Exo-earth/super-earth yield of jwst plus a starshade external occulter

摘要

We estimate the exo-Earth/super-Earth yield of an imaging mission that combines the James Webb Space Telescope (JWST) with a starshade external occulter under a realistic set of astrophysical assumptions. For the purpose of this study, we define "exo-Earth" and "super-Earth" as a planet of mass 1 to 2 M? and 2 to 10 M?, respectively, orbiting within the habitable zone (HZ) of a solar-type star. We show that for a survey strategy that relies on a single image as the basis for detection, roughly half of all exo-Earth/super-Earth detections will be false alarms for ?? = 0.1, 0.2, and 0.3. Here, a false alarm is a mistaken identification of a planet as an exo-Earth/super- Earth, and we define ?? as the frequency of exo-Earth/super-Earths orbiting sunlike stars. We then consider two different survey strategies designed to mitigate the false alarm problem. The first is to require that for each candidate exo-Earth/super-Earth, a sufficient number of detections are made to measure the orbit. When the orbit is known we can determine if the planet is in the habitable zone. With this strategy, we find that the number of exo-Earth/super- Earths found is, on average, 0.9, 1.9, and 2.7 for ?? = 0:1, 0.2, and 0.3. There is a 7sim;40% probability of finding zero exo-Earth/super-Earths for ?? =0:1. A second strategy can be employed if a space-based astrometry mission capable of submicroarcsecond precision has identified and measured the orbits and masses of the planets orbiting nearby stars. In this case, the occulter mission is much more efficient, because it surveys only the stars known to have exo-Earth/super- Earths. We find that with prior knowledge from a space-based astrometric survey of 60 nearby stars, JWST plus an external occulter can obtain spectra, as well as orbital solutions, for the majority (70% to 80%) of the exo-Earth/super- Earths orbiting these 60 stars. The yield of exo-Earth/super-Earths is approximately five times higher than the yield for the JWST plus occulter mission without prior astrometric information. With prior space-based astrometry, the probability that an imaging mission will find zero exo-Earth/super-Earths is reduced to <1% for the case of ? =0:1.