Stellar-like objects with effective temperatures of 2700K and below are referred to as “ultracool dwarfs”. This heterogeneous group includes both extremely low-mass stars and brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15% of the stellar-like objects in the vicinity of the Sun. Based on the small masses and sizes of their protoplanetary disks, core-accretion theory for ultracool dwarfs predicts a large, but heretofore undetected population of close-in terrestrial planets, ranging from metal-rich Mercury-sized planets to more hospitable volatile-rich Earth-sized planets. Here we report the discovery of three short-period Earth-sized planets transiting an ultracool dwarf star 12 parsecs away using data collected by the TRAPPIST telescope as part of an ongoing prototype transit survey. The inner two planets receive four and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Eleven orbits remain possible for the third planet based on our data, the most likely resulting in an irradiation significantly smaller than Earth's. The infrared brightness of the host star combined with its Jupiter-like size offer the possibility of thoroughly characterizingthe components of this nearby planetary system.
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