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From Exo-Earths To Exo-Venuses – Flux And Polarization Signatures Of Reflected Light

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Left column: total flux (or phase function). and right column: degrees of polarization of incident unpolarized starlight reflected by the model planet at four evolutionary stages as a function of α and λ. First row: Phase 1 (“Present Earth”); Second row: Phase 2 (“Thin Cloud Venus”). Line 3: Phase 3 (“Thick cloud Venus”). Line 4: Phase 4 (“Present Venus”). The phase function is normalized to equal the geometrical albedo AG of the planet at α = 0 ◦. — Astro-ph.EP

Terrestrial exoplanets in the habitable zone are everywhere. However, it is unclear which has an Earth-like climate or a Venus-like climate.

Distinguishing between different planet types is important for determining whether a planet is habitable. We investigate the possibility of polarimetry to distinguish between outer Earth and outer Venus. We present the calculated flux and polarization of stellar light reflected by exoplanets with atmospheres in evolutionary states from present-day Earth to present-day Venus. Cloud compositions range from pure water to 0.75 sulfuric acid solutions with wavelengths between 0.3 and 2.5 microns.

The polarization of the reflected light shows greater variation with planetary phase angle than with the total flux. Across the visible range, maximum polarization is reached in an Earth-like atmosphere with water clouds due to Rayleigh scattering above clouds and rainbows near 40 degree phase angles.

In the near-infrared, planets with CO2 atmospheres and thin water clouds, such as Venus, exhibit the most pronounced polarization properties due to scattering by small cloud droplets. Planets around Alpha Centauri A leave temporal variations in reflected luminous flux along the orbits of the planets in spatially unresolved star-planetary systems, on the order of 10E-13 W/m3, and polarization along the orbits of the planets. I would leave 10E-11 at a time.

The contrast between stars and planets is on the order of 10E-10. Current polarimeters cannot distinguish between possible evolutionary stages of spatially unresolved exoplanets. This is because a sensitivity close to 10E-10 is required to discriminate planetary signals in the background of unpolarized starlight.

Telescopes capable of reaching planet-star contrasts lower than 10E-9 should be able to observe changes in the planet’s resolved degree of polarization as a function of its phase angle, resulting in exo-Earth and exo-Venus. should be able to identify exo-Earths based on The unique polarization signature of clouds.

Gurav Mahapatra, Fuad Abiad, Loic Rossi, Daphne M. Stam

Comment: Postable in A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instruments and Methods for Astrophysics (astro-ph.IM)
Quoted as: arXiv:2301.11314 [astro-ph.EP] (or arXiv:2301.11314v1 [astro-ph.EP] for this version)
Submission history
From: Daphne Stam
[v1] Thursday, January 26, 2023 18:53:08 UTC (1,751 KB)
space biology

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