Nearby, low-mass M-dwarf stars are particularly attractive targets for transit surveys to detect small, rocky planets orbiting within their stellar habitable zones. Unlike their Sun-like counterparts, terrestrial planets transiting late M-dwarfs present a photometric signal that is of sufficient depth as to permit detection with ground-based observatories. Moreover, the low stellar luminosities imply that the habitable zones are located at small orbital distances from the central star, which in turn suggests detection of planets in their habitable zones should be facilitated by the higher geometric probability of a transit, and by the reduced orbital period and hence time span required to detect such an event.
Here, Dr. Charbonneau will lead a 3-year survey of the 2000 nearest late M-dwarfs with a cadence and sensitivity sufficient to detect habitable-zone planets with radii as small as twice that of the Earth. The survey will be conducted at the MEarth Observatory at Mt Hopkins in southern Arizona, which consists of 8 identical 40cm CCD-based telescopes located in a single roll-off enclosure. By sequentially surveying only nearby, well-characterized targets, MEarth overcomes the severe problems of false-positive contamination that have plagued extant wide-field surveys.
It is expected that the survey will either detect large, habitable-zone terrestrial exoplanets, or yield an upper limit (with 99% confidence) of 17% on their rate of occurrence. In the event of a detection, these planets will (1) provide the first constraints on models of the physical structure and composition of terrestrial exoplanets, (2) inform models of the planetary formation around low-mass stars, and (3) provide keystone targets for subsequent studies of their atmospheres, which in turn will permit an understanding of the atmospheric chemistry and dynamics for terrestrial exoplanets.
Through this project, postdoctoral fellows, graduate students and undergraduates will be trained through access to a cutting-edge research opportunity in the rapidly-evolving field of exoplanet science. The results will be disseminated broadly by Dr. Charbonneau and the project participants through a series of publications in refereed journals, articles in popular science magazines, talks at astronomical and interdisciplinary research conferences, and a project webpage geared for the general public.
One of the greatest questions in all of science is whether or not there are inhabited worlds other than our own. Low-mass stars, called M-dwarfs, are ideal targets to search for habitable exoplanets by seraching for the periodic dimming of the star light as the planet passes in front of the star: Due to the small physical sizes of the stars, the planet blocks enough of the star's area that the eclipse can be detected from the ground using small and hence inexpensive telescopes. Moreover, due to the low energy output of these stars, the distance of the habitable zone -- the location at which a planet would have liquid water on its surface -- is relatively close to the star, meaning that the orbital period of the planet is only one or two weeks (as opposed to 1 year for a Sun analog). The MEarth Observatory is dedicated to this search. It consists of 8 identical automated telescopes located in a remotely controlled roll-off enclosure atop Mt. Hopkins, Arizona, with instruments to automatically weather conditions. Using MEarth, we are conducting a survey of 2000 of the closest, smallest stars to look for super-Earths (planets similar in composition to the Earth, but physically larger than the Earth) orbiting within their stellar habitable zones. Since the stars are among our nearest neighbors, any planets we find would be amenable to spectroscopic study of their atmospheres to detect and identify molecules. The MEarth Team discovered the transiting super-Earth GJ1214b, a planet only 6 times the mass of the Earth, and already many groups world wide have undertaken follow-up investigations of this planet and its atmosphere. The most recent of these studies indicate the planet is aptly named a waterworld, since it may be mostly water in composition. In addition to the discovery of this exoplanet, the MEarth team has made several important contributions to stellar astrophysics. First, they have discovered 3 eclipsing systems in which one star is eclipsed by another star or by a brown dwarf. These discoveries have tested current models of the structure and evolution of stars substantially less massive than the Sun. Second, due to the unique precision and time coverage of the MEarth data, the MEarth team has directly determined the rotation period of many neary low-mass stars and found, contrary to some predictions, that many are slowly rotating, indicating that these low-mass stars spin down as they age. This will facilitiate searches for planets orbiting such stars by the complementary method known as the Doppler or wobble technique. The MEarth project is currently continuing its survey from Arizona and constructing a second facility in Chile to enable the search for planets orbiting stars visible only from the southern hemisphere.
