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Timeline for the Exoplanet Science Strategy
The exoplanet community aims to accomplish the parallel goals of increasing understanding of planetary systems as astrophysical objects and taking the next steps toward identifying habitable environments and biosignatures on extrasolar worlds. This chapter summarizes and restates the strategy envisioned by the Exoplanet Strategy Committee, separated here into near-term (< 5 years), medium-term (5-15 years), and long-term (15-20 years) goals.
NEAR-TERM ACTIVITIES
The field of exoplanet science is relatively new and remains a vibrant and evolving research area. In the near-term time scale (<5 years) and extended forward into the mid-term, support for innovative ideas in theory, observations, and instrumentation is needed. Exoplanet science is also an increasingly and necessarily interdisciplinary endeavor, spanning both astrophysics and planetary science, and currently in particular need of substantial input from stellar astrophysics and laboratory studies. Support in the form of consistently well-supported individual investigator grant programs, including opportunities for technology development, theoretical work, and interdisciplinary collaboration, is needed to maximize the yield of strategic programs in exoplanet science.
The James Webb Space Telescope (JWST) has the potential to provide infrared spectra of terrestrial-size planets in and near the liquid water habitable zones of M dwarfs, as well as a statistical sample of spectra for up to about 100 larger and hotter planets. The latter will enable the search for physical and chemical trends across objects, while the former will significantly advance understanding of planetary habitability. The spectra of warm terrestrials will answer the critical question of what kinds of atmospheres rocky planets around M dwarfs can form and retain. In terms of potentially habitable worlds, JWST may have the capability to detect molecules (such as H2O, CO2, and CH4, although most likely not O2) in the atmospheres of a handful of the most favorable planets. However, the precise outcome of JWST investigations in this area hinges on several unknown factors, including the noise floor of its instruments, the number of potentially habitable worlds that will be discovered around late M dwarfs within 10 pc by the Transiting Exoplanet Survey Satellite (TESS) and other surveys, the nature of these planets’ atmospheres, and the community’s willingness to invest hundreds of hours of observing time on individual planets.
Given the limited lifetime of JWST and the substantial interest in observing time from scientists across all areas of astrophysics and planetary science, the exoplanet spectroscopy work should be conducted as efficiently as is possible with full community involvement. JWST’s current launch timeline means that many ideal atmospheric characterization targets will have been discovered by TESS in advance of launch. The committee recommends
that NASA create a mechanism for community-driven legacy surveys early in the JWST mission that would allow exoplanet astronomers to self-organize to propose a survey of atmospheres that would benefit the full community.
Current workforce development will affect exoplanet science for years to come. The committee sees a need for current action to improve community practices in areas including but not limited to reducing harassment and discrimination on the basis of race, gender, sexual orientation, gender identity, and other marginalized identities, as well as addressing intersectional concerns. The committee considers the Astronomy and Astrophysics and Planetary Science Decadal Surveys to be appropriate venues to convene committees of experts to provide recommendations for improved workforce development.
MEDIUM-TERM ACTIVITIES
The committee identifies several large strategic priorities that must be begun now and will come to fruition in the medium-term time scale of 5-15 years. The committee reaffirms support of the exoplanet community for two major projects currently in progress: the Wide-Field Infrared Survey Telescope (WFIRST) and the U.S.-led giant segmented mirror telescopes (GSMTs: Giant Magellan Telescope and Thirty Meter Telescope).
