In August 1846, Urbain Le Verrier announced to the French Academy his prediction of a previously undiscovered planet. He had studied the motion of Uranus, and concluded using Newton’s Law of Gravitation that the small deviations he had witnessed in its orbit were due to gravitational perturbations made by the proposed planet. John Adams, also working on the same hypothesis, was barely beaten to the punch. The new planet was Neptune, and its discovery was a triumph of the scientific method. Its existence was predicted without ever having been observed (although it is said that Neptune had been observed in the past but never correctly identified).
One hundred and sixty five years later, astrophysicists have repeated Le Verrier’s method for the first time in an extrasolar planetary system. The KOI-872 system is one of the Kepler Space Telescope’s Objects of Interest, and its data is public. The Hunt for Exomoons using Kepler Project (HEK, which I’ve blogged about before) has analysed this data, and detected several transiting extrasolar planets in the system. Hidden inside the transit data from planet b is the gravitational tug of the hidden planet c:
The gravitational tug of planet c disturbs the orbit of planet b, making its transits shuffle around in time (known as Transit Timing Variation or TTV). TTV, and its cousin, Transit Duration Variation or TDV, are potentially capable of detecting exomoons, hence the HEK project’s interest in cataloguing them. The TTV and TDV generated by exomoons are pretty small, but the TTV seen here is vast – this variation indicates that planet c must have a mass close to that of Saturn, orbiting the star every 57 days. There are also signs of a Super Earth nestled in there as well (planet d, unconfirmed), with an orbital period of around a week. Here’s my somewhat piffling cartoon of the system as it stands, with distances from the Sun beneath:
The eccentricity of all planets is similar to that of the Solar System planets (i.e. almost zero), and all the planets appear to be orbiting in the same plane. This is more evidence that multiple planetary systems occur fairly frequently, and coplanar solar systems like ours are not a fluke. More than this, this is proof that the TTV signal can be detected, and that the HEK team are on the right track. Next stop, an exomoon!