Detection of a Transiting Low-density Super-earth

We present evidence for photometric transits of the low-mass planet HD 97658b across the disk of its host star, an early K dwarf. This planet was previously discovered in radial velocities (RVs) from Keck/HIRES as part of the Eta-Earth Survey. Using photometry from the Automated Photometric Telescopes at Fairborn Observatory, we detected four separate planetary egress events at times predicted from the RV orbit. We measured a transit depth of 1470 ± 260 ppm, a result that should be confirmed and refined with space-based photometry. We also collected additional KeckHIRES RV measurements that refined the transit ephemeris and planet mass. With an orbital period of 9.4957 ± 0.0022 days, HD 97658b is a close-in planet that had been classified as a ‘super-Earth’ based on its mass of 6.4± 0.7 M⊕. However, the planet radius of 2.93± 0.28 R⊕ implies a density of 1.40 −0.36 g cm −3 and suggests ‘sub-Neptune’ status. The low density can be explained by an extended atmosphere of volatiles such as hydrogen, helium, and water. HD 97658b is similar to GJ 1214b in mass, radius, and density, although HD 97658b has a higher equilibrium temperature of 510–720 K. The star HD 97658 (V = 7.8, K = 5.7) is among the brightest known to host a transiting planet, which will facilitate detailed follow-up measurements. Subject headings: planetary systems — planets and satellites: formation, interior — stars: individual (HD 97658) — techniques: photometric