XO‐3b: A Massive Planet in an Eccentric Orbit Transiting an F5 V Star

Abstract

We report the discovery of a massive planet (Mpsin i = 13.02 ± 0.64 MJ; total mass = 13.25 ± 0.64 MJ), large (1.95 ± 0.16 RJ) planet in a transiting, eccentric orbit (e = 0.260 ± 0.017) around a 10th magnitude F5 V star in the constellation Camelopardalis. We designate the planet XO-3b and the star XO-3, also known as GSC 03727–01064. The orbital period of XO-3b is 3.1915426 ± 0.00014 days. XO-3 lacks a trigonometric parallax; we estimate its distance to be 260 ± 23 pc. The radius of XO-3 is 2.13 ± 0.21 R☉, its mass is 1.41 ± 0.08 M☉, its vsin i = 18.54 ± 0.17 km s−1, and its metallicity is [ Fe/H ] = − 0.177 ± 0.027. This system is unusual for a number of reasons. XO-3b is one of the most massive planets discovered around any star for which the orbital period is less than 10 days. The mass is near the deuterium-burning limit of 13 MJ, which is a proposed boundary between planets and brown dwarfs. Although Burrows et al. propose that formation in a disk or formation in the interstellar medium in a manner similar to stars is a more logical way to differentiate planets and brown dwarfs, our current observations are not adequate to address this distinction. XO-3b is also unusual in that its eccentricity is large given its relatively short orbital period. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. Analysis of the transit light curve of XO-3b suggests that the spectroscopically derived parameters may be overestimated. Though relatively noisy, the light curves favor a smaller radius in order to better match the steepness of the ingress and egress. The light curve fits imply a planetary radius of 1.25 ± 0.15 RJ, which would correspond to a mass of 12.03 ± 0.46 MJ. A precise trigonometric parallax measurement or a very accurate light curve is needed to resolve the uncertainty in the planetary mass and radius.