Wednesday, February 01, 2006

2003 UB313, bigger than Pluto, so what is a planet?

The latest study of the small planet 2003 UB313, published today in Nature, reveals that it is bigger than Pluto, a team of astronomers says. The team used infrared measurements to determine its size, about 3000 kilometers across, making it significantly larger than Pluto, which measures 2300 kilometers. Infrared measurements at a wavelength of 1200 nm are more sensitive to size than visual light, which depends on knowing a body's color and what percentage of light it is reflecting. 2003 UB313 is so far out its color is very difficult to determine.

All this serves to smash further holes in our definition of a planet. Many researchers currently consider our four rocky inner planets, Mercury, Venus, Earth and Mars, and the four outer gas giants, Jupiter, Saturn, Uranus and Neptune to be typical planets. (See them all here.)

Pluto was recently demoted to being a Kuiper Belt Object, which is debris and smaller objects on the outskirts of the solar system. This upset a lot of people, including astronomers. A google search quickly shows the controversy. Astronomers responsible for the demotion claimed Pluto was just the largest and closest object of the Kuiper Belt. 2003 UB313 definitely lives farther out in the thick of the Kuiper Belt. But its large size makes its irritatingly outside of the standard boxes that define planets.

Other problem children include the large gas giants that circle around stars other than our own. These extrasolar planets can be much larger than Jupiter and can orbit much closer than Earth. And where do asteroids fit, such as the asteroid belt in our solar system, which didn't have the mass to become a rocky planet? Then there's brown dwarves, which have just enough mass to glow warmly in infrared light, but certainly not enough to start the fusion that makes a star.

So we have all these oddballs tugging on our definition of planets. The IAU working group on planets clearly has its work cut out.

Personally, I like Alan Stern's proposal best (you'll have to scroll way down): have lots of different kinds of planets, like we have for stars. For stars there are main-sequence, white dwarves, brown dwarves, neutron, pulsar, red giants, red dwarves and supergiants. They all have different characteristics, but nobody doubts there are, or were, stars.

For planets, there could be gas giants like Jupiter, rocky ones like Earth, and small outliers like Pluto and 2003 UB313. We just need the nomenclature to define them.

My view: if it can pull itself into a sphere under its own gravity and isn't a star, it's some kind of planet. Unless it's a comet. Rats.