DRVS also faces new challenges in detecting and characterizing extrasolar planets embedded in dust clouds. "There is a significant excess of the orbital mean velocity of the scatterers surrounding the Earth's orbit, over the terrestrial velocity, of the order of +25 to +30 %" . A similar difference in the mean orbital velocity of the extrasolar-planetary-dust with respect to the extrasolar planet would lead to a periodically varying difference in their radial velocities. For a circular orbit the two radial velocity curves would look like two sine waves with the same period and phase but different magnitudes. The differential radial velocity between the extrasolar planet and its dust envelope would vary during the orbit around the parent star from zero, when their orbits cross the line-of-sight from the observer to the star to several km s"1 at the two orthogonal positions in the orbit.
It may be recalled that radial velocity of the reflex motion of stars due to giant planets in close orbits varies over a range of tens of m s'1, and for Earthlike planets only a few cm s~\ Stellar "noise" (undesired signal) is a million (infrared) to a billion (visible) stronger than extrasolar planetary signal (desired signal). Precision radial velocity spectroscopy is required for measuring such small variations.
The periodic differential radial velocity between an extrasolar planet and its dust is several km s"1, that is 3 to 5 orders of magnitude greater than the stellar planetary situation. Moreover, the dust is only a few hundred times brighter, that is 3 orders of magnitude lower, than the stellar planetary case. Precision radial velocity spectroscopy will not be needed. Lower signal-to-noise ratio and smaller processing gain in the Doppler spectral correlation filter will be good enough for separating the intrinsic emission spectra of the extrasolar planet and the dust obscuring it. There is a significant difference in the Doppler shift of the stellar spectrum scattered from the extrasolar planet and the extrasolar-planetary-dust. The reflex motion of the parent star is very small and its spectrum has minimal Doppler shift. (It could in fact be used as a local frame of reference.) The mixture of the direct signal from the star because of incomplete nulling, the two components of the scattered stellar signal, and the intrinsic emissions from the extrasolar planet and the dust cloud, each one with its unique Doppler shift during the orbital cycle, will require a fairly intricate dynamic correlation filter. The extraction of a signal whose spectral signature could be postulated, such as, a specific molecule from a very noisy background would be less difficult.
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