M-type dwarf stars, or Red Dwarfs, are the smallest and most abundant stars in the Milky Way. As such many of these occur in the Kepler Field of View, the area of space being examined by the Kepler Space Telescope, and many of those are considered to be possible hosts for planets. The smallest of these is KOI-961 (Kepler Object of Interest-961), an object approximately 120 light years from Earth, with three potential orbiting planets. Unfortunately it is had to resolve the mass and radius of small, distant M-type stars, making it even harder to make any assessments of potential planets orbiting them. Thus KOI-961 remained effectively on the shelf, interesting, but unstudied due to lack of data.
That was until Kevin Apps, a British amateur astronomer, pointed out that the spectral properties of KOI-961 were almost identical to those of Barnard's Star, one of the closest stars to Earth at a distance of only 6 light years, and subsequently very well studied. This lead to a revised study of the system by scientists, resulting in the publication of a paper on the arXiv database at Cornell University Library formally describing the KOI-961 on 10 January 2011, a paper which has also been accepted for publication in The Astrophysical Journal, by a team of scientists lead by Philip Muirhead of the Department of Astronomy at the California Institute of Technology. The discovery was also announced at the 219th meeting of the American Astronomical Society by John Johnson, also of Caltech.
Muirhead et al. used data from the Kepler Space Telescope, combined with new data from the Keck I Telescope in Hawaii and the Hale Telescope at the Palomar Observatory. They calculated that KOI-961 was slightly metal-poor compared to Barnard's Star, but otherwise remarkably similar. By assuming that KOI-961 is essentially similar to Barnard's Star, with a radius of 0.199 that of our sun, and a mass 0.158 of the suns, Muirhead et al. were able to build a model of the KOI-961 system and its three candidate planets; the planets are referred to as candidates since they were detected by their transits of the star and have not been confirmed by other means; Muirhead et al. were able to demonstrate that no other explanation fits the available data very well, even though the dataset was made complicated by numerous multiple transits (transits by more than one planet at the same time).
KOI-961 has a volume only 170% of that of Jupiter, it is orbited by three planets, all smaller than the Earth, with the smallest probably about the same size as Mars. All of it's planets are close to the star and orbit with very short periods, making the system more like that of Jupiter than that of the Sun.
A comparison of the sizes and distances in the KOI-961 and Jovian systems. Image from NASA/Caltech.
KOI-961.01 orbits the star every 29 hours at a distance of 0.0116 AU, that is to say 0.0116 times the distance at which Earth orbits the sun. It has a radius approximately 78% of that of the Earth (though their is a big margin of error in this) and an estimated average equatorial temperature of 519 K (244 °C).
KOI-961.02 orbits the star every 10.8 hours at a distance of 0.0060 AU. It has an estimated radius of 0.73 times that of the Earth, and an estimated equatorial temperature of 720 K (447 °C).
KOI-961.03 orbits the star every 45 hours at a distance of 0.0154 AU. It has an estimated radius of 0.57 times that of the Earth (1.07 times that of Mars) and an estimated equatorial temperature of 450 K (177 °C).
An artist's impression of the KOI-961 system. Image from NASA/JPL-Caltech.
See also NASA may have discovered an exoplanet smaller than the Earth and Exoplanets on Sciency Thoughts YouTube.