Discussion of Planet Nine, the temporary name of a hypothetical planet in the solar system, is making me think big. It potentially explains the orbiting patterns of several Kuiper Belt Objects. We have never directly observed it but the models could guide us where to look. There is enthusiasm for this, not least because it would fill the slot in the Solar System that was left by downgrading the status of Pluto (too small!); Mike Brown, one of the people involved in both downgrading Pluto and the excitement about Planet Nine, is quoted describing it as: “the most planet-y of the planets in the whole Solar System”!
So, I wondered, could there be an alternative Earth which we have never seen because it lurks in the same orbit but on the far side of the Sun? It turns out that the theory of a Counter-Earth is nothing new. Something along these lines (Antichthon) was proposed back in the fifth century BC by Philolaus. In more modern times, it has apparently been a staple Sci Fi theme. I hadn’t remembered that, for example, it featured as part of the original story of the Cybermen; their planet, Mondas, was originally a sister planet to earth until ejected to the far reaches of the Solar System where they had to become cyborgs in order to survive.
The idea of another Earth, situated just where we can never see it, is quite appealing. However, astrophysics suggest that this is not the case. While we cannot look through the Sun and it is a bit like an inaccessible spot between your shoulder blades, we would expect to see the influence of an Earth-sized body on other parts of the system and on probes we have sent in that direction. No such peturbations have been observed. Of course, this is from the guys who spent most of the last century calling Pluto a planet and who are now on the trail of a never directly observed Planet Nine.
Watch this space?