Detecting exo-Earths with hypertelescopesin space: the Exo-Earth Discoverer concept

Abstract

"Densified-pupil multi-aperture imaging interferometers" also called "hypertelescopes" can provide direct images at their focal plane. Coronagraphic masking is applicable for nulling a star's image and searching associated planets. The scheme is applicable at the infra-red wavelengths where a planet has its best contrast, but also potentially at visible wavelengths where chlorophyll-like spectral features are to be searched. In the infra-red, where zodiacal and exo-zodiacal clouds tend to contaminate the image, the scheme provides a better rejection of this contamination than schemes using a beam-splitter and flat interference. For a typical Earth-like exo-planet the detection time is reduced 10 to 100 times. In the visible, planets are often resolved from their star by an interferometer's sub-aperture. Visible coronagraphy is then achievable in each sub-aperture before combining the beams and producing the high-resolution image. This relaxes the piston tolerances. Whether for sub-apertures or a densified pupil, or even a conventional telescope, multiple stages ofcoronagraphy are of interest, if equipped with as many deformable mirrors or transmissive plates. Located in the successive image planes, such adaptive correctors can phase the residual stellar speckles without affecting the planet peak. This generates a central peak of starlight, which can be masked without much obstructing the planet's wavefront. A design concept studied for an "Exo-Earth Discoverer", a hypertelescope variant proposed to NASA for its Terrestrial Planet Finder, involves 37 mirror elements of 1 m, driven by small solar sails and arranged to form a 40-300 m diluted spherical mirror. One or several focal combiners move along the focal surface. Optical solutions have been explored in preliminary detail.