A much lower density for the transiting extrasolar planet WASP-7

TL;DR

This study provides high-precision photometry of WASP-7, revealing a larger, less dense planet and a more evolved host star, refining the system's physical parameters and transit ephemeris.

Contribution

First high-precision photometry of WASP-7 using defocussing techniques, leading to revised planetary and stellar parameters and improved transit timing.

Findings

WASP-7's planetary radius is larger than previously measured.

The planet's density and surface gravity are significantly lower.

The host star is more evolved than earlier thought.

Abstract

We present the first high-precision photometry of the transiting extrasolar planetary system WASP-7, obtained using telescope defocussing techniques and reaching a scatter of 0.68 mmag per point. We find that the transit depth is greater and that the host star is more evolved than previously thought. The planet has a significantly larger radius (1.330 +/- 0.093 Rjup versus 0.915 +0.046 -0.040 Rjup) and much lower density (0.41 +/- 0.10 rhojup versus 1.26 +0.25 -0.21 rhojup) and surface gravity (13.4 +/- 2.6 m/s2 versus 26.4 +4.4 -4.0 m/s2) than previous measurements showed. Based on the revised properties it is no longer an outlier in planetary mass--radius and period--gravity diagrams. We also obtain a more precise transit ephemeris for the WASP-7 system.