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Appendix E: Report of the Panel on Exoplanets, Astrobiology, and the Solar System
Pages 284-304

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From page 284...
... Complementing our studies of individual worlds, multiple techniques have pieced together a broad understanding of exoplanet classes, enabling a new era of comparative planetary system science as we work toward a more complete census. Even though exciting progress has been made, significant key advances are still needed to place the solar system and our inhabited Earth in its cosmic context.
From page 285...
... Roughly a dozen planets are known with semimajor axes larger than 50 AU, and many have poorly constrained orbits and masses. Based on all available surveys, the occurrence rate of gas giants appears to peak near a few AU and then decline at larger separations, but these estimates depend on extrapolations of power laws in mass and semi-major axis, and the ~3–10 AU region is not yet thoroughly explored.
From page 286...
... At orbital periods shorter than Mercury's, intense stellar radiation has sculpted the mass-radius diagram, inflating hot Jupiters and producing a bimodal radius distribution for small planets, likely owing to atmospheric escape. While highly irradiated planets smaller than ~1.6 REarth have bulk densities consistent with a terrestrial composition, larger planets require significant fractions of volatiles, as do solar system ice giants.
From page 287...
... Exoplanet Characterization and Solar System Synergy Efforts to characterize and model exoplanet atmospheres have focused largely on giant and Neptune-size planets; atmospheric characterization of smaller planets has just begun. Comprehensive surveys of transiting planets across a range of mass, radius, orbits, and/or insolation levels have provided key insights into interior and atmospheric composition, as well as the atmospheric circulation, chemical, and radiative properties that regulate planetary atmospheres.
From page 288...
... Theory and observations now suggest that there are many evolving interactions between a planet, star, and planetary system that affect the likelihood that a planet can support a surface ocean -- and that a comprehensive, systems-level approach to habitability assessment is now needed. These studies have identified systems-level challenges to habitability for M dwarf HZ planets that are less likely to be experienced by planets orbiting in the HZ of more Sun-like stars, including radiation and stellar-wind-driven atmosphere and ocean loss, and gravitational interactions that modify orbits, rotation rate, and climate.
From page 289...
... surveys have primarily been limited to the most massive gas giants. The Nancy Grace Roman Space Telescope microlensing survey is poised to 6 In addition to community inputs in the form of white papers and presentations, the congressionally mandated reports by the National Academies of Sciences, Engineering, and Medicine, Exoplanet Science Strategy and An Astrobiology Strategy for the Search for Life in the Universe, were considered as inputs to the panel.
From page 290...
... To fully understand migration, the chemical and dynamical conditions of planets must be studied prior to, during, and after migration. By combining detailed mm wavelength observations of protoplanetary disks with cold planet demographics and a large number of well-studied individual planetary system architectures, we can relate atmospheric properties and locations of planets in mature systems to those in protoplanetary disks.
From page 292...
... For terrestrial planets, surface/atmosphere exchange mechanisms mediate atmospheric composition, and planetary magnetic fields can illuminate processes occurring deep in a planet's interior, while providing critical insights into how the planet's atmosphere interacts with the space environment. Meeting the goal of determining the bulk composition of a planet thus entails connecting the observable atmosphere, as sculpted by such processes, to deep atmospheric or surface processes and chemical composition.
From page 293...
... Phase-resolved observations from facilities such as JWST, and high-dispersion spectroscopy conducted from the ground, ideally in tandem with climate, photochemistry, and 3D atmospheric dynamical models, provides estimates of the atmospheric composition and dynamics, and insights into planetary rotation state. The microphysical and dynamical processes that govern the morphology and transport of clouds have yet to be untangled for giant exoplanets, and detailed models of these processes will be imperative for understanding formation and transport of clouds on all types of terrestrial planets with atmospheres.
From page 294...
... Roman CGI will obtain optical wavelength thermal emission spectra of young companion objects as a complement to JWST and ground-based, longer wavelength observations, and perhaps reflected light spectra of a few cool giants. Both types of observations will inform the properties of giant planets, helping to place them in context with low-mass brown dwarfs.
From page 295...
... to drive planetary atmosphere and ocean loss. The close-in HZ makes M dwarf HZ planets potentially more vulnerable to atmospheric loss, coronal mass ejection events, PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION E-12
From page 296...
... HOW CAN SIGNS OF LIFE BE IDENTIFIED AND INTERPRETED IN THE CONTEXT OF THEIR PLANETARY ENVIRONMENTS? Over the next 10 years, JWST and upcoming ground-based telescopes will have the opportunity to conduct the first searches for signs of life on terrestrial planets orbiting a handful of nearby M dwarf stars.
From page 297...
