Atmospheric characterization of cold exoplanets using a 1.5-m coronagraphic space telescope

¹ LESIA, Observatoire de Paris, CNRS, University Pierre et Marie Curie Paris 6 and University Denis Diderot Paris 7, 5 place Jules Janssen, 92195 Meudon, France
e-mail: anne-lise.maire@obspm.fr
² National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
e-mail: raphael.galicher@nrc-cnrc.gc.ca
³ Dépt. de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada
⁴ Groupement d’Intérêt Scientifique Partenariat Haute Résolution Angulaire Sol Espace (PHASE) between ONERA, Observatoire de Paris, IPAG, LAM, CNRS and University Denis Diderot Paris 7, France
⁵ LUTh, Observatoire de Paris, CNRS and University Denis Diderot Paris 7, 5 place Jules Janssen, 92195 Meudon, France
⁶ Dept. of Aeronautics and Astronautics, MIT, 77 Mass. Ave. 37-367, Cambridge, MA 02139, USA
⁷ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁸ JPL, California Institute of Technology, M/S 301-355, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Received: 3 February 2012
Accepted: 12 March 2012

Abstract

Context. High-contrast imaging is currently the only available technique for the study of the thermodynamical and compositional properties of exoplanets in long-period orbits, comparable to the range from Venus to Jupiter. The SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems) project is a coronagraphic space telescope dedicated to the spectro-polarimetric analysis of gaseous and icy giant planets as well as super-Earths at visible wavelengths. So far, studies for high-contrast imaging instruments have mainly focused on technical feasibility because of the challenging planet/star flux ratio of 10^-8−10^-10 required at short separations (200 mas or so) to image cold exoplanets. However, the main interest of such instruments, namely the analysis of planet atmospheric/surface properties, has remained largely unexplored.

Aims. The aim of this paper is to determine which planetary properties SPICES or an equivalent direct imaging mission can measure, considering realistic reflected planet spectra and instrument limitation.

Methods. We use numerical simulations of the SPICES instrument concept and theoretical planet spectra to carry out this performance study. We also define a criterion on the signal-to-noise ratio of the measured spectrum to determine under which conditions SPICES can retrieve planetary physical properties.

Results. We find that the characterization of the main planetary properties (identification of molecules, effect of metallicity, presence of clouds and type of surfaces) would require a median signal-to-noise ratio of at least 30. In the case of a solar-type star  ≤10 pc, SPICES will be able to study Jupiters and Neptunes up to  ~5 and  ~2 AU respectively, because of the drastic flux decrease with separation. It would also analyze cloud and surface coverage of super-Earths of radius 2.5 Earth radii at 1 AU. Finally, we determine the potential targets in terms of planet separation, radius and distance for several stellar types. For a Sun analog, we show that SPICES could characterize Jupiters (M  ≥  30 Earth masses) as small as 0.5 Jupiter radii at  ≲2 AU up to 10 pc, and super-Earths at 1−2 AU for the handful of stars that exist within 4−5 pc. Potentially, SPICES could perform analysis of a hypothetical Earth-size planet around α Cen A and B. However, these results depend on the planetary spectra we use, which are derived for a few planet parameters assuming a solar-type host star. Grids of model spectra are needed for a further performance analysis. Our results obtained for SPICES are also applicable to other small (1−2 m) coronagraphic space telescopes.