Physical properties of the 0.94-day period transiting planetary system WASP-18

TL;DR

This study provides high-precision photometric analysis of the WASP-18 system, determining its planetary and stellar properties with minimized errors, and discusses the possible existence of two populations of transiting planets based on mass and orbital eccentricity.

Contribution

First detailed photometric analysis of WASP-18 using telescope defocussing, achieving high precision and assessing systematic errors with multiple stellar models.

Findings

Mass of WASP-18b is approximately 10.43 Mjup.

Radius of WASP-18b is approximately 1.165 Rjup.

Most massive transiting planets tend to have eccentric orbits.

Abstract

We present high-precision photometry of five consecutive transits of WASP-18, an extrasolar planetary system with one of the shortest orbital periods known. Through the use of telescope defocussing we achieve a photometric precision of 0.47 to 0.83 mmag per observation over complete transit events. The data are analysed using the JKTEBOP code and three different sets of stellar evolutionary models. We find the mass and radius of the planet to be M_b = 10.43 +/- 0.30 +/- 0.24 Mjup R_b = 1.165 +/- 0.055 +/- 0.014 Rjup (statistical and systematic errors) respectively. The systematic errors in the orbital separation and the stellar and planetary masses, arising from the use of theoretical predictions, are of a similar size to the statistical errors and set a limit on our understanding of the WASP-18 system. We point out that seven of the nine known massive transiting planets (M_b > 3 Mjup) have eccentric orbits, whereas significant orbital eccentricity has been detected for only four of the 46 less massive planets. This may indicate that there are two different populations of transiting planets, but could also be explained by observational biases. Further radial velocity observations of low-mass planets will make it possible to choose between these two scenarios.