CHARACTERIZING EXTRA-SOLAR PLANETS WITH COLOR DIFFERENTIAL ASTROMETRY ON SPICA

SPICA coronagraph will be operating from 3.5 to 27 micron and primarily aims at directly imaging extra-solar planets of nearby stars. A strong limitation of the coronagraph for the 3.5 m telescope is its angular resolution which allows planet detectability farther than a few AUs at best. Color Differential Astrometry (CDA) is a spectrophotometric method which would push the detection range to very close-in planets and up to a few AUs, thus nicely complementing the imaging coronagraph. We present the method and show how it can be implemented within the host coronagraph without major impacts on its optical setup. We give preliminary estimates of its sensitivity according to different extra-solar planetary systems configurations. Critical instrumental requirements are discussed, especially regarding detector gain stability.