they are round and they can be classified as solitary entities as opposed to members of a group. Each historical planet exists in a region of space in which it is the only one of its kind.
Many astronomers see Pluto's designation as a planet as an accident of history. In 1930 when it was found it was thought to be the only one of its kind. We now know it is a member of a large population in the Kuiper belt. However, as Mike Brown explains in his website www.gps.caltech.edu/~mbrown/ planetlila/index.html, Pluto has been culturally accepted as a planet while at the same time it is culturally accepted that other things like large asteroids and other Kuiper belt objects smaller than Pluto are not planets. On this basis, new objects discovered orbiting our sun that are larger than Pluto should also be considered planets. The International Astronomical Union (IAU) is currently pontificating on the exact classification of the new object, but I will be surprised if it is decided not to designate 2003UB313 as a major planet.
All the nine established planets are named after Greek or Roman gods. It would seem appropriate therefore that such a name should also be assigned to the new planet. However, all of the significant Greek or Roman god names have been allocated to astronomical bodies. The IAU will not allow names already assigned to be used again. However, there are many mythological and spiritual traditions other than Greek and Roman and Brown and his colleagues have proposed a name to the IAU drawn from this wider category.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC