A Collage of Small Planets from the Lick-Carnegie Exoplanet Survey: Exploring the Super-Earth and Sub-Neptune Mass Regime

Analysis of new precision radial velocity (RV) measurements from the Lick Automated Planet Finder and Keck HIRES has yielded the discovery of three new exoplanet candidates orbiting the nearby stars HD 190007 and HD 216520. We also report new velocities from the APF and the Planet Finder Spectrograph and updated orbital fits for the known exoplanet host stars GJ 686 and HD 180617. Of the newly discovered planets, HD 190007 b has a period of P = 11.72 days, an RV semiamplitude of K = 5.64 ± 0.55 m s^-1, a minimum mass of M_pl = 16.46 ± 1.66 M_⊕, and orbits the slightly metal-rich, active K4V star HD 190007. For HD 216520 b, we find P = 35.45 days, K = 2.28 ± 0.20 m s^-1, and M_pl = 10.26 ± 0.99 M_⊕, while for HD 216520 c, P = 154.43 days, K = 1.29 ± 0.22 m s^-1, and M_pl = 9.44 ± 1.63 M_⊕. Both planets orbit the slightly metal-poor, inactive K0V star HD 216520. Our updated best-fit models for HD 180617 b and GJ 686 b are in good agreement with the published results. For HD 180617 b, we obtain P = 105.91 days and M_pl = 12.214 ± 1.05 M_⊕. For GJ 686 b, we find P = 15.53 days and M_pl = 6.624 ± 0.432 M_⊕. Using an injection-recovery exercise, we find that HD 190007 b and HD 216520 b are unlikely to have additional planets with masses and orbital periods within a factor of 2, in marked contrast to ∼85% of planets in this mass and period range discovered by Kepler. ^* This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.