Transiting planet candidate from K2 with the longest period

TL;DR

This paper reports the discovery and characterization of a long-period transiting planet candidate from K2 data, with a potential orbital period of around 10 years, representing the longest-period transiting planet candidate known.

Contribution

The study introduces a novel method for analyzing single transits to identify and characterize extremely long-period exoplanets using combined photometry and radial velocity data.

Findings

Planet candidate has a period of approximately 10 years.

Radial velocity data constrains the mass to below 13 Jupiter masses.

The candidate orbits a sub-giant star at about 4.5 AU.

Abstract

Context: We present the transit and follow-up of a single transit event from Campaign 14 of K2, EPIC248847494b, which has a duration of 54 hours and a 0.18% depth. Aims: Using photometric tools and conducting radial velocity follow-up, we vet and characterise this very strong candidate. Methods: Owing to the long, unknown period, standard follow-up methods needed to be adapted. The transit was fitted using Namaste, and the radial velocity slope was measured and compared to a grid of planet-like orbits with varying masses and periods. These used stellar parameters measured from spectra and the distance as measured by Gaia. Results: Orbiting around a sub-giant star with a radius of 2.70$\pm$0.12R$_{\rm Sol}$, the planet has a radius of 1.11$_{-0.07}^{+0.07}$R$_{\rm Jup}$ and a period of 3650$_{-1130}^{+1280}$ days. The radial velocity measurements constrain the mass to be lower than 13M$_{\rm Jup}$, which implies a planet-like object. Conclusions: We have found a planet at 4.5 AU from a single-transit event. After a full radial velocity follow-up campaign, if confirmed, it will be the longest-period transiting planet discovered.