The study of extrasolar planets and planet formation in general has exploded within the astrophysics community. In a 2016 survey of U.S. members of the American Astronomical Society, 21 percent of respondents listed exoplanets as their primary field of interest, 23 percent listed solar systems and planetary science, and 9 percent listed astrobiology (Pold et al., 2016). This excitement reflects a field in which important discoveries are happening at a rapid rate, with the prospect of discoveries on the horizon that could fundamentally alter the view of humanity’s place in the universe (Figure 1.1). With appropriate investment, researchers are on the cusp of learning fundamental truths about the galactic distribution of planets and planetary systems in which Earth and the Solar System reside. Are systems like the Solar System rare? Are planets like Earth rare? Does life exist on planets other than Earth and on planets orbiting other stars?
These questions have captured the imagination of the general public. Moreover, they have attracted a young and diverse cohort of scientists to this new field. With the accretion of a new community in progress and with opportunities for paradigm-shifting discoveries on the horizon, organized input from the exoplanet community to the decadal survey process is essential. The U.S. Congress and the National Aeronautics and Space Administration have commissioned this report to provide such a perspective for the reference of the upcoming decadal survey committee. The aim of this report is to highlight strategic priorities for large, coordinated efforts that will support the scientific goals of the broad exoplanet science community. Along the way, this report will capture a subset of the vibrant, creative work currently under way in this diverse field.
Exoplanet science over the coming decades aims to achieve two overarching goals:
- 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.
- 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, researchers need to distinguish between the signatures of life and those of nonbiological processes, and search for signatures of life on worlds orbiting other stars.
The 2010 astronomy and astrophysics decadal survey, New Worlds, New Horizons (NRC, 2010), states that
The search for life around other stars is a multi-stage process. … First, the frequency with which Earth-size planets occur in zones around stars where liquids such as water are stable on planetary surfaces must be measured. Stars will then be targeted that are sufficiently close to Earth that the light of the companion planets can be separated from the glare of the parent star and studied in great detail; this will allow us to find signatures of molecules that indicate a potentially habitable environment. (pp. 37 and 39)
In the intervening decade since the publication of that report, thousands of planets and high-quality planet candidates have been found orbiting stars other than the Sun. As summarized in Chapter 2, these discoveries show that
- Exoplanets are ubiquitous.
- Extrasolar planetary systems are diverse.
- Small planets with compositions that may resemble Earth and stellar irradiation consistent with the presence of surface liquid water are common.
Targets for planet characterization are abundant. The time has come for the next steps.
Given these discoveries, strategic investments over the coming decade would likely enable major progress toward the above goals. Profound questions that can be addressed include the following:
- What physical processes primarily determine the diverse outcomes of planet formation and how common are systems like the Solar System?
- What interior, surface, and atmospheric compositions typically result from these processes? In other words, what are the properties of planets orbiting stars in the solar neighborhood?
- What planetary, stellar, and planetary system properties determine habitability, and how do planet formation processes shape the distribution of habitable environments in the galaxy?
- How common are potential biosignature gases and signatures of disequilibrium chemistry in the atmospheres of extrasolar planets? Are theoretical suggestions that these signatures will be signposts of life consistent with the observed behavior of planetary atmospheres?
In this report, the committee describes a strategic plan to answer these questions through a combination of large, ambitious community-supported efforts and support for diverse, creative, community-driven investigator research. Projects capable of characterizing a full range of planets, including rocky bodies with sizes comparable to Earth, as well as the systems in which they reside, are needed. This strategy will allow the United States to take the next steps in the quest to understand the galactic diversity of worlds—their origins, histories, and atmospheric properties.
This report is organized as follows: Chapter 2 provides an overview of the current status of exoplanet science. Chapter 3 presents the scientific strategic plan to advance the two overarching goals over the next decade. Chapter 4 provides details about the technical and organizational requirements for the future investments highlighted in Chapter 3. Chapter 5 highlights the need for coordination among various organizations to achieve the strategic plan outlined here. Chapter 6 provides a timeline for the implementation of the strategic plan, broken down into near-, medium-, and long-term activities. Appendix A provides the committee’s statement of task, Appendix B presents the call for white papers issued by the committee and lists all white papers submitted to the committee, Appendix C describes the exoplanet detection techniques in more detail, Appendix D provides a biosignature table, Appendix E presents short biographical sketches of the committee members and National Academies staff, Appendix F summarizes the acronyms used in this report and defines some quantities that might be unfamiliar to the reader, and Appendix G provides a glossary of selected terminology.
Andrews, S.M., D.J. Wilner, Z. Zhu, T. Birnstiel, J.M. Carpenter, L.M. Pérez, X.-N. Bai, K.I. Öberg, A.M. Hughes, A. Isella, and L. Ricci. 2016. Ringed substructure and a gap at 1 AU in the nearest protoplanetary disk. Astrophysical Journal 820(2):40.
Marois, C., B. Zuckerman, Q.M. Konopacky, B. Macintosh, and T. Barman. 2010. Images of a fourth planet orbiting HR 8799. Nature 468:1380.
NRC (National Research Council). 2010. New Worlds, New Horizons in Astronomy and Astrophysics. The National Academies Press, Washington, D.C.
Pold, J., R. Ivie, I. Momcheva, and the AAS Demographic Committee. 2016. Workforce Survey of 2016 US AAS Members Summary Results. American Astronomical Society. https://aas.org/files/aas_members_workforce_survey.pdf.