Counter-Earth - Scientific Analysis

Scientific Analysis

The idea of a hypothetical planet always on the other side of the sun from the Earth has been a recurring theme in science fiction and fiction. To maintain 180 degree difference from the sun, (the theory went) the planet would have to have the same orbiting speed/path as earth, and so necessarily be the same distance from the sun and the same mass as earth, which would allow it to have the same (or very similar) atmospheric pressure and temperature range. Thus, what would make the planet undetectable to astronomers would also allow it to be habitable to beings at least similar to humans. The idea is said to have been particularly popular in the 1950s science fiction.

However, if such a planet actually existed, according to present scientific cosmology, it would have to be detectable from Earth for a number of reasons.

Its would have gravitational influence upon the other planets, comets and man-made probes of the Solar System. No such influence has been detected, and indeed space probes sent to Venus, Mars and other places could not have successfully flown by or landed on their targets if a Counter-Earth existed, as it was not accounted for in navigational calculation. Furthermore, Jupiter and Sun orbit a barycenter outside the Sun, and this orbital rotation is asynchronous with Earth's orbit, so a Counter-Earth could be even directly seen.

It must also be noted that the Earth's orbit is not a circle but an ellipse, and in respect of Kepler's second law, a planet revolves faster when it is close to the star. So if the Counter-Earth followed the Earth on the same orbit with half a year of delay, it would sometimes be visible from Earth. Rather, to be hidden from Earth, the Counter-Earth would have an orbit symmetrical to Earth's, not sharing the second focus.

The Sun-Jupiter Trojan asteroid system is an example of a stable Lagrange orbit. Equation 10 in section 14 of Lectures on Celestial Mechanics by Siegel and Moser shows the relation between the masses of the bodies and the distances between them in the case of a collinear orbit. However, these linear orbits are not as stable as, for example, the equilateral Lagrange orbits L₄ and L₅. Hilda asteroids do not visit L₃ of Jupiter-Sun system, though they do come close to it in their curious orbits.

Any planetary sized body 180 degrees from Earth should have been visible by NASA's STEREO coronagraph space probes (two spacecraft launched into orbits around the sun in 2006, one farther ahead of and one behind the Earth's orbit) during the first half of 2007. The separation of the STEREO spacecraft from Earth would give them a view of the Lagrangian "L3" point during the early phase of the mission. Later, as the spacecraft continued to separate, the L₃ point drifted out of the field of view. Given the sensitivity of STEREO's COR2 coronagraph, anything larger than 100 kilometres (62 mi) in diameter should have been detected.