Exoplanet Science Strategy (2018) / Chapter Skim
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4 Implementing the Exoplanet Science Strategy
4 Implementing the Exoplanet Science Strategy
Pages 65-129
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From page 65... ...
Thus, microlensing surveys require monitoring a few square degrees with a resolution of <0.3 arcsecond on time scales of a few days or less.
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From page 66... ...
Figure 4.2 shows the simulated detection of a Ganymede-mass planet located at 5.2 AU from its parent star, and the detection of a potentially habitable planet. WFIRST may also be able improve upon Kepler's estimate of η⊕, the mean number per star of rocky planets with between 1 and 1.5-2 Earth radii that reside in the habitable zone of their host star.
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From page 67... ...
Example simulated detection of an Earth-mass planet in the habitable zone of a Sun-like star. Although the expected number of detections of potentially habitable planets is small (given Kepler's current best estimate of η⊕)
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From page 68... ...
. Most direct imaging detections of exoplanets so far have relied on the combination of adaptive optics and coronagraphs on ground-based telescopes.
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From page 69... ...
From the detection and spectroscopic study of gas and ice giants in reflected light and thermal emission, to the search for biosignatures of rocky planets orbiting M-type stars, direct imaging provides complementary phase space coverage to indirect methods and is the only technique capable of spectroscopically characterizing nontransiting exoplanets (Fitzgerald et al., 2018, white paper)
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. In the right plot, the diamonds denote young giant planets discovered by current ground-based adaptive optics facilities.
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From page 71... ...
Finding: The GMT and TMT will enable profound advances in imaging and spectroscopy of entire planetary systems, over a wide range of masses, semimajor axes, and wavelengths, potentially including temperate Earth-size planets orbiting M-type stars. Thermal Infrared Studies Thermal infrared observations with GSMTs may allow detection of warm (T = 400-600 K)
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It multiplies the contrast ratio achieved by high-contrast imaging by the additional contrast realized by high-dispersion spectroscopy. Implemented on GSMTs, it has the potential to find and atmospherically characterize temperate terrestrial planets around the most nearby stars.
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From page 73... ...
Science and Technology Pathfinders on Existing 8 to 10 Meter Class Facilities Ground-based telescopes have been playing a leading role in exoplanet direct imaging science and technological development for the past two decades and will continue to have an indispensable role for the next decade and beyond. Extreme AO systems will advance wavefront control, coronagraphy, and post-processing, thereby augmenting the performance of and mitigating the risk for WFIRST-CGI, while validating performance requirements and motivating improvements to atmosphere models needed to unambiguously characterize Solar System analogues with HabEx/LUVOIR (Currie et al., 2018, white paper)
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From page 74... ...
, but because it does not require the "slant geometry" of transit spectroscopy, it is less sensitive to high-atmosphere clouds and hazes. Thus, directly imaged spectra potentially allow for the detection of molecular species in cloudy planets (Robinson et al., 2016)
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From page 75... ...
Characterization of Mature Exoplanets in Reflected Light Space-based direct imaging will be able to access mature giant planets, Saturn-size or smaller, from approximately 0.5 to 15 AU. Space-based coronagraph imaging is a powerful complement spectroscopic characterization with JWST, which will characterize giant planets close to their stars.
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From page 76... ...
For example, the masses of the planets seen in the directly imaged systems HR8799 and Beta Pictoris are constrained by dynamical interactions between multiple planets or dust disks (Fabrycky and Murray-Clay, 2010)
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for potentially Earth-like planets (small terrestrial planets in the classical HZ of their star) versus telescope-inscribed diameter for both offaxis telescopes that can employ more efficient coronagraphs, such as the vortex coronagraph (VC)
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Combined with characterization such as planetary mass measurements from other facilities, this mission could provide a comprehensive picture of the evolution of planetary systems and may provide the first evidence of a habitable Earth twin -- an epochal moment in human history. Recommendation: NASA should lead a large strategic direct imaging mission capable of measuring the reflected-light spectra of temperate terrestrial planets orbiting Sun-like stars.
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From page 79... ...