... The next decade will present several opportunities to characterize terrestrial exoplanets and undertake the very first search for biosignatures on a handful of planets orbiting nearby M dwarfs. Owing to their host stars' super-luminous pre-main sequence phase, activity, and the proximity of the HZ to the star, M dwarf planets likely undergo a very different evolutionary history -- which may include atmosphere and ocean loss -- than planets orbiting more Sun-like stars, and may allow us to expand our understanding of biospheres for different stellar hosts.
From page 298...
... This profound question has echoed down through the millennia, and the answer is now within our scientific and technical grasp. Ground-based surveys have transformed the search for life from a philosophical question to a near-term scientific observable by providing a handful of high-priority M dwarf terrestrial planets amenable to spectroscopic atmospheric characterization with JWST and the ELTs.
From page 299...
... The capabilities needed to study the environments and possible biospheres of habitable planets orbiting more Sun-like stars are not met by any existing or currently selected facilities, but developing the scientific community and technological capabilities required to do so would enable huge advances in multiple aspects of exoplanet science and astrophysics. This Discovery Area would benefit significantly from collaboration across disciplinary boundaries, and ongoing support for enabling observations, theory and laboratory work.
From page 300...
... E-Q2d: How does a planet's interaction with its host star and planetary system influence its atmospheric properties over all time scales? E-Q2e: How do giant planets fit within a continuum of our understanding of all substellar objects?
From page 301...
... to search for biosignatures mas, OWA ~ 500 mas, ~100s of stars, R Q4c, E- ~ 150 spectroscopy for potentially DA dozens of Earth analogs) Astrometry E-Q1, E- Gaia, Roman WFI supplement: Near-IR astrometry to measure Q2a, E- population studies overlapping with substellar object masses/orbits; masses Q2b, E- Kepler, cold gas giants in TESS and and orbits of temperate planets orbiting Q2c, E- nearby systems FGKM stars Q2e, E Q3d, E DA Polarization E-Q1d, E- Roman: polarization of disks Direct imaging to probe polarized ocean Q1e, E- glint on terrestrial planets Q2c, E- Ground-based instruments, including on Q2e, E- ELTs: polarization signatures of disks Q3c, E- and giant planets Q3d, E Q4, E-DA Microlensing E-Q1a Roman population studies PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION E-18
From page 302...
... for photoevaporation and inferring stellar mass loss rates UV observations E-Q2a, E- HST limited UV transit capability UV space telescope: R > 1000 of planets and host Q2c, E- spectroscopy; monitor atmospheric stars Q2d, E- escape; high-contrast imaging of planets Q3c to detect UV absorbers; time-resolved UV stellar flux High-resolution E-Q2a, E- R > ~1e5 O/IR spectroscopy (8–10 m High-contrast vis-NIR reflected light O/IR spectroscopy Q2.c, E- telescopes) : giant planet characterization spectroscopy of mature planets Q2d, E- (~dozens)
From page 303...
... Solar system E-Q1b, E- HST, ground-based telescopes Large time allocations and/or improved small-body Q1c, E- detection algorithms; continued characterization Q1d detection and spectroscopic characterization of small bodies in UV/IR; rotation rates and orbital characteristics; small KBO binary fraction Characterization E-Q2, E- Venus atmospheric composition Ice giants: atmospheric and interior of solar system Q3b, E- structure and composition planets Q3d, E- Atmospheric escape Mars Q4b Venus: atmosphere entry probes Habitability E-Q3b, E- Dragonfly: Titan; Europa Clipper: Venus: atmospheric chemistry assays relevant solar Q4a Europa system Enceladus: future missions environments JUICE: Galilean moons Earth: detectable characteristics of Earth Mars2020: Mars environments through time Ground-based observations: Venus Interdisciplinary E-Q3, E- Identification of novel biosignatures, comprehensive multifactorial framework for theory, laboratory, Q4a, E- habitability assessment, identification of biosignature false positives and negatives field Q4b, E- and their observational discriminants. Probabilistic framework for biosignatures DA assessment.
From page 304...
... Marley, et al., 2016, The need for laboratory work to aid in the understanding of exoplanetary atmospheres, arXiv preprint arXiv:1602.06305. NOTE: IWA/OWA: inner and outer working angles for optimum starlight suppression in direct imaging systems; R: spectral resolution; P: planetary orbital period; CGI: coronagraphic instrument; E/PRV: extreme/precision radial velocity; NEID: NN-EXPLORE exoplanet investigations with Doppler spectroscopy; GPI: giant planet imager; SPHERE: Spectro-Polarimetric High-Contrast Exoplanet Research; Roman WFI: Roman Wide-Field Imager; CME: coronal mass ejection.


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