Understanding the structure of planetary systems is crucial both to an understanding of planets as astrophysical objects and to the evaluation of potential habitable environments. Current knowledge of exoplanet occurrence rates shows that Earth-size planets are common enough in the Solar System neighborhood to plan for their atmospheric characterization. However, this information does not show the histories or formation contexts of these bodies. Fundamentally, systems like the Solar System remain largely inaccessible to current detection techniques, and it is still not known whether this system architecture is common or rare. Planet statistical demographics from Kepler have launched a reimagining of planet formation scenarios. Information about planets in a new phase space—one that encompasses solar-system-like architectures—would do the same. The committee thus endorses the WFIRST microlensing survey. This survey will substantially broaden the view of the structures of planetary systems and their diversity as well as the range of physical processes that determine planet compositions. The committee notes that Solar System bodies are a touchstone for understanding of physical planet properties, so evaluation of potential biosignatures on exoplanets will rely on a clear understanding of how the Solar System’s properties compare to those of other systems. The WFIRST microlensing survey is thus important for both primary goals of this report. The committee further supports flying the WFIRST coronagraph and demonstrating its capabilities on exoplanet targets, both a technology demonstration on the path to future missions capable of imaging terrestrial exoplanets, and to place tighter constraints on the typical levels of exozodiacal light, which will affect the capabilities of such missions.
The 3-fold improvement in angular resolution, 10-fold improvement in light-collecting capabilities, and 80-fold improvement in sensitivity to point sources provided by the next generation of GSMTs will open up new vistas of exoplanet exploration. 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 dwarfs, direct imaging and high-resolution spectroscopy on GSMTs will be capable of spectroscopically characterizing transiting and nontransiting exoplanets. The committee notes here the particular synergy of the GSMTs (which could detect O2 in the atmospheres of several temperate terrestrial planets) and JWST (which likely cannot detect O2, but could detect other gases such as H2O, CO2, and CH4, which are essential to evaluating whether the oxygen is biogenic).
While many small-scale efforts are best supported by openly advertised, competitive individual investigator opportunities, the committee finds one particular area in need of strategic investment from now through the midterm time scale. Mass is a fundamental planetary property, necessary to understand bulk compositions and system architectures as well as to interpret atmospheric spectra. The committee finds that radial velocity measurement is the technique most likely to provide masses for a substantial number of Neptune, super-Earth, and terrestrial-mass planets. However, the success of efforts to improve radial velocity precision to the required level is not assured. In addition to improvements in instrument capabilities, the varied velocity signals produced by surface processes on stars will need to be understood at a substantially better level. The committee considers this problem too large to be addressed by principal investigators in possession of individual investigator grants and thus recommends that
NASA and the National Science Foundation establish an extreme precision radial velocity initiative to support and organize these efforts in order to maximize the science yield of future missions and the GSMTs. The committee emphasizes that a single co-located center is not recommended for this endeavor. Rather, the varied expertise of investigators at a range of institutions will be needed. The committee suggests that progress in precision measurement of masses through the radial velocity technique would benefit from NASA’s established ability to organize large groups of investigators in pursuit of demanding, unprecedented, and clearly defined goals.
LONG-TERM ACTIVITIES
Direct imaging of exoplanets requires angularly resolving the planets from their host stars and directly detecting photons from the planets. Separating the planet’s image from its host star is fundamentally limited by the theoretical diffraction limit, which is set by the observing wavelength and the telescope diameter. The large flux ratios and small angular separations imply that directly imaging and spectroscopically characterizing planets close to their host star requires dedicated high-contrast facilities, which is where the committee sets its sights for the long-term time scale (15-20 years from now).
Lynx
An X-ray mission such as Lynx would provide information about the high-energy radiation and stellar wind fluxes received by planets from their host stars. These stellar inputs are important for understanding atmospheric escape, evolution, and photochemistry. The committee considers this science interesting but finds that, unlike the other three proposed missions, this science case does not address the currently most central questions in exoplanet science.
Origins Space Telescope
A cooled near-to-far infrared (IR) mission such as the Origins Space Telescope (OST) would advance exoplanet science both by providing inputs to the study of planet formation through investigations of protoplanetary disks and by allowing planetary atmospheric characterization via the transit method. For the study of protoplanetary disks, the committee considers such a mission to be potentially transformative given its far-IR coverage. High-spectral-resolution investigation of water lines would allow study of the spatial distribution of water across disks. Measurements of hydrogen deuteride (HD) lines would allow direct measurement of hydrogen masses of disks. Both would provide important information about the conditions under which planets form.
