THE NASA-UC-UH ETA-EARTH PROGRAM. IV. A LOW-MASS PLANET ORBITING AN M DWARF 3.6 PC FROM EARTH

We report the discovery of a low-mass planet orbiting Gl 15A based on radial velocities from the Eta-Earth Survey using HIRES at Keck Observatory. Gl 15 Ab is a planet with minimum mass M sin i = 5.35 +/- 0.75M(circle plus), orbital period P = 11.4433 +/- 0.0016 days, and an orbit that is consistent with circular. We characterize the host star using a variety of techniques. Photometric observations at Fairborn Observatory show no evidence for rotational modulation of spots at the orbital period to a limit of similar to 0.1 mmag, thus supporting the existence of the planet. We detect a second RV signal with a period of 44 days that we attribute to rotational modulation of stellar surface features, as confirmed by optical photometry and the Ca II H & K activity indicator. Using infrared spectroscopy from Palomar-TripleSpec, we measure an M2 V spectral type and a sub-solar metallicity ([M/H] = -0.22, [Fe/H] = -0.32). We measure a stellar radius of 0.3863 +/- 0.0021R(circle dot) based on interferometry from CHARA.