Orbiting Space Interferometer (OSI): A first generation space interferometer

The technical requirements and performance of a first generation space interferometer is discussed. The performance of an interferometer, sensitivity, field of view, dynamic range, astrometric accuracy, etc, in space is set by what cannot be achieved for a ground-based instrument. For the Orbiting Space Interferometer (OSI), the nominal performance parameters are 20 mag sensitivity, field of view of approximately 500*500 pixels, a 1000:1 dynamic range in the image with one milliarcsec resolution, and an astrometric accuracy of 0.1 milliarcsec for wide angle astrometry and 10 microarcsec accuracy for narrow field astrometry (few degrees). OSI is a fully phased interferometer where all critical optical paths are controlled to 0.05 wavelengths. The instrument uses two guide interferometers locked on bright stars several degrees away to provide the spacecraft attitude information needed to keep the fringes from the faint science object stable on the detector

Exo-Earth/Super-Earth Yield of JWST plus a Starshade External Occulter

We examine the scientific viability of an imaging mission to find exo-Earths combining the James Webb Space Telescope (JWST) with a starshade external occulter under a realistic set of astrophysical assumptions. We define an exo-Earth as a planet of 1 to 10 Earth masses orbiting in the habitable zone (HZ) of a solar-type star. We show that for a survey strategy that relies on a single image to detect an exo-Earth, roughly half of all exo-Earth detections will be false alarms. Here, a false alarm is a mistaken identification of a planet as an exo-Earth. We consider two survey strategies designed to mitigate the false alarm problem. The first is to require that for each potential exo-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-Earths found is on average 0.9, 1.9 and 2.7 for {\eta}_Earth = 0.1, 0.2 and 0.3. Here, {\eta}_Earth is the frequency of exo-Earths orbiting solar-type stars. There is a ~40% probability of finding zero exo-Earths for {\eta}_Earth = 0.1. A second strategy can be employed if a space astrometry mission has identified and measured the orbits and masses of the planets orbiting nearby stars. We find that with prior space-based astrometry from a survey of 60 nearby stars, JWST plus an external occulter can obtain orbital solutions for the majority (70% to 80%) of the exo-Earths orbiting these 60 stars. The exo-Earth yield is approximately five times higher than the yield for the JWST plus occulter mission without prior astrometry. With prior astrometry, the probability that an imaging mission will find zero exo-Earths is reduced to below 1% for the case of {\eta}_Earth = 0.1.Comment: Accepted by PASP. To appear in February 2010 issue. 15 pages, 2 figure

The Synergy of Direct Imaging and Astrometry for Orbit Determination of exo-Earths

The holy grail of exoplanet searches is an exo-Earth, an Earth mass planet in the habitable zone around a nearby star. Mass is the most important parameter of a planet and can only be measured by observing the motion of the star around the planet-star center of mass. A single image of a planet, however, does not provide evidence that the planet is Earth mass or that it is in a habitable zone orbit. The planet's orbit, however, can be measured either by imaging the planet at multiple epochs or by measuring the position of the star at multiple epochs by space-based astrometry. The measurement of an exo-planet's orbit by direct imaging is complicated by a number of factors: (1) the inner working angle (IWA); (2) the apparent brightness of the planet depending on the orbital phase; (3) confusion arising from the presence of multiple planets; and (4) the planet-star contrast. In this paper we address the question: "Can a prior astrometric mission that can identify which stars have Earthlike planets significantly improve the science yield of a mission to image exo-Earths?" We find that the Occulting Ozone Observatory (a small external occulter mission that cannot measure spectra) could confirm the orbits of ~4 to ~5 times as many exo-Earths if an astrometric mission preceded it to identify which stars had such planets. We find that in the case of an internal coronagraph, a survey of the nearest ~60 stars could be done with a telescope of half the size if an astrometric mission had first identified the presence of Earth-like planets in the habitable zone and measured their orbital parameters.Comment: ApJ, in press; 28 pages, 8 figure