For the direct study of exoplanets, OST’s primary strength is in atmospheric characterization through transit spectroscopy in both primary and secondary eclipse. Like JWST, OST’s mid-IR wavelength coverage allows secondary eclipse measurements to probe thermal emission from temperate atmospheres and detect a variety of key molecules using transmission and emission spectroscopy. Given sensitivity constraints, OST would be able to characterize terrestrial-size planets in the liquid water habitable zone around mid- to late M-dwarfs but not around earlier-type stars, including Sun-like stars.
The committee finds that OST will likely provide only a modest increase in the number of habitable-zone M-dwarf exoplanets that can be characterized compared to JWST. The currently proposed aperture, spectral resolution, and wavelength coverage of OST do not differ substantially from JWST, and thus improvements over JWST in OST’s ability to characterize atmospheres are primarily predicated on an improved instrumental noise floor. Since detector stability for transit spectroscopy was not a technology driver for JWST’s design, such an improvement is plausible, but not guaranteed.
The committee is excited about exploring the atmospheres of terrestrial planets in the habitable zones of M dwarfs. These planets may host life and, given the large of abundance of M dwarfs, may even be the most common habitable environments. However, the committee has reservations about currently investing in an OST-like mission for the purposes of exoplanet science given its potentially modest improvement in atmospheric characterization
when compared with JWST. In addition, the habitable zone of M dwarfs might not in fact be a habitable environment given its extreme exposure to high-energy stellar irradiation. Observations by JWST will address whether terrestrial planet atmospheres can survive under these conditions. Earth-like orbits around Sun-like stars need not be the only habitable environments, but Earth’s biosphere provides the only known example of a place where life can arise. OST would not open up the significant discovery space to characterize Earth-like planets around more Sun-like stars.
HabEx and LUVOIR
The committee considers a large space-based, direct imaging mission, capable of directly detecting and characterizing terrestrial planets in reflected light around Sun-like stars at near-ultraviolet, optical, and near-infrared wavelengths, to be the primary long-term priority for NASA exoplanet science. Such a mission would explore the atmospheres of planets with a range of sizes and effective temperatures, and image multiple planets in each system, enabling comparative exoplanetology. A direct imaging mission would also be sensitive to terrestrial planets in the habitable zones of stars similar to the Sun, environments for which Earth provides a proof of concept that habitability is possible, but which are accessible only from space.
Measurements over the past decade have dramatically reduced three major risk factors identified in the 2010 Astronomy and Astrophysics Decadal Survey (NRC, 2010) for a planet-imaging mission. First, abundance statistics from Kepler strongly suggest that terrestrial planets are common in the habitable zones of stars similar to the Sun. Second, Large Binocular Telescope Interferometer measurements and upper limits for exozodiacal dust in habitable zones indicate that dust is unlikely to prevent optical characterization of planets in most systems. Finally, coronagraph and starshade technologies have advanced substantially, and designing starlight suppression systems that perform at levels necessary for an imaging mission is now practical.
Molecular features of interest for atmospheric characterization are present from the UV to mid-IR, and which features are the best probes of atmospheric physics and chemistry will depend on what properties of planetary atmospheres turn out to be most common. For planets orbiting more Sun-like stars using direct imaging, the UV to near-IR wavelength range is the most accessible. This region is promising for identifying biosignature gases because it hosts multiple features of molecular oxygen (0.2, 0.69, 0.76, and 1.27 microns; see Table D.1 in Appendix D), as well as its photochemically produced by-product ozone (0.2-0.3 and 0.5-0.7 microns) and collisionally induced O2 absorption in atmospheric pressures higher than Earth’s (0.3-1.27 microns). In the past 5 years, astrobiologists have developed and improved the understanding of how to more credibly interpret potential biosignatures in the context of their planetary and stellar environments, and O2 is the best studied example for this paradigm. The assessment of an O2 detection as a biosignature will be strengthened by searching for observational discriminants that can systematically rule out abiotic mechanisms that could also form it and searching for other environmental characteristics, such as the simultaneous presence of CH4 or N2O that could make the biological interpretation more credible. Features of one such disequilibrium partner—namely, CH4—are present at 0.79 microns and longer, making them potentially accessible to an optical mission depending on atmospheric concentrations (although the strongest methane features are in the near and mid IR). False positive complements to molecular oxygen, such as carbon dioxide, carbon monoxide, and absorption from oxygen collisions, which in large abundances may suggest oxygen production through photolysis or photochemistry, also have features at wavelengths shorter than 1.8 microns (see Appendix D).
