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3 Comparative Planetology and Multiparameter Habitability Assessment
Pages 44-63

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From page 44...
... Further, the post2015 discovery of Earth-sized planets orbiting M-dwarf stars in their habitable zones -- a term that has, at times, been used to mean different things in different communities (see below) -- has transformed the field of exoplanet studies from a statistical exercise predicting that such planets are common, to providing specific targets amenable to near-term telescopic study.
From page 45...
... Recent discoveries of exo-Venuses -- terrestrial-size planets (i.e., approximately 1.6 Earth radii) in an orbit closer to the parent star than the inner edge of the habitable zone -- and habitable zone terrestrial planets have also enabled an exciting new era of comparative planetology.
From page 46...
... , M-dwarf star, and Proxima Centauri b's short orbital period FIGURE 3.2  NASA's Kepler mission discovered approximately 50 planet candidates within the habitable zone of their host stars (Thompson et al.
From page 47...
... Given the ubiquity of M dwarfs in the Milky Way, these nearby discoveries (summarized in Table 3.1) suggest that the galaxy is teeming with planets orbiting in their host star's habitable zones.
From page 48...
... It is also worth noting that Kepler/K2 and ground-based surveys are probing very different types of M-dwarf stars and, yet, both are finding that potentially habitable planets are common. Finding: The availability of near-term data on the atmospheres of terrestrial exoplanets orbiting M-dwarf stars will enable the first observational tests of their potential habitability.
From page 49...
... In the longer term, direct imaging observations of planetary atmospheres and surfaces will be needed to build upon these initial transmission assays of planetary upper atmospheres, by probing near-surface atmospheres and constraining planetary surface conditions. Transmission observations are not an option for more Sun-like F-, G-, and K-dwarf stars, due to the larger stellar size and more distant habitable zone, which diminishes the planet signal and the probability that it transits its star.
From page 50...
... Other work on redefining the limits of the habitable zone has looked at the habitable zone limits for terrestrial planets larger than Earth (Kopparapu et al.
From page 51...
... 2012) , as jovians can affect volatile delivery to forming terrestrial planets (e.g., eccentric jovian planets can inhibit water delivery to forming rocky planets; Raymond et al.
From page 52...
... COMPARATIVE PLANETOLOGY The discovery of over 3,700 confirmed planets beyond the solar system opens up an exciting opportunity to understand a diversity of planetary characteristics and processes. Many of these new worlds have no analog in the solar system and can enrich our understanding of the characteristics of the planetary system, star, and planet that contribute to, or detract from, planetary habitability.
From page 53...
... Such diversity of habitable planets in the solar system may also exist in other stellar systems with multiple "habitable" planets as well. Their comparative planetology is going to be just as important to reveal whether life exists on any of them.
From page 54...
... Looking into the future, there is much to be learned not only about Earth, but also about exoplanets, from Venus. Ocean Worlds: Comparing Surface and Subsurface Oceans Some of the most exciting recent work in the ocean worlds community has come from comparing these bodies to each other.
From page 55...
... Theoretical models of rocky planet formation and evolution, including the maintenance or loss of habitability and volatile transport within forming planetary systems, will soon be confronted with new data on the atmospheric compositions of a number of potentially habitable worlds, leading to theoretical refinements incorporating those observations. The Kepler mission revolutionized knowledge of exoplanet demographics and has helped to determine the frequency of potentially habitable worlds around Sun-like and low-mass stars (G-, K-, and M-dwarf stars)
From page 56...
... Solar System and Exoplanet Synergies in Comparative Planetology Recent acquisition of observational constraints on the atmospheres of habitable-zone exoplanets opens a new interdisciplinary field of comparative planetology for habitable-zone planets. The potential for growth is large and requires expanded collaboration between scientists studying bodies in the solar system and those who observe and model exoplanets.
From page 57...
... Finding: Comparative planetology between the solar system and exoplanetary systems is • a powerful approach to understanding the processes and properties that impact planetary habitability; • essential to inform experiments, modeling, and mission planning in astrobiology; and • fundamentally crossdivisional and requires collaboration between astrobiologists, planetary scientists, and exoplanet astronomers and is therefore ideally suited to a research coordination network. Statistical Methods for Comparative Planetology A wide range of problems in astrobiology have benefited from recent progress in statistical methods.
From page 58...
... 2016. A ter restrial planet candidate in a temperate orbit around Proxima Centauri.
From page 59...
... 2017. A statistical comparative planetology approach to the hunt for habitable exoplanets and life beyond the solar system.
From page 60...
... 2017. Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1.
From page 61...
... Planets formed in habitable zones of M dwarf stars probably are deficient in volatiles. Astrophysical Journal Letters 660(2)
From page 62...
... 2018. "Addressing the Habit ability of Europa with the Europa Clipper Mission." White paper submitted to the Committee on an Astrobiology Science Strategy for the Search for Life in the Universe.
From page 63...
... Astrophysical Journal Supplement Series 235(2)


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