MOST Spacebased Photometry of the Transiting Exoplanet System HD 209458: Transit Timing to Search for Additional Planets

TL;DR

This study uses space-based photometry to precisely measure transit times of HD 209458, finding no evidence of additional planets and constraining their possible masses, thus refining our understanding of the system.

Contribution

First space-based, nearly continuous transit timing data for HD 209458, providing stringent limits on additional planets down to below Earth mass.

Findings

No significant transit timing variations detected.

Limits on additional planets' masses below Earth mass.

No evidence of perturbing planets causing HD 209458b's low density.

Abstract

We report on the measurement of transit times for the HD 209458 planetary system from photometry obtained with the MOST (Microvariability & Oscillations of STars) space telescope. Deviations from a constant orbital period can indicate the presence of additional planets in the system that are yet undetected, potentially with masses approaching an Earth mass. The MOST data sets of HD 209458 from 2004 and 2005 represent unprecedented time coverage with nearly continuous observations spanning 14 and 43 days and monitoring 3 transits and 12 consecutive transits, respectively. The transit times we obtain show no variations on three scales: (a) no long-term change in P since before 2004 at the 25 ms level, (b) no trend in transit timings during the 2005 run, and (c) no individual transit timing deviations above 80 sec level. Together with previously published transit times from Agol & Steffen (2007), this allows us to place limits on the presence of additional close-in planets in the system, in some cases down to below an Earth mass. This result, along with previous radial velocity work, now eliminates the possibility that a perturbing planet could be responsible for the additional heat source needed to explain HD 209458b's anomalous low density.