MOST detects transits of HD 97658b, a warm, likely volatile-rich super-Earth

TL;DR

MOST space telescope confirmed HD 97658b as a transiting super-Earth with a rocky core and light-element atmosphere, providing precise measurements of its size, mass, and density, and clarifying previous unverified signals.

Contribution

First direct detection of transits of HD 97658b confirming its planetary nature and detailed characterization of its physical properties.

Findings

Transit depth indicates a 2.34 Earth radii super-Earth.

Derived planet density suggests a rocky core with a light-element atmosphere.

Confirmed transit ephemeris aligns with radial velocity data.

Abstract

Through photometric monitoring of the extended transit window of HD 97658b with the MOST space telescope, we have found that this exoplanet transits with an ephemeris consistent with that predicted from radial velocity measurements. The mid-transit times are $5.6\sigma$ earlier than those of the unverified transit-like signals reported in 2011, and we find no connection between the two sets of events. The transit depth together with our determined stellar radius ($R_\star = 0.703^{+0.039}_{-0.034} R_\odot$) indicates a 2.34$^{+0.18}_{-0.15}$ $R_{\oplus}$ super-Earth. When combined with the radial velocity determined mass of 7.86 $\pm 0.73$ $M_{\oplus}$, our radius measure allows us to derive a planet density of 3.44$^{+0.91}_{-0.82}$ g cm$^{-3}$. Models suggest that a planet with our measured density has a rocky core that is enveloped in an atmosphere composed of lighter elements. The star of the HD 97658 system is the second brightest known to host a transiting super-Earth, facilitating follow-up studies of this not easily daunted, warm and likely volatile-rich exoplanet.