12 planets? 15? 19?

The International Astronomical Union (the body officially responsible since 1919 for naming planets, stars etc.) has proposed a plan for dealing with the problem of Pluto. What problem? Well, we all learned that there are 9 planets in the solar system – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. However, Pluto’s always been an anomaly. It’s far smaller than any other planet (indeed, it’s smaller than 9 moons, including our own Moon), it’s basically a lump of ice, it doesn’t orbit in the same plane as the other planets, and its orbit actually crosses that of Neptune.

What really pushed the issue, though, was the discovery in recent years of several “trans-neptunian objects” – large icy bodies orbiting in roughly the same area as Pluto. And, in particular, the discovery in 2003 of the object called UB313 (or “Xena”), which is actually larger than Pluto. If it’s bigger than Pluto then surely it has at least as much right to be called a planet? The problem was that no one really wanted to think of it as a planet, because it’s just one of (literally) billions of similar bodies in the outer regions of the solar system.

Now, according to the BBC, after 2 years of deliberation, the IAU have proposed a basic definition of a planet – it must be round owing to its gravity (i.e. it’s got to be pretty big) and it must orbit a star (so round moons don’t count, because they orbit planets). The new problem, though, is that the solar system suddenly contains at least 12 planets – the eight “classical” planets Mercury to Neptune, Pluto and Pluto’s “moon” Charon (which so large in comparison to Pluto that they’re often regarded as a twin planet) and Xena (which are together to be known as the “plutons”), and even more strangely Ceres. Until now, Ceres has just been the largest of the asteroids. However, it’s large enough to be gravitationally rounded and orbits the Sun, so it counts.

So, the solar system now goes: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto/Charon, Xena. And it gets even more confusing, because there are several other bodies already known that will probably pass this new definition – the four trans-neptunian bodies Sedna, Orcus, Quaoar and 2003 EL61, and the asteroids Vesta, Pallas and Hygiea. The solar system looks set to keep getting bigger.

Update
New Scientist has more details on the story, including a nice summary of the proposed new definitions of planet, pluton and so forth, and a diagram of the new solar system.

pax et bonum

Article posted at 09:49am on 16 August 2006
Category: Science, General

I’ve been reading about this. My own thought is that if Pluto keeps its official “planet” status, then the other, similar orbiting objects should be named, as well. I think this is more likely to happen than stripping Pluto of its title. I’m here from Romancing the Paradox, btw. I enjoy your blog.

Jennifer () (URL)
2:32pm on 16 August 2006