When the discovery of ‘a new planet called Smiley’ hit the headlines a month ago, it brought a few smiles to astronomers’ faces. The media reaction suggested that the number of planets in our Solar System had leapt abruptly from nine to ten. In fact, the new object cannot really be called a planet, and it won’t end up being called Smiley. But the discovery has generated its own excitement among astronomers. It confirms that there is a whole new class of objects orbiting the Sun, in addition to the planets and the comets. And these ‘ice dwarfs’ may hold the key to the way that the Solar System was formed.
David Jewitt and Jane Luu had been searching for something like Smiley for five years before its discovery on 30 August. They were looking for dim and distant objects beyond the orbit of Pluto, using a very sensitive electronic light detector attached to a telescope at the Mauna Kea Observatory in Hawaii. Such charge-coupled device, or CCD, detectors can scan only a very small part of the sky at a time, so Jewitt and Luu had to spend years checking out tiny patches of sky, one by one.
In August, they struck lucky while searching through the stars in the constellation of Pisces. Jewitt and Luu picked out a faint speck of light that moved very gradually against the background stars. An actual star would be too distant to appear to move in the course of a few minutes, so the object had to be reflecting sunlight – meaning it must lie in the Solar System. And as Jewitt…
