THE HUNT FOR THE MISSING PLANET

Astronomers are now regularly detecting planets thousands of light years from Earth – but have we missed one in our own Solar System?

- COLIN STUART

My Very Easy Method Just Speeds Up Naming Planets'. This used to be a way to remember the names of the planets in our Solar System. Today, however, every diligent schoolchild knows there are only eight planets, not nine. The fact that Pluto was dropped from the list just 19 years ago hints that our picture of the outer Solar System is far from settled. Pluto's demotion to dwarf planet status came on the back of the discovery of other worlds, some as large as Pluto, orbiting further from the Sun. If Pluto was a planet, so must these objects be. A strict admission policy to the planet club was drawn up by astronomers in 2006, and Pluto and its band of icy companions were refused entry to this exclusive group. But there's growing evidence that there may actually be another planet lurking in the far reaches of our Solar System - it's thought to be a 'super-Earth', a planet with a mass higher than Earth but below Uranus and Neptune.

The strong hint that there might be another big discovery to be made in the Solar System came in 2003, when astronomers found Sedna, a seemingly impossible minor planet, marooned between the Kuiper Belt and the Oort Cloud. There was simply no explanation for how this tiny world came to be stranded in a cosmic no man's land. The most likely scenario is that it started out life somewhere else before being kicked into its present orbit.