POLARIZATION SPECTRA OF EXTRASOLAR PLANETS

POLARIZATION SPECTRA OF EXTRASOLAR PLANETSDaphne M. StamAstronomical Institute ”Anton Pannekoek”, University of Amsterdam,Kruislaan 403, 1098 SJ Amsterdam, The NetherlandsEmail: dstam@science.uva.nlWe present numerically simulated visible to near–infrared spectra of the flux (irradiance) and degree oflinear polarization of starlight that has been reflected byJupiter–like extrasolar planets. The results indicate thatpolarizationspectra are more sensitive to the compositionand structureof the planetarymodel atmospherethan fluxspectra. We conclude that polarization observations willnot only be very useful for detecting extrasolar planets,but also for characterizing their atmospheres.Key words: Extrasolar planets; atmospheres; polariza-tion; radiative transfer.Indirect detection methods, in which a planet’s influenceon its parent star is measured, have proved to be verysuccessful in finding extrasolar planets. Such methods,however, cannot provide information on important phys-ical characteristics of a planet, such as the compositionand structure of its atmosphere. Such characteristics canonly be derived from radiation of the planet itself. Plan-etary radiation is composed of thermal (infrared) radia-tion emitted by the planet, and stellar (visible to near–infrared) radiation that is reflected by the planet. The rel-ative contribution of each type of radiation to the plane-tary spectrum will depend strongly on age of the planet,and on the distance of the planet to its star. Because anextrasolar planet will be extremely faint compared to itsstar, direct detection methods will have to rely on specialtechniques to separate the planetaryradiation fromthat ofthe star. A promising technique appear to be polarizationobservations.There are a number of advantages of polarization obser-vations: Such observations can distinguish between directstarlight, which will be unpolarized, and reflectedstarlight, which will generally be polarized;