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Discovering Earth Analog Candidates in the Near-UV

Presentation #1125 in the session “Open Engagement Session B”.

Published onMar 17, 2021
Discovering Earth Analog Candidates in the Near-UV

Habitable planets may be found on Sun-like stars. In such systems, an Earth analog can be sought that shares Earth’s albedo spectrum. We propose using an anomalous characteristic of the albedo of Earth to identify Earth 2.0 candidates. We present a specialized telescope adept at making the requisite observation. In the portion of solar spectrum where black body radiation has its peak intensity in green then decreasing into the shorter wavelength region toward the UV, the correlated spectral emission of Earth’s albedo shows a unique gain in emitted energy above 400 nm when compared to all other planets in our Solar System. Earth is luminescent in the near-UV band. The earth may be “a pale blue dot,” but surprisingly it is bright in the near-UV. By way of contrast, Neptune, a blue planet, loses flux in the near-UV in proportion to the Sun’s spectrum. The gain in reflected energy by Earth in the region of interest is attributable to Raleigh scattering in the atmosphere and a lowering of absorption in oceans. In the former, we see a more productive albedo scattering from Earth’s atmospheric gas abundances such as nitrogen. In the latter, the absorption curve from the Sun spectrum in water dies out into the blue. We used Earth observing satellite data to document the Earth albedo spectrum. Although photons are scarce when observing exoplanets with G-Class parent stars in the region of interest from 350-550 nm, there are more photons than in longer wavelengths. Gathering photons within perhaps four binned bands of 50 nm width we could test for a differential gain relative to a parent star in the near-UV. If a directly observed exoplanet shows a differential gain relative to its G-Class parent star in the near-UV, the differential between parent star and child planet is a signal indicating an Earth analog candidate. The novel telescope proposed to make this observation uses a Gabor Zone Plate (GZP) as its primary objective. We show how this instrument enjoys a feature that isolates a star and its exoplanetary system from all proximate stars by use of its secondary spectrometer. The optical physics are a direct descendant from Newton’s dual prism experiment. The slit between the primary and secondary along with a secondary dispersive optic such as a grating excludes all sources outside confined angles encompassing the target system alone. Within the target, direct spectrographic observation of individual albedos is feasible. We choose the near-UV not simply for its utility in running the proposed differential flux test for an Earth analog. We show that coronagraphy in this band is facilitated by higher contrast absorption lines compared to longer wavelengths. The troughs of the star absorption lines give access to peaks of albedo lines, dropping the contrast ratio by two to nearly three orders of magnitude across selected Fraunhofer lines. Further coronagraphy is facilitated by a unique circular line of foci coming from the GZP primary. We show how this permits Angular Differential Imaging without physically rotating the telescope. A third coronagraph called BLOC (Bifurcated Light Optical Coronagraph) is being investigated. This last design uses the nulling interferometric method with the improvement that the null occurs only over an inner diameter extinguishing the host star but does not touch the region where exoplanets can be observed. The proposed telescope called DUET (Dual Use Exoplanet Telescope) is intrinsically spectrographic and has the alternative use of taking indirect radial velocity measurements by Doppler shift. It has been tested on an optical bench. We present our laboratory results.

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