The committee acknowledges that this focus on molecular oxygen may well be an Earth-centric view of biosignatures and the properties of life on other worlds, but as researchers embark on this journey of exploration, the committee considers oxygen, with its disequilibrium complements methane (possible to detect concurrently given some mission architectures) and nitrous oxide (available only with follow-up observations at wavelengths longer than 2.1 microns), a compelling place to start.
When evaluating the science potential of proposed optical imagers, several capabilities must be considered.
Aperture
The primary benefit of a larger aperture telescope is that, in general, it allows characterization of planets orbiting stars at larger distances from the Sun, increasing the sample of accessible objects. How many objects are needed for transformative science?
For Neptune- and Jupiter-size planets, several atmospheres have already been characterized and a statistical sample is needed. Hot Jupiter atmospheric characterization shows that, even in the case of these hydrogen- and helium-dominated planets, a sample of 10 objects contains substantial spectral diversity but this is not enough objects to derive statistically significant trends that explain their physical and chemical differences. Of order 50 objects or more would be needed to improve this understanding. If multiple planets are imaged in the same system, however, a smaller sample of giants would become interesting. Currently, only the directly imaged system HR 8799 contains multiple planets for which concurrent atmospheric characterization is possible. A sample of more than several systems with multiple characterized planets would enable tests of planet formation by allowing comparison of atmospheric compositions at a range of orbital distances.
For super-Earth-size planets, less information is currently available than for giants, and a sample of at least 10 would be illuminating. A substantially better physical understanding would likely result from a larger sample containing several objects both smaller and larger than the radius valley at about 1.8 times the radius of Earth which has been measured for short-period planets, as well as objects on orbits with periods greater than 25 days, which are less likely to be affected by atmospheric evaporation than their short-period brethren. Given current estimates of the occurrence rates of hot, warm, and cold super-Earths, Neptunes, and gas giants, both the Habitable Exoplanet Imaging Mission (HabEx) and Large UV/Optical/IR Surveyor (LUVOIR) will be able to detect and characterize several hundred such planets, thus yielding a statistically significant sample of planets with which to perform, for the first time, comparative exoplanetology over a broad range of planet masses and temperatures.
The committee concludes that the characterization of the atmospheres of terrestrial planets in the habitable zones of Sun-like stars is the most compelling opportunity in the field of exoplanets. The current knowledge of terrestrial planet occurrence rates indicates that targets for such characterization are within reach of technologies under study by HabEx and LUVOIR Science and Technology Definition Teams (STDTs). However, the modest uncertainties in the occurrence rates and the risk of small number fluctuations requires a conservative approach to designing a mission that can guarantee at least a few terrestrial planet atmospheres. That is, the minimum expected yield for a mission should be on order of 10 terrestrial planet atmospheres to be certain that at least one, and very likely at least a few, will be observed. The committee finds that the HabEx “A” mission concept, as it is scoped at the time of this writing (4 m primary mirror diameter), meets this threshold criterion. Measurements of the atmospheres of a few terrestrial planets would give a first glimpse into the diversity of these worlds and the committee would consider this to be a major advance.
