Is the Solar System a cosmic rarity or a galactic commonplace? How do Earth-like planets form, and what determines whether they are habitable? Is there life on other worlds? The answers to some of the greatest questions about humanity’s place in the cosmos are at hand. But what path should be taken to answer these questions?
The past decade has delivered remarkable discoveries in the study of exoplanets. Scientists have learned that most, if not all, stars host planets, and that small planets are ubiquitous. They have directly imaged young gas-giant exoplanets. They have probed the atmospheres of more than a hundred worlds and detected molecules and clouds. They have measured the rate of occurrence of terrestrial planets at distances from their stars where surface oceans might be possible. They have even identified a handful of such worlds transiting nearby small stars, calling out with each passing orbit to investigate if they too host life. Hand-in-hand with these advances, a theoretical understanding of the myriad of processes that dictate the formation and evolution of planets has matured, spurred on by the avalanche of unexpected discoveries. Appreciation of the factors that make a planet hospitable to life has grown in sophistication, as has understanding of the context for biosignatures, the remotely detectable aspects of a planet’s atmosphere or surface that reveal the presence of life.
There are two overarching goals in exoplanet science, as follows.
Goal 1 is to understand the formation and evolution of planetary systems as products of the process of star formation, and characterize and explain the diversity of planetary system architectures, planetary compositions, and planetary environments produced by these processes. This leads to three scientific findings that will guide an implementation strategy:
Finding: Current knowledge of the demographics and characteristics of planets and their systems is substantially incomplete. Advancing an understanding of the formation and evolution of planets requires two surveys: First, it requires a survey for planets where the census is most incomplete, which includes the parameter space occupied by most planets of the Solar System. Second, it requires the characterization of the atmospheres and bulk compositions of planets spanning a broad range of masses and orbits.
Finding: An understanding of planet formation requires a census of protoplanetary disks, young planets, and mature planetary systems across a wide range of planet-star separations.
Finding: Characterizing the masses, radii, and atmospheres of a large number of exoplanets with a range of physical and orbital parameters for a diverse set of parent stars will yield fundamentally new insights into the formation and evolution of planets and the physics and chemistry of planetary environments.
Goal 2 is to learn enough about the properties of exoplanets to identify potentially habitable environments and their frequency, and connect these environments to the planetary systems in which they reside. Furthermore, scientists need to distinguish between the signatures of life and those of nonbiological processes, and search for signatures of life on worlds orbiting other stars. This goal, in turn, leads to two guiding scientific findings:
Finding: The concept of the habitable zone has provided a first-order technique for identifying exoplanets that may be able to harbor life. A multiparameter holistic approach to studying exoplanet habitability, using both theory and observation, is ultimately required for target selection for biosignature searches.
Finding: Inferring the presence of life on an exoplanet from remote sensing of a biosignature will require a comprehensive framework for assessing biosignatures. 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.
The quest to characterize potentially habitable planets and search for atmospheric biosignatures presents two paths, both of which demand exploration. In the near term, temperate rocky planets orbiting the closest small stars (known as M dwarfs) can be studied with facilities under construction. Several such planets are known and more will be discovered by the recently launched Transiting Exoplanet Survey Satellite (TESS) mission. However, understanding the foreign environment surrounding an M dwarf requires a substantial extrapolation of scientists’ knowledge of habitability informed by the Solar System. Ultimately, the exoplanet community needs to develop the means to study potentially habitable planets orbiting more Sun-like stars. Developing this capability will require bold investments and a longer time scale to bear fruit, but along the way it will foster the development of the scientific community and the technological capacity to understand a myriad of worlds that currently elude us. The requirements to pursue an Earth-Sun analogue imply an imager in space:
Finding: A coronagraphic or starshade-based direct imaging mission is the only path currently identified to characterize Earth-size planets in the habitable zones of a large sample of nearby Sun-like stars in reflected light.
Finding: Recently acquired knowledge of the frequency of occurrence of small planets, and advances in the technologies needed to directly image them, have significantly reduced uncertainties associated with a large direct imaging mission.
