Can a Stellar Disturbance Explain Kepler-78b's Orbit

Can Kozai-Lidov cycles explain Kepler-78b?

Author:

Rice

Abstract:

Kepler-78b is one of a growing sample of planets similar, in composition and size, to the Earth. It was first detected with NASA's \emph{Kepler} spacecraft and then characterised in more detail using radial velocity follow-up observations. Not only is its size very similar to that of the Earth (1.2R⊕), it also has a very similar density (5.6 g cm−2). What makes this planet particularly interesting is that it orbits its host star every 8.5 hours, giving it an orbital distance of only 0.0089 au. What we investigate here is whether or not such a planet could have been perturbed into this orbit by an outer companion on an inclined orbit. In this scenario, the outer perturber causes the inner orbit to undergo Kozai-Lidov cycles which, if the periapse comes sufficiently close to the host star, can then lead to the planet being tidally circularised into a close orbit. We find that this process can indeed produce such very-close-in planets within the age of the host star (∼600−900 Myr), but it is more likely to find such ultra-short-period planets around slightly older stars (greater than 1 Gyr). However, given the size of the Kepler sample and the likely binarity, our results suggest that Kepler-78b may indeed have been perturbed into its current orbit by an outer stellar companion. The likelihood of this happening, however, is low enough that other processes - such as planet-planet scattering - could also be responsible.