The Unusual Transiting Extra-Solar Planet Orbiting XO-3

We report the discovery of a massive planet (≈12 M_JUP) in an eccentric orbit (e ≈ 0.2) around the F6 star XO-3. This planet transits the 10^th magnitude star every 3.19 days. 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 M_JUP which is a proposed boundary between planets and brown dwarfs. Burrows et al. (2001) 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. Finally, the star XO-3 is somewhat metal poor, with [Fe/H] ≈ -0.25. We will present our observations leading to this discovery and discuss the implications of this system for models of planet formation.