Detection of a Transiting Low-Density Super-Earth

Creators: Henry, Gregory W.; Howard, Andrew W.; Marcy, Geoffrey W.; Fischer, Debra A.; Johnson, John Asher

Abstract

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 Keck-HIRES 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^(+0.53)_(-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.

Additional Information

8 pages, 7 figures, submitted to ApJ The ApJ paper has been withdrawn by the authors. We thank the many observers who contributed to the measurements reported here. We gratefully acknowledge the efforts and dedication of the Keck Observatory staff, especially Scott Dahm, Hien Tran, and Grant Hill for support of HIRES and Greg Wirth for support of remote observing. We thank Heather Knutson, Josh Winn, and Jonathan Fortney for helpful discussions. We are grateful to the time assignment committees of the University of California and NASA. G.W.H. acknowledges support from NASA, NSF, Tennessee State University, and the State of Tennessee through its Centers of Excellence program. G.W.M. acknowledges NASA grant NNX06AH52G. D.A. F. acknowledges support from NSF grant AST-1036283 and NASA grant NNX08AF42G. This work made use of the SIMBAD database (operated at CDS, Strasbourg, France), NASA's Astrophysics Data System Bibliographic Services, and the NASA Star and Exoplanet Database (NStED). Finally, the authors wish to extend special thanks to those of Hawai'ian ancestry on whose sacred mountain of Mauna Kea we are privileged to be guests. Without their generous hospitality, the Keck observations presented herein would not have been possible.

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