Empirical Constraints on the Oblateness of an Exoplanet

TL;DR

This study uses precise photometry to empirically constrain the shape of exoplanet HD 189733b, finding it less oblate than Saturn and setting limits on its quadrupole moment, thus informing planetary structure models.

Contribution

First empirical constraints on exoplanet oblateness using transit photometry, including a novel fast algorithm for modeling oblate planet transits.

Findings

HD 189733b is less oblate than Saturn.

No detectable oblateness-induced anomalies were observed.

Upper limit on planetary quadrupole moment J2 < 0.068.

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 observable manifestations 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 (J2 < 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.