, also directly measuring visible-wavelength scattered light, reducing the uncertainty of extrasolar zodiacal light impacting a future planet-imaging mission. Finding: The WFIRST-CGI at current capabilities will carry out important measurements of extrasolar zodia cal dust around nearby stars at greater sensitivity than any other current or near-term facility.
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The notional HabEx and LUVOIR represent points in a continuum of exoplanet science. All large, strategic, direct imaging mission architectures are capable of transformative science in the integrated study of planetary systems.
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. NASA funded two studies of probe-class missions, Exoplanet Direct Imaging: Coronagraph (Exo-C; a 1.4 m telescope with an internal coronagraph)
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By 2020 the ExEP should know how well each coronagraph design would work with all architectures considered for future exoplanet missions. Coronagraph and wavefront control technologies are critical to future direct imaging missions.
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's detectors are expected to establish the state of the art in MIR detection sensitivity, and future MIR direct imaging is likely to require detectors that exceed it. It is likely that the detection sensitivity gap can be closed in the next decade, as a range of choices are close to meeting the requirements.
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From page 84... ...
These missions will provide many of the targets for atmospheric characterization efforts, which rely on high SNR to detect the minute signature of atmospheric absorption, emission, or scattering in a differential sense against the bright background of a much larger host star. Below, the committee outlines first the upcoming transit survey missions and how they will alter the landscape of the known transiting exoplanet population toward fulfilling the goals of the Exoplanet Science Strategy.
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. Its observing strategy is being designed to back a science goal of detecting Earth-size habitable-zone planets orbiting Sun-like stars.
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While large space-based direct imaging missions are currently being conceived of that would obtain spectra of terrestrial and gas giant planets at wider separations, transit spectroscopy techniques will remain the primary mode for atmospheric characterization over the next decade and beyond. James Webb Space Telescope (JWST)
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From page 87... ...
The Exoplanet Science Strategy outlined in this report provides some of the framework for a shared strategic vision in atmospheric characterization with JWST. For the first time JWST will bring exoplanet atmospheric characterization efforts from a regime of limited observations to one of high-fidelity spectroscopic investigations of a comparative sample (Cowan et al., 2015)
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From page 88... ...
Nevertheless, the committee notes that Earth's thermal emission peaks at 10 microns and the spectra of temperate terrestrial planets drop precipitously beyond 18 microns due to the decline of thermal blackbody emission and strong water absorption. JWST does have photometric capabilities that extend beyond the MIRI Low Resolution Spectrometer cutoff, and since observations of potentially habitable planets at these wavelengths will likely be limited by photon counting noise, OST spectroscopy would offer only a modest improvement over JWST in this area.
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From page 89... ...
The short wavelength range is important for exploring planetary mass loss and characterizing aerosol scattering, and a substantial number of gas-phase absorbers for potentially habitable planets will also be accessible in the optical and near infrared, including carbon dioxide, water, and the biosignature combination of molecular oxygen, ozone, and methane. The addition of a high-resolution optical or infrared spectrograph on LUVOIR would yield a substantial advance for transit observations using the cross-correlation technique because of the absence of contaminating telluric lines.
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From page 90... ...
or thermal emission spectrum (red box) , of which the latter is also sensitive to nontransiting planets and can break the orbital inclination degeneracy.
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From page 91... ...
Masses are also needed to constrain surface gravities and atmospheric scale heights, which are key boundary conditions for interpreting spectra obtained through either the transit or direct imaging techniques. Thus, exoplanet mass measurements are important for the success of JWST, ARIEL, the GSMT exoplanet studies, and a direct imaging mission.
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From page 92... ...
. The desire to measure the masses of giant planets beyond the snow line, to measure the masses for directly imaged giant planets, and to illuminate the formation histories of planetary systems that include directly imaged terrestrials motivates radial velocity monitoring over decades (Montet et al., 2014; Howard and Fulton, 2016)
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. The dashed lines show where the radial velocity semi-amplitude of a 1 ME planet would equal either 1 or 0.09 m/s; with the stated precision it would be possible to detect a 1 ME planet to the left of the line.
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From page 94... ...
to push down to a precision of several centimeters per second. Telluric Contamination Spectral contamination from telluric lines is a problem for EPRV measurements because their variation in strength and position over time scales ranging from minutes to seasons can lead to spurious radial velocity shifts much larger than the sought-after planetary signals.
