Non-detection of previously reported transits of HD 97658b with MOST photometry

TL;DR

Follow-up photometry with MOST did not detect transits of HD 97658b, challenging previous reports, and set upper limits on the planet's size while refining its orbital parameters.

Contribution

This study provides the first non-detection of transits for HD 97658b, constrains its maximum size, and improves orbital parameters using new photometric and radial velocity data.

Findings

No transits detected at reported ephemeris

Transit depth limit corresponds to 2.09 R_Earth

Radial velocity data supports 9.5-day orbital period

Abstract

The radial velocity-discovered exoplanet HD 97658b was recently announced to transit, with a derived planetary radius of 2.93 \pm 0.28 R_{Earth}. As a transiting super-Earth orbiting a bright star, this planet would make an attractive candidate for additional observations, including studies of its atmospheric properties. We present and analyze follow-up photometric observations of the HD 97658 system acquired with the MOST space telescope. Our results show no transit with the depth and ephemeris reported in the announcement paper. For the same ephemeris, we rule out transits for a planet with radius larger than 2.09 R_{Earth}, corresponding to the reported 3\sigma lower limit. We also report new radial velocity measurements which continue to support the existence of an exoplanet with a period of 9.5 days, and obtain improved orbital parameters.