Earth as an Exoplanet

Abstract

Earth is the only planet known to harbor life and, as a result, the search for habitable and inhabited planets beyond the Solar System commonly focuses on analogs to our planet. However, Earth's atmosphere and surface environment have evolved substantially in the last 4.5 billion years. A combination of in situ geological and biogeochemical modeling studies of our planet have provided glimpses of environments that, while technically belonging to our Earth, are seemingly alien worlds. For modern Earth, observations from ground-based facilities, satellites, and spacecraft have yielded a rich collection of data that can be used to effectively view our planet within the context of exoplanet characterization. Application of planetary and exoplanetary remote sensing techniques to these datasets then enables the development of approaches for detecting signatures of habitability and life on other worlds. In addition, an array of models have been used to simulate exoplanet-like datasets for the distant Earth, thereby providing insights that are often complementary to those from existing observations. Understanding the myriad ways Earth has been habitable and inhabited, coupled with remote sensing approaches honed on the distant Earth, provides a key guide to recognizing potentially life-bearing environments in other planetary systems.Look again at that dot. That's here. That's home. That's us. [. . . ] [E]very saint and sinner in the history of our species lived there -on a mote of dust suspended in a sunbeam. -Carl Sagan 1 "Sun-like" typically refers to main sequence stars with spectral type F, G, or K. Such stars range from 30% larger and 6× more luminous than our Sun down to 30% smaller and 13× less luminous than the Sun. Sun-like stars are often contrasted to "late type" stars, which are red, cool main sequence M stars that can be smaller than 10% the size of our Sun and can have luminosities as small as 1-2% that of the Sun.

Starshade rendezvous: exoplanet sensitivity and observing strategy

“…Our assumed geometric albedo for Earth-like planets (0.2) only matches the Earth's reflectivity for scattering angles β ≲ 90°; for more grazing angles, it is an underestimate. 17,18 The function Φ L ðβÞ is normalized to unity at maximum illumination (β ¼ 0), takes the value of 1∕π at maximum elongation (β ¼ π∕2), and vanishes smoothly as the planet eclipses the star (β → π). The normalization of the reflected light at maximum illumination, taking into account the ðr pl ∕R pl Þ 2 dependence and Φ L ðβÞ, is the geometric albedo A G .…”

Colors of a Second Earth. Ii. Effects of Clouds on Photometric Characterization of Earth-Like Exoplanets

“…Therefore, in this section we combine the models and empirical data of the surface and the cloud distribution so as to reproduce the diurnal light curves in multi-bands observed by EPOXI. In addition to the forward calculation presented in Paper I, numerous other groups have simulated the scattered light of the Earth (Ford et al 2001;Tinetti et al 2006a,b;Montañés-Rodríguez et al 2006;Pallé et al 2008;Arnold et al 2009;Doughty & Wolf 2010;Robinson et al 2011).…”

Starshade Rendezvous: exoplanet orbit constraints from multi-epoch direct imaging

“…We model Earth-like exoplanets assuming they scatter light isotropically using a Lambertian illumination phase function with a geometric albedo of 0.2 (Ref. 15). We model the star as a blackbody radiator with the parameters provided in ExoCat.…”