Characterizing the cool KOIs. III. KOI 961: A small star with large proper motion and three small planets

Philip S. Muirhead, John Asher Johnson, Kevin Apps, Joshua A. Carter, Timothy D. Morton, Daniel C. Fabrycky, John Sebastian Pineda, Michael Bottom, Bárbara Rojas-Ayala, Everett Schlawin, Katherine Hamren, Kevin R. Covey, Justin R. Crepp, Keivan G. Stassun, Joshua Pepper, Leslie Hebb, Evan N. Kirby, Andrew W. Howard, Howard T. Isaacson, Geoffrey W. MarcyDavid Levitan, Tanio Diaz-Santos, Lee Armus, James P. Lloyd

Research output: Contribution to journalArticlepeer-review

187 Scopus citations


We characterize the star KOI 961, an M dwarf with transit signals indicative of three short-period exoplanets discovered by the Kepler mission. We proceed by comparing KOI 961 to Barnard's Star, a nearby, well-characterized mid-M dwarf. We compare colors, optical and near-infrared spectra, and find remarkable agreement between the two, implying similar effective temperatures and metallicities. Both are metal-poor compared to the Solar neighborhood, have low projected rotational velocity, high absolute radial velocity, large proper motion, and no quiescent Hα emission - all of which are consistent with being old M dwarfs. We combine empirical measurements of Barnard's Star and expectations from evolutionary isochrones to estimate KOI 961's mass (0.13 ± 0.05 M ), radius (0.17 ± 0.04 R ), and luminosity (2.40 × 10-3.0 ± 0.3 L ). We calculate KOI 961's distance (38.7 ± 6.3 pc) and space motions, which, like Barnard's Star, are consistent with a high scale-height population in the Milky Way. We perform an independent multi-transit fit to the public Kepler light curve and significantly revise the transit parameters for the three planets. We calculate the false-positive probability for each planet candidate, and find a less than 1% chance that any one of the transiting signals is due to a background or hierarchical eclipsing binary, validating the planetary nature of the transits. The best-fitting radii for all three planets are less than 1R , with KOI 961.03 being Mars-sized (RP = 0.57 ± 0.18 R ), and they represent some of the smallest exoplanets detected to date.

Original languageEnglish (US)
Article number144
JournalAstrophysical Journal
Issue number2
StatePublished - Mar 10 2012
Externally publishedYes


  • planetary systems
  • stars: fundamental parameters
  • stars: individual (Barnard's Star, KOI 961)
  • stars: late-type
  • stars: low-mass

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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