Empirical Constraints on the Oblateness of an Exoplanet
Author(s)Carter, Joshua Adam; Winn, Joshua Nathan
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We show that the gas giant exoplanet HD 189733b is less oblate than Saturn, based on Spitzer Space Telescope photometry of seven transits. The observablemanifestations of oblatenesswould have been slight anomalies during the ingress and egress phases, as well as variations in the transit depth due to spin precession. Our nondetection of these effects gives the first empirical constraints on the shape of an exoplanet. The results are consistent with the theoretical expectation that the planetary rotation period and orbital period are synchronized, in which case the oblateness would be an order of magnitude smaller than our upper limits. Conversely, if HD 189733b is assumed to be in a synchronous, zero-obliquity state, then the data give an upper bound on the quadrupole moment of the planet (J[subscript 2] < 0.068 with 95% confidence) that is too weak to constrain the interior structure of the planet. An Appendix describes a fast algorithm for computing the transit light curve of an oblate planet, which was necessary for our analysis.
DepartmentMassachusetts Institute of Technology. Department of Physics; MIT Kavli Institute for Astrophysics and Space Research
American Astronomical Society
Carter, Joshua A. and Joshua N. Winn. "Empirical Constraints on the Oblateness of an Exoplanet. 2010 Astrophysical Journal, v.709, p.1219-2129. ©2010 The American Astronomical Society.
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