The NASA-UC Eta-Earth Program: II. A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses

We report the discovery of HD 156668b, an extrasolar planet with a minimum mass of M_P sin i = 4.15 M_Earth. This planet was discovered through Keplerian modeling of precise radial velocities from Keck-HIRES and is the second super-Earth to emerge from the NASA-UC Eta-Earth Survey. The best-fit orbit is consistent with circular and has a period of P = 4.6455 d. The Doppler semi-amplitude of this planet, K = 1.89 m/s, is among the lowest ever detected, on par with the detection of GJ 581e using HARPS. A longer period (P ~ 2.3 yr), low-amplitude signal of unknown origin was also detected in the radial velocities and was filtered out of the data while fitting the short-period planet. Additional data are required to determine if the long-period signal is due to a second planet, stellar activity, or another source. Photometric observations using the Automated Photometric Telescopes at Fairborn Observatory show that HD 156668 (an old, quiet K3 dwarf) is photometrically constant over the radial velocity period to 0.1 mmag, supporting the existence of the planet. No transits were detected down to a photometric limit of ~3 mmag, ruling out transiting planets dominated by extremely bloated atmospheres, but not precluding a transiting solid/liquid planet with a modest atmosphere.