Empirical Constraints on the Oblateness of an Exoplanet
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Winn_Empirical Constraints Oblateness.PDF
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Author(s)
Carter, Joshua Adam
Winn, Joshua Nathan
Date Issued
February 2010
Journal
Astrophysical Journal
Publisher
American Astronomical Society
Citation
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.
Version
Author's final manuscript
Abstract
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.
MIT Department
Massachusetts Institute of Technology. Department of Physics
MIT Kavli Institute for Astrophysics and Space Research
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DOI of Published Version
http://dx.doi.org/10.1088/0004-637X/709/2/1219
