Is there a ‘Planet X’?

Forget about X-ray vision and gamma ray defence beams. Planet X isn’t a superhero home world but a hypothetical 10th planet in our solar system. “I think this question tends to be intriguing because of X – the unknown,” says astronomer John Percy. “But right now, we just don’t know if there is another planet lurking on the edges of the solar system.” Percy says there are thousands of small objects orbiting the Sun beyond Neptune and Pluto. Called Kuiper Belt Objects, astronomers do not consider these to be planets because of their relatively small size – the largest, Quaoar, is half the size of Pluto. They are icy and disintegrate if diverted by another cosmic body into an orbit closer to the Sun. If this occurs, he adds, they appear to us as comets.