PSJC #63 Apr 2 2010

Ryan Hardy

On the Orbit of Exoplanet WASP-12b

We observed two secondary eclipses of the exoplanet WASP-12b using the Infrared Array Camera on the Spitzer Space Telescope. The close proximity of WASP-12b to its G-type star results in extreme tidal forces capable of inducing apsidal precession with a period as short as a few decades. This precession would be measurable if the orbit had a significant eccentricity. The ground-based secondary eclipse phase reported by Lopez-Morales et al. (0.510 ± 0.002) implies eccentricity at the 4.5-sigma level, and the spectroscopic orbit of Hebb et al. has eccentricity 0.049 ± 0.015, a 3-sigma result, and predicts an eclipse phase of 0.509 ± 0.007. Our eclipse phases are 0.5012 ± 0.0006 (3.6 and 5.8 micron) and 0.5007 ± 0.0007 (4.5 and 8.0 micron). These values are inconsistent with the ground-based data, but marginally consistent with the spectroscopic orbit. Considering the unlikely possibility that precession brought the long axis of the orbit into alignment during our observations, a model considering these points and transit times from professional and amateur observers estimates orbital precession at omega_dot = 0.02 ± 0.01 deg/d. This implies a tidal Love number, k2p, of 0.15 ± 0.08, indicating a very centrally condensed planet. However, if the orbit is actually eccentric, we have observed it at a remarkably special time to find eclipse phases consistent with apsidal alignment. Future observations can decide between these possibilities.