Recommendation: The National Aeronautics and Space Administration (NASA) should lead a large strategic direct imaging mission capable of measuring the reflected-light spectra of temperate terrestrial planets orbiting Sun-like stars. (Chapter 4)
Ground-based astronomy will also play a pivotal role. The two U.S.-led giant segmented mirror telescopes, the Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT), will unveil an incredible discovery space in the study of planet formation, mature gas giants, and even terrestrial worlds:
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.
Finding: The technology roadmap to enable the full science potential of GMT and TMT in exoplanet studies is in need of investments, leveraging the existing network of U.S. centers and laboratories and current 8-10 meter class facilities.
Finding: GMT and TMT, equipped with high-resolution optical and infrared spectrographs, will be powerful tools for studying the atmospheres of transiting and nontransiting close-in planets, and have the potential to detect molecular oxygen in temperate terrestrial planets transiting the closest and smallest stars.
Finding: The detection of young planets in disks will provide the ground truth for the time scale of planet formation and permit studies of the dynamical interaction between disks and planets. With the high spatial resolution of the GMT and TMT, researchers will be able to search the inner parts of planet-forming systems.
Recommendation: The National Science Foundation (NSF) should invest in both the GMT and TMT and their exoplanet instrumentation to provide all-sky access to the U.S. community. (Chapter 4)
As found above, an essential input to inform an understanding of planet formation is a statistical census of the population of planets. While radial velocity surveys and transits, notably the revolutionary Kepler mission, have characterized the remarkable population of planets relatively close to their stars, knowledge of planets in the outer reaches of planetary systems is woefully incomplete. The 2010 Decadal Survey realized this, and hence it strongly recommended the Wide-Field Infrared Survey Telescope (WFIRST) mission.
Finding: A microlensing survey would complement the statistical surveys of exoplanets begun by transits and radial velocities by searching for planets with separations of greater than 1 AU (including free-floating planets) and planets with masses greater than that of Earth. A wide-field, near-infrared, space-based mission is needed to provide a similar sample size of planets as found by Kepler.
Finding: A number of activities, including precursor and concurrent observations using ground- and space-based facilities, would optimize the scientific yield of the WFIRST microlensing survey.
Through its coronagraphic instrument, WFIRST will also play an extremely valuable role in enabling a large direct imaging mission, both through retiring technical risk and by providing more sensitive constraints than are currently available for a potentially troublesome impediment for imaging missions—namely, exozodiacal dust.
Finding: Flying a capable coronagraph on WFIRST will provide significant risk reduction and technological advancement for future coronagraph missions. The greatest value compared to ground testing will come from observations and analysis of actual exoplanets, and in a flexible architecture that will allow testing of newly developed algorithms and methods.
Finding: The WFIRST-Coronagraph Instrument at current capabilities will carry out important measurements of extrasolar zodiacal dust around nearby stars at greater sensitivity than any other current or near-term facility.
Recommendation: NASA should launch WFIRST to conduct its microlensing survey of distant planets and to demonstrate the technique of coronagraphic spectroscopy on exoplanet targets. (Chapter 4)
Mass is the most fundamental property of a planet, and knowledge of a planet’s mass (along with a knowledge of its radius) is essential to understand its bulk composition and to interpret spectroscopic features in its atmosphere. If scientists seek to study Earth-like planets orbiting Sun-like stars, they need to push mass measurements to the sensitivity required for such worlds.
Finding: The radial velocity method will continue to provide essential mass, orbit, and census information to support both transiting and directly imaged exoplanet science for the foreseeable future.
Finding: Radial velocity measurements are currently limited by variations in the stellar photosphere, instrumental stability and calibration, and spectral contamination from telluric lines. Progress will require new instruments installed on large telescopes, substantial allocations of observing time, advanced statistical methods for data analysis informed by theoretical modeling, and collaboration between observers, instrument builders, stellar astrophysicists, heliophysicists, and statisticians.
Recommendation: NASA and NSF should establish a strategic initiative in extremely precise radial velocities to develop methods and facilities for measuring the masses of temperate terrestrial planets orbiting Sun-like stars. (Chapter 4)
For the first time, the James Webb Space Telescope (JWST) will bring exoplanet atmospheric characterization efforts from a regime of limited observations to one of high-fidelity spectroscopic investigations of a comparative sample. The entire exoplanet research community would benefit from a strategic and systematic survey of exoplanet atmospheres with JWST, which has the potential to guide future observing strategies for years, if not decades.
