MOST Spacebased Photometry of the Transiting Exoplanet System HD 189733: Precise Timing Measurements for Transits Across an Active Star

TL;DR

This study used space-based photometry to precisely measure transit times of HD 189733b, finding no significant variations and thus ruling out certain additional planets, while also addressing challenges posed by stellar activity.

Contribution

First precise transit timing measurements of HD 189733b from space, demonstrating the method's effectiveness despite stellar activity, and constraining potential additional planets.

Findings

No transit timing variations above ±45 seconds.

Ruling out super-Earths in specific resonances.

Insights into measuring transits on active, spotted stars.

Abstract

We have measured transit times for HD 189733b passing in front of its bright (V = 7.67) chromospherically active and spotted parent star. Nearly continuous broadband optical photometry of this system was obtained with the MOST (Microvariability & Oscillations of STars) space telescope during 21 days in August 2006, monitoring 10 consecutive transits. We have used these data to search for deviations from a constant orbital period which can indicate the presence of additional planets in the system that are as yet undetected by Doppler searches. There are no transit timing variations above the level of ${\pm}45$ s, ruling out super-Earths (of masses $1 - 4 M_{\earth}$) in the 1:2 and 2:3 inner resonances and planets of 20 $M_{\earth}$ in the 2:1 outer resonance of the known planet. We also discuss complications in measuring transit times for a planet that transits an active star with large star spots, and how the transits can help constrain and test spot models. This has implications for the large number of such systems expected to be discovered by the CoRoT and Kepler missions.