Trial of a Time Geek

Over the years, there have been numerous claims to have found a 10^th planet in our solar system. They have been put aside one after the other because the bodies discovered were so small - clearly smaller than Pluto, the outermost and smallest planet - that astronomers couldn't accept the idea that they should be called planets.

No more.

Astronomers in the United States have announced the discovery of the 10th planet to orbit our Sun[, the Beeb said on Saturday].

The largest object found in our Solar System since Neptune was discovered in 1846, it was first seen in 2003 but has only now been confirmed as a planet.

Designated 2003 UB313, it is about 3,000km across, a world of rock and ice and somewhat larger than Pluto.

Yep. Larger than Pluto. So if Pluto is a planet, which it's accepted it is, then this is definitely a planet. And then there were 10.

2003 UB313 was first observed in October of 2003, but it's so far away - three times farther away than Pluto - that they couldn't detect its motion across the sky until January, 2005: 15 months later.

It has a highly-inclined orbit, set at an angle of 44^o to the plane of the innermost eight planets. (Pluto also has an eccentric orbit but it's inclination is only 17^o.) and right now is about 97 AU away. (AU = "astronomical unit," a measure of distance just about equal to the average distance of the Earth from the Sun, about 150 million kilometers or 93 million miles. If you want a precise definition, NASA has one.)

Pluto and 2003 UB313 are both Kuiper Belt objects, as were the previous pretenders to 10^th planet status. The new discovery, however, has raised the possibility that there are other larger-than-Pluto objects lurking out there yet to be discovered.