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From page 95... ...
Ground-based measurements should be carried out leading up to the launch of a direct imaging space mission, irrespective of whether the coronagraph technology used involves an internal or an external occulter. Importantly, NASA has focused its precious resources on measuring the masses of discovered planets (which the committee emphasizes is an important goal)
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Recommendation: NASA and NSF should establish a strategic initiative in extremely precise radial velocities (EPRVs) to develop methods and facilities for measuring the masses of temperate terrestrial planets orbiting Sun-like stars.
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From page 97... ...
Finding: High-precision, narrow-angle astrometry could play a role in the identification and mass measure ment of Earth-like planets around Sun-like stars, particularly if the radial velocity technique is ultimately limited by stellar variability. THE NEED FOR DETAILED STELLAR CHARACTERIZATION An understanding of exoplanets is inextricably linked to an understanding of the stars they orbit.
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From page 98... ...
. Impact of High-Energy Stellar Radiation on Exoplanet Atmospheres A better understanding of the high-energy emission from host stars will allow for better interpretation of atmospheric features in exoplanetary spectra, and will inform the target strategy for an imaging mission.
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From page 99... ...
For habitable-zone terrestrial planets orbiting M dwarfs, the high-energy photon and proton flux is even more critical to understand, as the fluxes are at least five times stronger than the fluxes received at 1 AU from a solar-type star (France et al., 2016)
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100 EXOPLANET SCIENCE STRATEGY FIGURE 4.13 GALEX snapshot observations of a large sample of M stars display a drop in median values of FUV and NUV flux with increasing age, yet observed activity levels span one to two orders of magnitude. SOURCE: Shkolnik and Barman (2014)
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From page 101... ...
and photochemical planet atmosphere models for all types of exoplanets, from Earths to Jupiters, need input stellar UV fluxes (Segura et al., 2010; Line et al., 2010; Kaltenegger et al., 2011; Hu et al., 2012; Kopparapu et al., 2012; Moses et al., 2013) across planet formation and evolution time scales.
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From page 102... ...
This is valuable in particular to the discovery and characterization of self-luminous giant exoplanets with direct imaging, for which the age of the system determines the inferred mass of the companion through planetary model atmospheres (e.g., Bowler, 2016)
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A cold space telescope, high-resolution spatially resolved infrared spectroscopy, and ground-based mm interferometry would enable significant advances and permit meaningful comparisons to studies of comets, asteroids, and meteorites from the Solar System. Observations of young planets in disks will provide the ground truth for the otherwise indirectly inferred time scales of planet formation and permit studies of the dynamical interactions between disks and planets.
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From page 104... ...
. and direct imaging studies, there are indications of massive planets around very young stars (less than 5 Myr old; Kraus and Ireland, 2012; Sallum et al., 2015; Donati et al., 2016; Johns-Krull et al., 2016; Keppler et al., 2018)
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From page 105... ...
With the high spatial resolution of the GMT and TMT, researchers will be able to search the inner parts of planet-forming systems. At the end of its prime mission in 2018, LBTI will provide limits that will just reach a level that helps inform the size of a direct imaging mission (see the section "The Case for Imaging," earlier in this chapter)
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From page 106... ...
Extracting meaningful constraints relies on comparing the observed mass and emitted flux to models of the planet formation process. A second question relates to the inflated radii of many hot Jupiters.
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From page 107... ...
As described previously in the section "Stellar Composition, Planet Formation, and the Delivery of Volatiles" in Chapter 3, evidence suggests that some volatile-rich Earth-size planets may have formed in outer orbits and then migrated inward. Future key work will focus on models for terrestrial planet formation and migration around different types of stars, which can illuminate the initial composition of the terrestrial planet, and interactions between other components of the system that can affect volatile delivery.
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From page 108... ...
Thermochemical equilibrium approaches are computationally cheap and may be appropriate for hot atmospheres, in which reaction rates are fast. Chemical kinetics codes, which perform time-dependent calculations of reaction rates, photolysis reactions, and vertical mixing and evolve the atmosphere to a steady state, are computationally intensive, and are typically only applied in one-dimensional (1D)
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From page 109... ...
