Discovery of a Transiting Planet Near the Snow-Line

TL;DR

This paper reports the discovery of Kepler-421b, a Uranus-sized exoplanet near the snow-line with a long orbital period, providing insights into planet formation at icy boundaries.

Contribution

The discovery of a transiting Uranus-sized planet near the snow-line with detailed validation and formation analysis is a novel contribution.

Findings

Kepler-421b has a 704.2-day orbit and is validated with >4 sigma confidence.

It receives insolation similar to a body at ~2AU in the Solar System.

Modeling suggests it formed beyond the snow-line about 3 million years after disk formation.

Abstract

In most theories of planet formation, the snow-line represents a boundary between the emergence of the interior rocky planets and the exterior ice giants. The wide separation of the snow-line makes the discovery of transiting worlds challenging, yet transits would allow for detailed subsequent characterization. We present the discovery of Kepler-421b, a Uranus-sized exoplanet transiting a G9/K0 dwarf once every 704.2 days in a near-circular orbit. Using public Kepler photometry, we demonstrate that the two observed transits can be uniquely attributed to the 704.2 day period. Detailed light curve analysis with BLENDER validates the planetary nature of Kepler-421b to >4 sigmas confidence. Kepler-421b receives the same insolation as a body at ~2AU in the Solar System and for a Uranian albedo would have an effective temperature of ~180K. Using a time-dependent model for the protoplanetary disk, we estimate that Kepler-421b's present semi-major axis was beyond the snow-line after ~3Myr, indicating that Kepler-421b may have formed at its observed location.