Exoplanet Science Strategy (2018) / Chapter Skim
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3 Outlining the Exoplanet Science Strategy
3 Outlining the Exoplanet Science Strategy
Pages 42-64
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From page 42... ...
To achieve this goal, the exoplanet science community needs to do the following: • Determine the range of planetary system architectures by surveying planets at a variety of orbital separations and searching for patterns in the structures of multiplanet systems; • Characterize the diversity of bulk compositions and atmospheric compositions; • Identify the parameters that determine which stars can form certain types of planetary systems; and • Identify relationships between the planet formation process and the resulting planetary evolution, bulk composition, and atmospheric properties. A key goal of the theory of planet formation is to produce population synthesis models that tie observationally constrained properties of stars and circumstellar disks to the observed population of planets.
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From page 43... ...
. These results support the core accretion model of planet formation, in which disks hosting more solid material more easily form solid cores massive enough to accrete giant envelopes.
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From page 44... ...
Furthermore, Gaia will be sensitive to planets orbiting stars more massive (and, thus, at different evolutionary states) than those probed by radial velocity surveys.
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From page 45... ...
Given that the existence of a giant planet may or may not affect the habitability of the terrestrial world, this feature of direct imaging surveys is quite important. Direct imaging facilities will deliver detailed characterization of a comparatively smaller number of planetary systems, which will provide important constraints on the physics of planetary atmospheres and interiors, and on the architectures of planetary systems, including the frequency with which terrestrial planets have outer giant planets.
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From page 46... ...
Therefore, the atmospheric compositions of giant planets should be broadly similar to those of their host stars. On the other hand, the core accretion model is based on the assumption that giant planets form in a region of the disk that is rich in solid planetesimals.
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From page 47... ...
It should be possible to determine the carbon, nitrogen, and oxygen abundances, which is important because these elements make up the bulk of the solid planetesimals that are crucial for giant planet formation. Figure 3.3 shows what is known about the atmospheric metallicities of the solar-system giant planets, and exoplanets.
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From page 48... ...
for a large sample of distant, young, self-luminous planets will constrain the roles of the core accretion and disk instability mechanisms. Exploration via direct imaging of the boundary between wide-separation giant planets and brown dwarf binary companions will also inform whether a population of companions formed through gravitational instability exists (Kratter et al., 2010)
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From page 49... ...
As discussed in Chapter 4, in the section "The Case for Imaging," flying a large space-based direct imaging mission will be the most productive strategy for performing atmospheric characterization of a large number of terrestrial planets over a range of orbital separations. Precise measurements of the masses and radii of terrestrial planets will play an essential role in exploring their interior structures.
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From page 50... ...
Terrestrial planets can also be impacted by biological activity. Therefore, atmospheric compositions are an important diagnostic of planetary geophysics, habitability, and life (see the following section)
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From page 51... ...
. These studies will be complemented by statistical surveys on larger samples of terrestrial planets that provide information on planet demographics and bulk properties for terrestrial planets both interior to and outside the habitable zone.
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, those that require modeling interpretation, possibly constrained by observations (green) , and the properties or processes that are accessible primarily through theoretical modeling (orange)
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From page 53... ...
Moreover, the current understanding of the evolution of terrestrial exoplanets, with a diversity of compositions and host star environments, will need to improve. A key component will be understanding degassing for terrestrial planets of different compositions, including potentially more volatile-rich migrated planets that may be found orbiting M dwarfs, both inside and far from the nominal habitable zone (Gillon et al., 2017; Luger et al., 2017; Grimm et al., 2018; Berta-Thompson et al., 2015; Dittmann et al., 2017; Meadows and Barnes, 2018)
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From page 54... ...
For example, while initial modeling of the formation of M-dwarf terrestrial planets suggested that they might form with little water (Raymond et al., 2007; Lissauer, 2007) , recent measurements of the density of Earth-size planets orbiting the M8V TRAPPIST-1 star suggest that they are instead volatile rich (Grimm et al., 2018)
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From page 55... ...
. Observations and models that can confirm the presence of an atmosphere, as may be possible with transmission spectroscopy or direct imaging, or identify the factors most likely to contribute to atmospheric loss (such as stellar XUV flux and evolution)
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More generally, the early Earth provides information on alien habitable environments with different atmospheric compositions and climates (Lyons et al., 2014) , populated by metabolisms with potential biosignatures (Pilcher, 2003; Arney et al., 2018)
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From page 57... ...
Models that couple interactions between the planetary interior, surface, ocean, atmosphere, and biosphere can be used to look at the survivability of potential biosignatures in different contexts. Agnostic biosignatures look for patterns of complexity or aspects of the environment that cannot be explained by normal planetary processes such as volcanism or photochemistry.
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From page 58... ...
Such a framework would need to consider the context of the stellar and planetary environment, and include an understanding of false negatives, false positives, and their observational discriminants. Discovering Potentially Habitable Planets and Searching for Life on Them The methods and time scale for identifying potential habitable exoplanets and searching for atmospheric biosignatures are different depending upon the mass and size of the host star.
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From page 59... ...
If the planet exhibits the partial cloud cover characteristic of convective condensate clouds on Earth, then direct imaging observations may also probe to the surface, even in the presence of clouds. Requirements for Credible Interpretation of Biosignatures In summary, the interpretation of biosignatures will likely need the following: • An expanded interdisciplinary modeling, laboratory, and field effort to understand multifactorial habitability assessment and biosignatures assessment frameworks; • Studies of a wide range of planetary atmospheres, from gas giants to uninhabitable terrestrials, to improve understanding of the physical and chemical processes that modify planetary environments and provide the context for biosignature interpretation; • An improved understanding of planet formation deep enough to allow use of a planet's system architecture, and the compositions of its sibling planets, as discriminants for biosignature assessment.
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2015. The occurrence of potentially habitable planets orbiting M dwarfs estimated from the full Kepler dataset and an empirical measurement of the detection sensitivity.
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Astrophysical Journal Letters 793(2)
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Astrophysical Journal Letters 143:L16. Olson, S.L., E.W.
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Astrophysical Journal Letters 819(1)
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From page 64... ...
Astrophysical Journal Letters 785(2)
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