The goals of Exo-C are to obtain images of known gas giant planets orbiting nearby stars, discover and image smaller, less massive planets, and discover and characterize circumstellar dust disks around these same stars. These exoplanets and disks will be detected in the reflected light of their central star. The color and spectrum of the exoplanets will reveal information about the composition of their atmospheres. When combined with precise velocity measurements the images will constrain the mass of the imaged planets, thus characterizing the architecture of a dozen or so exoplanetary systems.
Almost all of the extrasolar planets detected to date have been found by indirect means. Many have been found from measurements of the regular variation in the motion of their primary star towards and away from Earth as the star is tugged by a planet’s gravity. For almost all such ‘radial velocity’ planets our only knowledge of the nature of the planet is a limit on its mass. Even more planets have been detected by the ‘transit’ method which discovers planets that pass in front of their primary star. The size of such planets can be measured from the amount which the star dims during transit. For the subset of planets detected by both radial-velocity and transit, the bulk density of the planet can be determined.
The atmospheric composition of a planet is fundamentally important because it can give clues to the processes that led to the formation and evolution of the planet. For example Jupiter’s atmosphere contains about three times more carbon and nitrogen, in the forms of methane and ammonia, than does the sun. Saturn’s atmosphere has about ten times more of these species. Astronomers believe that bombardment of the growing planets early in their history by planetesimals from a disk around the sun led to these enhancements. For extrasolar planets we would like to understand their atmospheres by recognizing and constraining the position of cloud layers, measuring atmospheric composition, and comparing the results to our own solar system.
In a few very hot planets, measurements to date during transit have allowed the properties of the upper atmosphere to be measured via spectroscopy. Exo-C will be able to directly image the light scattered by the atmospheres of cool giant planets like those in our solar system. This light will have penetrated to deeper atmospheric regions and is more representative of the bulk atmospheric composition than transit measurements. Will exoplanet atmospheres resemble those of our familiar solar system examples or will completely new types of atmospheres be seen ?
Debris dust has been found orbiting about 20% of nearby stars from far-infrared observations made with the Spitzer, Herschel, and WISE telescopes. This dust is released by ongoing collisions in belts of asteroidal and cometary bodies. In addition to revealing the location of these belts, debris disks can include rings, gaps, warps, and clumps forced by the gravity of unseen planets. With contrast improved 1000× over the Hubble Space Telescope, Exo-C will detect debris disks as tenuous as our Kuiper Belt, enabling comparative studies of dust disks across a range of host star masses and ages. Hundreds of debris disk targets will be surveyed, including radial velocity planet systems where gravitationally sculpted disk structures may be seen. A smaller survey of young protoplanetary disks will also be done. Polarimetric imaging will be used to measure dust properties in a subset of the targets.
Exo-C’s resolution will be sufficient to image the habitable zones of 10 Sun-like stars and another 40 more luminous stars. A survey of these targets will search for reflected light from dust in these habitable zones, down to limits within a factor of 5-10 of the levels found in our own Solar System. When dust is found, the Exo-C images will constrain the reflectivity of this dust, thus helping to define a noise source against which future missions will observe Earth-like extrasolar planets. In the nearest examples, Exo-C images may show asymmetric structures indicative of planetary perturbations to the dust distribution.