Finding: The combination of transiting planet detection with TESS, mass measurements with radial velocities, and atmospheric characterization with JWST will be transformative for understanding the nature and origins of close-in planets. Future space missions with broader wavelength coverage, a larger collecting area, or reduced instrumental noise compared to JWST would have greater reach to potentially habitable planets.
Recommendation: NASA should create a mechanism for community-driven legacy surveys of exoplanet atmospheres early in the JWST mission. (Chapter 4)
The identification of life on an exoplanet will not be accomplished by a single team of researchers, nor by a single method. It will happen only when researchers bring together the combined insights of astrophysicists, planetary scientists, Earth scientists, and heliophysicists, and provide them the opportunity and resources to collaborate.
Finding: The search for life outside the Solar System is a fundamentally interdisciplinary endeavor. The Nexus for Exoplanet Systems Science (NExSS) research coordination network encourages the cross-disciplinary and cross-divisional collaborations needed to support NASA exoplanet research and missions.
Recommendation: Building on the NExSS model, NASA should support a cross-divisional exoplanet research coordination network that includes additional membership opportunities via dedicated proposal calls for interdisciplinary research. (Chapter 4)
Present and future NASA missions promise a wealth of measurements that contain the answers to the two overarching goals of understanding planets and searching for life. But the scientific implications of these data will not be fully realized without a thriving and engaged community in related fields of theoretical, laboratory, and observational science.
Finding: Theoretical models are essential to plan and interpret observations of exoplanets, and are enabled by robust support via individual investigator grants.
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. Mechanisms to increase collaboration between exoplanet astronomers and experimental physicists and chemists would help overcome this barrier.
Finding: Understanding of exoplanets is limited by measurements of the properties of the parent stars, including stellar mass, radius, distance, binarity, rotation period, age, composition, emergent spectrum, and variability.
Recommendation: NASA should support a robust individual investigator program that includes grants for theoretical, laboratory, and ground-based telescopic investigations; otherwise, the full scientific yield of exoplanet missions will not be realized. (Chapter 4)
The search for life on other worlds is both a profound and a profoundly difficult endeavor, and the likelihood of success is maximized by marshaling, developing, and supporting all available talent. As a growing field, exoplanetary astronomy is particularly dependent on the effective development and retention of junior scientists because it is now putting into place the cohort of scientists who will be the senior leadership for many decades. Discrimination and harassment, as known to exist in the greater scientific workforce, likely affect the exoplanet community and serve as barriers to the participation of people from certain demographic groups. The Exoplanet Science Strategy therefore includes a strategy for developing and maintaining its human capital, including addressing its demographics and standards of professional conduct.
Finding: To maximize scientific potential and opportunities for excellence, institutions and organizations can enable full participation by a diverse workforce by taking concrete steps to eliminate discrimination and harassment and to proactively recruit and retain scientists from underrepresented groups.
Finding: Development and dissemination of concrete recommendations to improve equity and inclusion and combat discrimination and harassment would be valuable for building the creative, interdisciplinary teams needed to maximize progress in exoplanet science over the coming decades.
The great potential of exoplanet science demands large commitments. The work cannot be done by a single institution, but rather will engage federal partners and not-for-profit partners, and a consideration of international collaborations.
Finding: By continuing to find novel ways of partnering with each other, and by removing or reducing institutional barriers to such partnerships, agencies may be able to better address some of the most profound scientific questions outlined in this study, which often require instruments, telescopes, or missions that are too ambitious or expensive for any individual agency to fund, build, and operate alone.
For generations, humans have looked up at the stars and wondered whether we are alone. Wonder at this very question unites us. This Exoplanet Science Strategy describes how researchers can aim to address this question in a generation. It is unknown whether this generation will be the first to learn that life is common throughout the galaxy, or the first to discern hints of a cosmic lonesomeness. What we do know is that we can be the first with the technological and scientific ability to answer the question, if we so choose.