  Star-like objects with effective temperatures of less than 2,700 kelvin are
referred to as ultracool dwarfs. This heterogeneous group includes stars of
extremely low mass as well as brown dwarfs (substellar objects not massive
enough to sustain hydrogen fusion), and represents about 15 per cent of the
population of astronomical objects near the Sun. Core-accretion theory predicts
that, given the small masses of these ultracool dwarfs, and the small sizes of
their protoplanetary disk, there should be a large but hitherto undetected
population of terrestrial planets orbiting them - ranging from metal-rich
Mercury-sized planets to more hospitable volatile-rich Earth-sized planets.
Here we report observations of three short-period Earth-sized planets
transiting an ultracool dwarf star only 12 parsecs away. The inner two planets
receive four times and two times the irradiation of Earth, respectively,
placing them close to the inner edge of the habitable zone of the star. Our
data suggest that 11 orbits remain possible for the third planet, the most
likely resulting in irradiation significantly less than that received by Earth.
The infrared brightness of the host star, combined with its Jupiter-like size,
offers the possibility of thoroughly characterizing the components of this
nearby planetary system.