Spectral Retrievals While spectral retrievals are one of the specific types of modeling approaches, they deserve their own discussion because of their recent rise in popularity within the exoplanet community. Statistical techniques are applied to determine best-fit parameters and confidence intervals based on the level of agreement between observed spectra and large suites of forward models run in a Monte Carlo framework.
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From page 110... ...
The set of expertise needed to respond to all of these needs is diverse and spans multiple research communities. Finding: The limited laboratory and ab initio data covering the parameter space relevant to exoplanets is a barrier to accurate models of exoplanet atmospheres and interiors.
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From page 111... ...
These will be challenging to obtain, but could be pursued with transmission spectroscopy, thermal phase curves, secondary eclipses, infrared photometry, or direct imaging spectroscopy (Meadows et al., 2018)
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The planets orbiting Sun-like stars that will be studied by direct imaging missions could be discovered by the EPRV or astrometric methods, or discovered by the missions themselves; either way, advanced EPRV or astrometry will be essential to measure the masses of the directly imaged planets.
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From page 113... ...
A direct imaging mission promises the most capability for habitability and biosignature searches of habitablezone terrestrial planets orbiting FGKM stars from ultraviolet to near-infrared wavelengths. Direct imaging can probe to the planetary surface and so is more definitive for habitability detection than transmission, which cannot probe the near-surface atmospheres.
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From page 114... ...
on the impact of terrestrial planet interiors on planetary atmospheric and surface conditions, essentially working to understand the formation and nature of secondary outgassed atmospheres, which was jointly supported by the NAI and NSF; the Exoplanet Biosignatures Workshop (2016) , which produced six community scientific publications that greatly advanced understanding of the significance of false positives and agnostic biosignatures, and fostered the development of the comprehensive framework for biosignature assessment (Kiang et al., 2018; Schwieterman et al., 2018; Meadows et al., 2018; Catling et al., 2018; Walker et al., 2018; Fujii et al., 2018)
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From page 115... ...
The Exoplanet Science Strategy, therefore, includes a strategy for developing and maintaining human capital, including addressing demographics and standards of professional conduct, and identifies areas requiring further research. Equity and Inclusion Several recent reports provide evidence that women and people of color are underrepresented in the professional astronomy workforce.
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14 See American Astronomical Society, "AAS Code of Ethics," last updated October 11, 2017, https://aas.org/ethics#bullying. 15 SeeCortina and Magley (2003)
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The committee endorses the IA2015 recommendation that, "The decadal survey should address issues of policy making and leadership diversity imbalances as recommendations that can be acted upon by policy makers." To achieve this goal, the Astronomy and Astrophysics and the Planetary Science Decadal Surveys will need to consult with experts beyond the astrophysics and planetary science communities and with members of underrepresented and marginalized groups. As the exoplanet community continues to research and implement the recommendations cited above, these 17 See AAS Group Wiki, "Inclusive Astronomy: The Nashville Recommendations," American Astronomical Society, https://tiki.aas.org/ tiki-index.php?
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Monthly Notices of the Royal Astronomical Society 450(2)
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˜ -0.3 and the need for a large high-precision sample. Monthly Notices of the Royal Astronomical Society 476(3)
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Publications of the Astronomical Society of the Pacific 127(949)
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American Astronomical Society, DPS meeting #48, id.212.03. Fischer, D.A., G
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Accepted by the Monthly Notices of the Royal Astronomical Society. Johns, M., P
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Monthly Notices of the Royal Astronomical Society 344(4)
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Publications of the Astronomical Society of the Pacific 130(986)
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Monthly Notices of the Royal Astronomical Society 401(3)
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2018. "Alpha Elements' Effects on Planet Formation and the Hunt for Extragalactic Planets." Presentation to the American Astronomical Society, Meeting #231, id#427.01.
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. Pre sentation to the American Astronomical Society, Meeting #231, id#228.04.
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Monthly Notices of the Royal Astronomical Society 431(3)
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White paper submitted to the Committee on Exoplanet Science Strategy, National Academies of Sciences, Engineering, and Medicine, Washington, D.C.
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