Colors of an Earth-like exoplanet

Abstract

Context. Understanding the total flux and polarization signals of Earth-like planets and their spectral and temporal variability is essential for the future characterization of such exoplanets. Aims. We provide computed total (F) and linearly (Q and U) and circularly (V) polarized fluxes, and the degree of polarization P of sunlight that is reflected by a model Earth, to be used for instrument designs, optimizing observational strategies, and/or developing retrieval algorithms. Methods. We modeled a realistic Earth-like planet using one year of daily Earth-observation data: cloud parameters (distribution, optical thickness, top pressure, and particle effective radius), and surface parameters (distribution, surface type, and albedo). The Stokes vector of the disk-averaged reflected sunlight was computed for phase angles α from 0° to 180°, and for wavelengths λ from 350 to 865 nm. Results. The total flux F is one order of magnitude higher than the polarized flux Q, and Q is two and four orders of magnitude higher than U and V, respectively. Without clouds, the peak-to-peak daily variations due to the planetary rotation increase with increasing λ for F, Q, and P, while they decrease for U and V. Clouds modify but do not completely suppress the variations that are due to rotating surface features. With clouds, the variation in F increases with increasing λ, while in Q, it decreases with increasing λ, except at the largest phase angles. In earlier work, it was shown that with oceans, Q changes color from blue through white to red. The α where the color changes increases with increasing cloud coverage. Here, we show that this unique color change in Q also occurs when the oceans are partly replaced by continents, with or without clouds. The degree of polarization P shows a similar color change. Our computed fluxes and degree of polarization will be made publicly available.

The Fundamental Connections between the Solar System and Exoplanetary Science

“…In many ways, it makes sense to focus our efforts on planets of a similar size, mass, and insolation flux to Earth, because Earth is the only known globally habitable and inhabited planet. For that reason, studies of Earth are of critical importance in shaping the future direction of exoplanetary science (e.g., Fan et al, 2019;Groot et al, 2020;Horner & Jones, 2010;Unterborn et al, 2016). One of the great challenges that such a focus on Earth-similar planets poses is the determination of which factors in Earth's characteristics and history are universally required for habitability and life .…”

LOUPE: observing Earth from the Moon to prepare for detecting life on Earth-like exoplanets

“…We compute F (equation ( 2.1 )) of the visible and illuminated disc of a model Earth at a given phase angle α and wavelength λ by dividing the disc into pixels with specific surface–atmosphere models (e.g. ocean-cloudy, forest-clear, desert-clear), computing the reflected Stokes vector for each pixel using an adding-doubling radiative transfer algorithm [ 13 , 14 , 25 ], and summing up the local vectors to obtain the disc-integrated, planetary Stokes vector. An example of the simulated flux and polarization for an unresolved planet is shown in figure 1 .…”

“…Note that at α = 0°, P L is virtually zero. For these images, data described in [ 25 ] were used. (Online version in colour.)…”

“… polarimetric accuracy 1% <10 −3 Accuracy of measurements limited by systematic errors. Planetary properties affect the polarization signal at below 1% level [ 25 ]. relative photometry 3% 1% To track the diurnal changes in flux with a sufficient SNR [ 5 ].…”

“…To keep a first proof-of-concept version of LOUPE as simple as possible, key trade-offs may be undertaken, such as limiting the instrument not to measure the circular polarization V , but only F , Q and U . This trade-off is justified by the fact that, although V has the potential to be considered a biosignature of homochiral life [ 29 ], V of light reflected by an (exo)planet is several orders of magnitude lower than the linearly polarized flux [ 25 , 35 , 36 ] and neglecting it introduces no significant errors in F , Q and U [ 37 ]. Some other capabilities of LOUPE are functionally optional as well, such as radiometry, though its addition could provide data for climate research.…”