At the same time, the committee has every expectation that the characteristics of a few planets will not be representative of the full class of objects, given the already known diversity of planets in the Solar System and beyond. A mission capable of characterizing a statistical sample of approximately 50 terrestrial planet atmospheres would allow for a thorough exploration of the types of terrestrial atmospheres that exist, enabling dramatically more science. This is particularly critical for the search for evidence of life through biosignatures, where comparative planetology will likely be essential for interpreting detections. The committee finds that the LUVOIR “A” mission concept, as it is scoped at the time of this writing (15 m primary mirror diameter), would enable this ambitious objective.
Wavelength Range
Although oxygen alone is not a definitive biosignature, the committee finds that detection of molecular oxygen in a terrestrial planet’s atmosphere would be a monumental discovery, and it would justify significant further observation of the planet’s environment to assess the likelihood that the O2 was biological in origin. Simultaneous detection of molecular oxygen and methane, a disequilibrium pair, would be yet more exciting since this would be a more direct suggestion of atmospheric alteration by life. Detectability of oxygen or methane by a proposed
mission will necessarily depend on atmospheric abundances. Earth’s complement of oxygen and methane have, for example, varied substantially over time, making exoplanet expectations difficult to impossible to define. Similar atmospheric assumptions should be made when comparing mission architectures.
For the purposes of exoplanetology, a number of molecular features have the potential to be important. In particular, the waveband 0.2-1.8 microns hosts features of O2, O3, O4, CH4, CO2, CO, N2O, H2, SO2, H2O, and features from aerosols such as H2O, H2SO4, and hydrocarbons, and could support surface liquid water detection via phase-dependent mapping to search for ocean glint. The waveband 5-20 microns hosts features of O3, CO2, CH4, N2O, SO2, and H2O, and could reveal surface or brightness temperatures. See Appendix D for additional information, including wavelength details, and the role of each molecule as either a biosignature, false positive discriminant, or habitability indicator.
Spectral Resolution
Exoplanet atmosphere studies have demonstrated that low-resolution spectra (R = 100) can be interpreted with substantially more confidence than photometric data. Photometric points can be quite valuable for providing baseline information about atmospheric structures, but the committee advises that only resolved wavelength ranges be included when evaluating the potential for missions to identify molecular features.
Field of View
Although a telescope capable of imaging Earths can necessarily image larger planets as well, comparative planetology of a range of objects requires that an imager be sensitive to a range of orbital separations, not solely the habitable zone. Large-separation planets at fixed size and albedo are fainter than their close-in counterparts. A field of view large enough to encompass the faintest detectable giant planets for many systems would maximize the planetary yield of an imaging mission.
THE JOURNEY AHEAD
Finding, verifying, and exploring the characteristics of life on other worlds will be a many-step process. The committee recommends balancing a measured, step-by-step strategy that will allow the building of a nuanced understanding of planetary systems with bold steps targeted toward the current fledgling understanding of planetary habitability. Optical imaging of habitable-zone terrestrial planets would advance both of these objectives. Planning further major steps in the search for life could be precipitous in this journey of exploration. Researchers do not yet know how strange these newly discovered worlds will be, nor can they predict the diversity of detectable extrasolar life. If the next generation of space telescopes detects a signature of molecular oxygen on a habitable-zone terrestrial planet orbiting another star, the race will be on to characterize that planet and its system in as much detail as possible. What is the planet’s orbit? What other planets are in the system and what are their properties? What other molecules can be detected in the planet’s atmosphere across the widest possible wavelength range? Are disequilibrium gases such as methane also present? In other words, what is the context for that detection of oxygen, and can it be explained only by life?
The committee’s Exoplanet Science Strategy affirms that the answer to one of humanity’s greatest questions is within reach. For generations, humans have looked up at the stars and wondered whether or not we are alone. We do not know whether our generation will be the first to learn that life exists elsewhere in the galaxy. What we do know is that we can be the first with the technology, the scientific ability, and the sheer unrelenting drive to take the bold steps toward answering that great question. This may be a long journey, but if we choose to embark upon it, we will ultimately find our place in the Cosmos.
REFERENCE
NRC (National Research Council). 2010. New Worlds, New Horizons in Astronomy and Astrophysics. The National Academies Press, Washington, D.C.
