Kepler-503b: An Object at the Hydrogen Burning Mass Limit Orbiting a Subgiant Star

TL;DR

This study reclassifies Kepler-503b from a planet to a brown dwarf or low-mass star, using spectroscopic and Gaia data, highlighting the importance of radial velocity measurements in exoplanet validation.

Contribution

It provides the first detailed characterization of Kepler-503b as a hydrogen-burning mass object near the brown dwarf/stellar boundary, using APOGEE and Gaia data.

Findings

Kepler-503b's mass is approximately 0.075 solar masses.

The system's age is estimated at about 6.7 billion years.

Kepler-503b is at the hydrogen burning limit, near the brown dwarf/stellar boundary.

Abstract

Using spectroscopic radial velocities with the APOGEE instrument and Gaia distance estimates, we demonstrate that Kepler-503b, currently considered a validated Kepler planet, is in fact a brown-dwarf/low-mass star in a nearly circular 7.2-day orbit around a subgiant star. Using a mass estimate for the primary star derived from stellar models, we derive a companion mass and radius of $0.075\pm0.003 \ M_{\odot}$ ($78.6\pm3.1 \ M_{Jup}$) and $0.099^{+0.006}_{-0.004}\ R_{\odot}$ ($0.96^{+0.06}_{-0.04}\ R_{Jup}$), respectively. Assuming the system is coeval, the evolutionary state of the primary indicates the age is $\sim6.7$ Gyr. Kepler-503b sits right at the hydrogen burning mass limit, straddling the boundary between brown dwarfs and very low-mass stars. More precise radial velocities and secondary eclipse spectroscopy with James Webb Space Telescope will provide improved measurements of the physical parameters and age of this important system to better constrain and understand the physics of these objects and their spectra. This system emphasizes the value of radial velocity observations to distinguish a genuine planet from astrophysical false positives, and is the first result from the SDSS-IV monitoring of Kepler planet candidates with the multi-object APOGEE instrument.