Validation and Initial Characterization of the Long Period Planet Kepler-1654 b

TL;DR

This paper confirms Kepler-1654 b as a long-period, transiting gas giant orbiting a G2V star, with detailed characterization including its orbit, size, and mass constraints, highlighting its potential and challenges for atmospheric studies with JWST.

Contribution

The study provides the first confirmation and detailed characterization of Kepler-1654 b, a long-period transiting gas giant, including its orbit, size, and mass limits, expanding knowledge of such rare systems.

Findings

Kepler-1654 b has a 1047.84-day orbit and a radius of 0.82 R_Jup.

Radial velocity data constrain the planet's mass to less than 0.5 M_Jup.

The planet's low temperature (~200K) poses challenges for transmission spectroscopy.

Abstract

Fewer than 20 transiting Kepler planets have periods longer than one year. Our early search of the Kepler light curves revealed one such system, Kepler-1654 b (originally KIC~8410697b), which shows exactly two transit events and whose second transit occurred only 5 days before the failure of the second of two reaction wheels brought the primary Kepler mission to an end. A number of authors have also examined light curves from the Kepler mission searching for long period planets and identified this candidate. Starting in Sept. 2014 we began an observational program of imaging, reconnaissance spectroscopy and precision radial velocity measurements which confirm with a high degree of confidence that Kepler-1654 b is a {\it bona fide} transiting planet orbiting a mature G2V star (T$_{eff}= 5580$K, [Fe/H]=-0.08) with a semi-major axis of 2.03 AU, a period of 1047.84 days and a radius of 0.82$\pm$0.02 R$_{Jup}$. Radial Velocity (RV) measurements using Keck's HIRES spectrometer obtained over 2.5 years set a limit to the planet's mass of $<0.5\ (3\sigma$) M$_{Jup}$. The bulk density of the planet is similar to that of Saturn or possibly lower. We assess the suitability of temperate gas giants like Kepler-1654b for transit spectroscopy with the James Webb Space Telescope since their relatively cold equilibrium temperatures (T$_{pl}\sim 200$K) make them interesting from the standpoint of exo-planet atmospheric physics. Unfortunately, these low temperatures also make the atmospheric scale heights small and thus transmission spectroscopy challenging. Finally, the long time between transits can make scheduling JWST observations difficult---as is the case with Kepler-1654b.