Exoplanets are orders of magnitude fainter than their parent stars. A coronagraph is a set of optical elements that suppresses the star's light to create a region where a dim planet can be extracted. Due to optical errors in any system, all coronagraphs must be designed together with wavefront control via one or more deformable mirrors (DMs). The combined coronagraph and wavefront control system is characterized by the contrast, inner working angle, and stability achieved. Contrast is the degree to which the instrument can suppress scattered and diffracted starlight in order to reveal a faint companion. Inner Working Angle (IWA) is the smallest angle on the sky at which it can reach its designed contrast. This angle is typically only a few times larger than the theoretical diffraction limit of the telescope. The resulting residual stellar halo must also be stable over the time scale of an observation, so that the halo can be subtracted to reveal an exoplanet or disk.
In principle, with a well-characterized and stable point spread function, various subtraction methods that have been developed and used on both ground and space images can be employed to average the background photon noise and extract faint planets that are below the raw contrast level. The recent history of planet imaging shows that recovering planets with up to factors of 10 fainter contrast than the background is regularly accomplished, both on large ground telescopes and from the Hubble Space Telescope. We thus characterize the coronagraph instrument by its detection limit, that is, the limiting magnitude of a recoverable planet relative to the star from the combination of the coronagraph and wavefront control and data processing.
Only a very small number of exoplanets have been imaged to date from the ground. We have obtained spectra on an even smaller number. All of them are at great distances from their parent star and are very large. Additionally, the limitations in contrast of a ground-based instrument means that all of these planets are also extremely young — less than a hundred million years — and shining through their residual interior heat, rather than reflecting starlight. WFIRST provides the first opportunity to observe and characterize planets like those in our solar system lying from 3 to 10 AU from their parent star. The coronagraph on WFIRST operating at a contrast of 1 ppb and an inner working angle of less than 0.2” will detect at least a dozen new planets in this range and characterize over a dozen of the known radial velocity planets.
The majority of these planets will also be characterized using R~70 spectra in the wavelength range 400-1000 nm via an integral field spectrograph. These spectra will allow the detection of features expected due to methane, water, and alkali metals, reveal the signature of Rayleigh scattering, and easily distinguish between different classes of planets (i.e., Neptunes versus Jupiters), and planets with very different metallicities.
The figure (left) is a scatter plot showing model planets of all types distributed around the brightest 200 or so stars within 30 pc along with the contrast and inner working angle limits of increasingly aggressive coronagraphs. Planets are distributed consistent with the radius distribution found by Kepler and extrapolated to larger semi-major axis and lower mass. There are roughly 4 planets per star with a mix of hydrogen-envelope gas giants, rocky planets at 1 to 1.4 Earth radii, and icy Neptune and Saturn like planets. The plot shows a snapshot of the random selection of planets in their orbit as viewed from Earth. At the 1 ppb contrast level and a 0.2 arcsec IWA, a substantial number of new gas and ice giants are accessible, thus increasing the number of known giant planets. Should further analysis and experiment show that 10-10 detection contrast or smaller IWA is achievable, then the number of possible planets to be discovered increases significantly, including a small number of water planets and Super-Earths.
The coronagraph will also be sensitive to debris disks with a few times the solar system's level of dust in the habitable zones and asteroid belts of nearby (~10 pc) sun-like stars. The high sensitivity and spatial resolution (0.05 arcsec is 0.5 AU at 10 pc) of WFIRST images will map the large-scale structure of these disks, revealing asymmetries, asteroid belts, and gaps due to unseen planets. WFIRST will make the most sensitive measurements yet of the amount of dust in or near the habitable zones of nearby stars. This is important for assessing the difficulty of imaging Earth-like planets with future missions as well as for understanding nearby planetary systems. Finally, spectrophotometry of these disks at the full range of available wavelengths from 400-1000 nm provides constraints on dust grain size and composition.