Astronomers are thinking of redefining planets, as a result of newly discovered objects in the solar system. They reckon that this will increase the number of planets to 12, will Ceres, Charon and the new UB313 (Xena) being added.

But by their own criteria, they should also include the Moon. It's round, and it orbits a star. No, I hear you cry, it orbits the Earth. But the Earth orbits the Sun, and look at it as though you were in a spaceship above the Earth/Moon orbit.

The Moon's orbit round the Earth NEVER goes backwards relative to the Earth's orbit round the Sun. The average orbital speed of the Earth is about 1.6 million miles a day. The Moon's average orbital speed around the Earth is about 55 thousand miles a day- so even when orbiting "backwards" relative to Earth it has only reduced in Sun orbital speed by about 3.5%.

So from your spaceship, the two planets seem to be going at roughly the same speed together, with the Moon wiggling along an orbit that takes it sometimes ahead of the Earth, sometimes behind, sometimes inside its orbit and sometimes outside. It's a twin planet system. What's more, the Moon is larger than Pluto.

So fair's fair; if astronomers want to keep Pluto, they've got to promote the Moon.