Can Kozai-Lidov cycles explain Kepler-78b?

TL;DR

This paper explores whether Kozai-Lidov cycles caused by an outer companion could explain the ultra-short-period orbit of Kepler-78b, considering tidal effects and stellar age.

Contribution

It demonstrates that Kozai-Lidov cycles can produce ultra-close-in planets like Kepler-78b within stellar lifetimes, suggesting a possible formation pathway involving outer stellar companions.

Findings

Kozai-Lidov cycles can produce ultra-short-period planets within 600-900 Myr.

Such planets are more likely around stars older than 1 Gyr.

The probability of Kozai-Lidov-induced orbit migration for Kepler-78b is low, implying other processes may also be involved.

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 ($1.2 R_\oplus$), 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 ($\sim 600 - 900$ Myr), but it is more likely to find such ultra-short-period planets around slightly older stars ($> 1$ 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.