Why Alpha Centauri is a Particularly Good Target for Direct Imaging of Exoplanets.

Abstract:

Several mission concepts are being studied to directly image planets around nearby stars. It is commonly thought that directly imaging a potentially habitable exoplanet around a Sun-like star requires space telescopes with apertures of at least 1m. A notable exception to this is Alpha Centauri (A and B), which is an extreme outlier among FGKM stars in terms of apparent habitable zone size: the habitable zones are ~3x wider in apparent size than around any other FGKM star and are thus in theory accessible to much smaller telescopes. Alpha Centauri is also an extreme outlier in terms of how bright its planets are. For example, an Earth-like planet would be several magnitudes brighter around Alpha Centauri than around any other Sun-like star. Alpha Centauri lies in the galactic plane but its high brightness and proper motion make confusion with background stars unlikely, and extinction by our own galaxy makes confusion with extragalactic sources unlikely. The high brightness also helps any planet stand out against exozodiacal light. Alpha Centauri B has a (probable) planet, which establishes that a planetary system has formed in the system despite the multiplicity of the system. Dynamical simulations show that the habitable zones around both Alpha Centauri A and B are stable, and RV measurements rule out any large planets in the system that may have disrupted the formation of habitable planets. Recently developed high contrast techniques enable the detection of planets in multi-star systems, enabling imaging planets around Alpha Centauri. In particular, a small ~30-45cm visible light space telescope equipped with a modern high performance coronagraph or starshade is sufficient to directly image any potentially habitable planet that may exist in the system.