Measuring the Oblateness and Rotation of Transiting Extrasolar Giant Planets

Abstract

We investigate the prospects for characterizing extrasolar giant planets by measuring planetary oblateness from transit photometry and inferring planetary rotational periods. The rotation rates of planets in the solar system vary widely, reflecting the planets' diverse formational and evolutionary histories. A measured oblateness, assumed composition, and equation of state yields a rotation rate from the Darwin-Radau relation. The lightcurve of a transiting oblate planet should differ significantly from that of a spherical one with the same cross-sectional area under identical stellar and orbital conditions. However, if the stellar and orbital parameters are not known a priori, fitting for them allows changes in the stellar radius, planetary radius, impact parameter, and stellar limb darkening parameters to mimic the transit signature of an oblate planet, diminishing the oblateness signature. Thus even if HD209458b had an oblateness of 0.1 instead of our predicted 0.003, it would introduce a detectable departure from a model spherical lightcurve at the level of only one part in 10^5. Planets with nonzero obliquity break this degeneracy because their ingress lightcurve is asymmetric relative to that from egress, and their best-case detectability is of order 10^{-4}. However, the measured rotation rate for these objects is non-unique due to degeneracy between obliquity and oblateness and the unknown component of obliquity along the line of sight. Detectability of oblateness is maximized for planets transiting near an impact parameter of 0.7 regardless of obliquity. Future measurements of oblateness will be challenging because the signal is near the photometric limits of current hardware and inherent stellar noise levels.