We propose a low-cost mission concept called TOLIMAN dedicated to the detection of Earth-mass exoplanets in temperate orbits around the two nearest Sun-like stars: Alpha Centauri (a Cen) A & B. Our baseline mission concept uses a 9 cm telescope to obtain relative astrometry measurements of the binary at sufficient precision to detect planets down to sub-Earth mass throughout the Habitable Zones of both a Cen A & B. Although thousands of planetary systems are known, astronomers have scant knowledge of temperate rocky planets; not a single Earth-analog orbiting a Sun-like star has been detected within 10 pc. The impact of the detection of a single temperate rocky planet orbiting one of the nearest Sun-like stars can hardly be overstated; such a discovery would be a target of keen astrobiological interest and will shape the future of direct imaging and characterization efforts for years to come. High precision astrometry can detect the small angular displacement of a star due to gravitational perturbation by its orbiting planets. Although the target stars are generally very bright, the final signal-to-noise is entirely limited by the requirement to observe fainter field stars over wide areas of the sky. The TOLIMAN mission concept sidesteps these problems for the special case of planets orbiting bright binary stars. Two bright stars occupying a narrow field leverages strong fundamental gains in signal-to-noise allowing compelling science within a modest mission profile. However, the telescope needs to be equipped with a Diffractive Pupil, which serves as an absolute astrometric ruler on board by populating the image plane with diffraction features. This enables our mission to deliver sub-Earth mass astrometric detection capability as well as contemporaneous high-precision spectrophotometric monitoring. Our mission is a pathfinder for advancing the technical readiness of diffractive pupil technologies that will empower astrometric detection and characterization of Earth-like planets around nearby Sun-like stars by the future space observatories and by the next generation of scientists.© California Institute of Technology 2019. All rights reserved. Government sponsorship acknowledged.