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DRVS and extrasolar planetary dust noise reduction Rajendra VikramSingh

3611 Lupine Avenue, Palo Alto, CA 94303, USA.

Noise due to dust around extrasolar planets is not resolved by astrometry. Planetary-signal to dust-noise ratio is also not diminished by interferometry. DRVS (Differential Radial Velocity Spectroscopy) based on the difference in their Doppler shifts, allows the weak spectrum of an extrasolar planet to be filtered out from its parent star's spectrum [1,2,3,4]. The dust noise spectrum is essentially the same across the narrow spectral window within which a particular spectral line is Doppler shifted during each planetary orbit and Doppler filtering as such does not improve the signal/noise ratio. However, careful selection of strong and narrow spectral lines for inclusion in the matched filter, controlling their total number for processing gain while minimizing their combined bandwidth to reduce dust noise contribution, could optimize the planetary-signal/dust-noise ratio.


Interplanetary dust scatters the stellar emission and is also a source of intrinsic radiation. Dust emission is several hundred times stronger than emission from planet Earth [5]. Leinert states "The appearance of the solar absorption line spectrum in the zodiacal light proves that it is sunlight scattered by slowly moving particles. The motion of the scattering dust particles, apart from a slight broadening of the lines, leads to a Doppler shift of about 0.3 Angstroms" [6]. "Zodiacal light intensity (/) strongly depends on the heliocentric distance (R), the increase towards the Sun, being I(R)~R e"2 3 [7]. Dumont and Levasseur-Regourd [8] write " To the first order, the zodiacal cloud is homogeneous and steady; it has a solar-like spectrum". Dumont [9] further declares "Doppler spectrometry can be used to retrieve the mean orbital velocity, v, of the interplanetary scatterers in the region of the terrestrial orbit". Dumont computes the difference of mean orbital velocity of the scatterers at 1 A.U. with respect to Earth from observational data to be +9.1 ± 5 km s"1 [10] and +7.6 ± 5 km s"1 [11],


Precision radial velocity spectroscopy has been successfully applied to indirect detection of giant extrasolar planets in tight orbits around their parent stars by measuring the periodic Doppler shift of the stellar spectrum due to reflex motion of the parent star Orbital characteristics of most of the extrasolar planets discovered so far are surprisingly different from those of the planets in the solar system. The chemical composition, particle size distibution, and dynamics of dust in the environs of extrasolar planets may also be different from the solar system dust. Scattered starlight is likely to be the dominant feature of their dust spectra, similar to the appearance of the Sun's spectrum in the zodiacal light due to scattering by dust.


Precision astrometry will be more sensitive, due to its wideband collection of photons, in locating and tracking dust clouds in other stellar systems, but is unlikely to be able to identify a planet obscured by a dust cloud in a closely related orbit. Interferometers based beyond the solar system dust and providing a million times or even greater nulling of the signal from the parent star of an extrasolar planet may be operational someday. In the near future, Earth based interferometers are expected to provide a nulling factor >1000, thereby reducing the stellar infrared flux to within a hundred to a thousand times the extrasolar planet's flux. The nulled stellar flux will then be of almost the same order of magnitude as flux from extrasolar planetary dust. According to Beichman [12] "dust in target solar system three hundred times brighter than planet". The dust surrounding an extrasolar planet obscures the line-of-sight to the extrasolar planet and cannot be nulled by interferometric spatial filtering. Emissions from dust in the vicinity of the extrasolar planet thus emerge as a major source of "noise" (undesired signal) against extrasolar planetary emissions (desired signal).

Noise from other interplanetary dust sources may however be minimized by observing the extrasolar planet in an appropriate epoch during which these dust sources do not lie along or near the line-of-sight to the extrasolar planet. It is also conceivable that short periods of direct observation of an extrasolar planet through relatively dust free holes in the extrasolar interplanetary dust may occur, and if recognized as such it may be possible to program further observations during these short epochs.


Probable detection of reflected starlight from the planet Peg 51 has been published [4], Attempts to observe changes in the stellar spectrum during the near transit of the planet Peg 51 across the line of sight have not shown any conclusive results so far, but other transiting extrasolar planets are likely to be discovered in the near term and may prove more fruitful. Extraction of spectral signatures of specific molecules, e.g., methane, from extrasolar planets which may be buried in the glare of their parent stars has also been attempted but produced uncertain answers up until now [13]. A combination of interferometric spatial filtering and differential Doppler filtering using dynamic correlation filters tracking the planet's periodically changing radial velocity during its orbital motion is more likely to succeed. The extrasolar planetary signals are quite weak but large aperture interferometers coupled with high resolution spectrographs, namely, Keck (Hawaii), LBT (Arizona), and ESO (Chile) will be available soon.


The difference in the mean orbital velocity of the scattering dust with respect to the extrasolar planet leads to a periodically varying difference in their radial velocities, whose magnitude goes from zero when their orbits cross the line-of-sight to the star, to a maximum at the two orthogonal orbital positions. The stellar spectrum reflected from the extrasolar planet will have a Doppler shift different in shape and magnitude than the same stellar spectrum reflected from the orbiting dust cloud.

Correlation filters using very narrow windows and a large number of spectral lines may manage to separate these two differentially Doppler shifted copies of the stellar spectrum and confirm the separate existence of the extrasolar planet and the dust cloud envelope. If the scattered signal were strong enough, observing changes in spectral line shapes could be an alternative approach. The spectrum of a specific molecular species showing a similar differential Doppler separation would indicate its presence on the extrasolar planet as well as in the dust cloud. If no differential Doppler separation occurs the molecular species would be located only on either the extrasolar planet or in the dust cloud.

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