Abstract:
Star-like objects with effective temperatures of less than 2,700 kelvin
are referred to as ‘ultracool dwarfs’1. 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 Sun2. Core-accretion theory predicts that, given
the small masses of these ultracool dwarfs, and the small sizes of
their protoplanetary disks3,4, there should be a large but hitherto
undetected population of terrestrial planets orbiting them5—
ranging from metal-rich Mercury-sized planets6 to more hospitable
volatile-rich Earth-sized planets7. 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 star8. 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.
