  By targeting nearby M dwarfs, a transit search using modest equipment is
capable of discovering planets as small as 2 Earth radii in the habitable zones
of their host stars. The MEarth Project, a future transit search, aims to
employ a network of ground-based robotic telescopes to monitor M dwarfs in the
northern hemisphere with sufficient precision and cadence to detect such
planets. Here we investigate the design requirements for the MEarth Project. We
evaluate the optimal bandpass, and the necessary field of view, telescope
aperture, and telescope time allocation on a star-by-star basis, as is possible
for the well-characterized nearby M dwarfs. Through these considerations, 1,976
late M dwarfs (R < 0.33 Rsun) emerge as favorable targets for transit
monitoring. Based on an observational cadence and on total telescope time
allocation tailored to recover 90% of transit signals from planets in habitable
zone orbits, we find that a network of ten 30 cm telescopes could survey these
1,976 M dwarfs in less than 3 years. A null result from this survey would set
an upper limit (at 99% confidence) of 17% for the rate of occurrence of planets
larger than 2 Earth radii in the habitable zones of late M dwarfs, and even
stronger constraints for planets lying closer than the habitable zone. If the
true occurrence rate of habitable planets is 10%, the expected yield would be
2.6 planets.
