A long-period substellar object exhibiting a single transit in Kepler

TL;DR

This paper reports the discovery of a long-period substellar object with a single, lengthy transit in Kepler data, suggesting it could be a planet or brown dwarf, and highlights the potential for future mass determination.

Contribution

It presents the detection and characterization of an extremely long-period transiting object, expanding knowledge of substellar companions with very long orbital periods.

Findings

Transit duration ~45 hours indicating a long orbital period

Object size estimated at 0.910 R_J, with uncertain mass

Radial velocity suggests the companion is not a star

Abstract

We report the detection of a single transit-like signal in the Kepler data of the slightly evolved F star KIC4918810. The transit duration is ~45 hours, and while the orbital period ($P\sim10$ years) is not well constrained, it is one of the longest among companions known to transit. We calculate the size of the transiting object to be $R_P = 0.910$ $R_J$. Objects of this size vary by orders of magnitude in their densities, encompassing masses between that of Saturn ($0.3$ $M_J$) and stars above the hydrogen-burning limit (~80 $M_J$). Radial-velocity observations reveal that the companion is unlikely to be a star. The mass posterior is bimodal, indicating a mass of either ~0.24 $M_J$ or ~26 $M_J$. Continued spectroscopic monitoring should either constrain the mass to be planetary or detect the orbital motion, the latter of which would yield a benchmark long-period brown dwarf with a measured mass, radius, and age.