Asked by: James Masters, East Grinstead
The idea that there might be another planet in our Solar System on an orbit that keeps it permanently behind the Sun dates back at least 2400 years to the Greek philosopher Philolaus. He postulated the existence of a ‘counter-Earth’ called Antichthon. It’s also caught the imagination of science fiction writers, including Gerry Anderson of Thunderbirds fame, who made a film based on the idea in the late 1960s. Melancholia and Another Earth, both from 2011, also explore these themes.
Sadly, the science doesn’t stack up: the gravitational action of the other planets in the Solar System would distort Antichthon’s orbit until it came out of perfect alignment with the Earth. Astronomers also have very precise data on the position of the planets and space-probes, and they show no evidence of missing planets.