Key Questions

Origin of the Solar System

Over the next decade, the fields of planetary science and astrobiology will be driven by key questions that can be categorized into four broad themes: the origins of the solar system, understanding the system’s planets and other bodies, life and habitability, and planetary systems elsewhere in the galaxy. The decadal survey report is organized around 12 key questions; the first three of which (see Table 3) are related to the origin of the solar system.

In the beginning was a rotating molecular cloud composed of gas and dust that slowly, under the influence of the gravitational attraction among its components, collapsed to form the protosun. The leftover materials self-organized into a disk-shaped structure—the protoplanetary disk (Figure 18; also see Figure 22)—which eventually evolved into the system of planets, moons, and other bodies that we observe today. But many questions remain

FIGURE 18 (Above) An artist’s impression of the disk of gas and dust surrounding a newly formed star.

about the processes that were involved in that evolution. How, for instance, did tiny bits of solid matter gradually accrete, forming larger and larger pieces, eventually creating large rocky and icy planetesimals, which would become the building blocks of the planets? Answering such questions will require a combination of theory, observations, laboratory studies, and spacecraft missions.

The outer part of the solar system, from Jupiter to the Kuiper belt and beyond, holds the key to understanding the formation and early evolution of the entire system. Researchers are still trying to perfect models that explain the formation of such gas giant planets as Jupiter and Saturn along with the rings and moons that circle them. The somewhat smaller Uranus and Neptune are distinct from both the gas giants and the inner, rocky planets, and they pose different questions. Are they simply failed gas giants, for instance, or did they form in a different way?

The inner solar system contains the four rocky planets Mercury, Venus, Earth, and Mars, along with assorted moons, the dwarf planet Ceres, and multiple asteroids and other bodies found in the asteroid belt and elsewhere in the inner planet region. They appear to have been formed through gradual accretion combined with other processes such as collisional impacts, and one of the key questions motivating the planetary scientists who study this issue is how typical this outcome is with planetary systems in general. Is the structure of the inner part of the solar system an inevitable result of planetary formation processes, or does it reflect random events that might have played out very differently in an alternative reality? The answer will have implications for how common are Earth-like planets elsewhere in the universe.

TABLE 3 Key Questions Concerning the Solar System’s Origin

Worlds and Processes

After the solar system assumed approximately the configuration that we see today, it continued to change, in part due to continuing collisions among the bodies and in part due to the evolution of the planets’ surfaces and interior, atmospheres and climate, and magnetic fields (Figure 19). The committee devoted five of its key questions (see Table 4) to the theme of understanding processes shaping planetary bodies.

An important topic is the role played by various small remnant bodies—rocky, carbonaceous, and icy—as they dynamically evolved and bombarded the planets, moons, and other bodies. It is thought, for instance, that they influenced the migration over time of the giant planets and played an important role in the transport of materials—for example, volatiles and organics—from where they formed to other parts of the solar system. Such transport processes may have conveyed the water that ultimately formed Earth’s oceans.

Internal forces drive much of the evolution of the solar system’s larger bodies. For example, Mercury, Venus, Earth, and Mars have interiors that have changed over time and helped shape the planets’ surface features. Knowing more about those internal structures and the roles

TABLE 4 Key Questions Concerning Worlds and Processes

they have played in forming the surface are important for understanding such things as why Earth and Venus have followed such different evolutionary paths.

Similarly, there is much still to be learned about the atmospheres of the solar system’s solid bodies—that is, the terrestrial planets, the dwarf planets (e.g., Pluto), and various of the larger moons (e.g., Titan). How, for instance, did the atmospheres of solid bodies form, and why did some of them end up with dense atmospheres while others did not? Answering these questions is critical to understanding the habitability of the various bodies, including understanding the processes that led to the emergence of life on early Earth.

The interiors, atmospheres, and magnetospheres of the giant planets are significantly different from those of the terrestrial planets, and understanding the processes that influenced their structure, evolution, and dynamics is a completely separate task.

A final issue is how the solar system’s various moons and ring systems were formed and how they interact with their host planets. For instance, the gravitation tidal forces on moons can heat up their interiors, sometimes leading to active volcanos and subsurface oceans.

Life and Habitability

The presence of life on every corner of Earth—from the hottest, driest deserts to the coldest parts of Antarctica and even to a few thousand meters below the sea floor—indicates just how adaptable life is. This in turn points to the key question driving much of astrobiological research: Given this adaptability, is there life beyond Earth and its environs? The committee devoted three of its key questions (see Table 5) to issues relating to extraterrestrial life.

Figuring out where and if life might exist elsewhere requires addressing three broad issues. The first is understanding as completely as possible what conditions and processes led to the emergence and evolution of life on Earth. Developing knowledge about how ancient prebiotic chemical pathways co-evolved with Earth’s early environment to give rise to life can provide insights into the conditions necessary for life to evolve elsewhere. In particular, recent discoveries have shown that life can exist in a much wider range of environments than previously imagined, using metabolic strategies and adaptations that have only recently been understood.

With this evolving knowledge about life on Earth and its development, the next step in the search for life elsewhere is to search the solar system for where conditions might have been suitable for life to have evolved—while keeping in mind that not all life may follow the patterns we see on Earth.

TABLE 5 Key Questions Concerning Life and Habitability

Determining whether a body is habitable—that is, capable of sustaining life, but not necessarily inhabited—requires understanding the factors controlling habitability. This requires recognizing that habitability is not a yes/no proposition but rather a continuum from more habitable to less habitable and then to not habitable, and also that a planetary environment can transition from habitable to not habitable, and vice versa, over space and time. Continuing research is needed to determine which bodies in the solar system are habitable—or may have been habitable at some point in the past—and to what degree.

Last, the search for life beyond Earth requires looking for evidence of past or present life elsewhere in the solar system. This requires careful consideration of exactly what constitutes “evidence of life” (Figure 20). Currently, there is a great deal of ongoing work on technologies that can be used to detect biosignatures. Outstanding questions include what sorts of signatures indicate life (or the absence of life) and how to avoid both false positives and false negatives.

Exoplanets

The past decade has seen an explosion of knowledge about planetary systems around other stars, with more than 5,000 exoplanets (Figure 21) having been discovered and new technologies making it possible not only to detect smaller and smaller planets but also to characterize these exoplanets and their atmospheres. The committee devoted its 12th and final question (see Table 6) to the relationship between studies of the solar system and extrasolar planetary systems.

Among the important discoveries over the past decade have been the realization that our galaxy holds more planets than stars, the recognition that planetary systems are extremely diverse, and the finding that at least some exoplanets may be of the right size and at the right distance from their stars to be potentially habitable by life as we know it.

Besides exoplanets and exoplanetary systems, astronomers have also observed protoplanetary disks around distant stars (Figure 22), some of which can be seen to be in the process of forming planets. Indeed, observations of such disks around young stars have begun to reveal the entire planet formation process, from the earliest accumulation of grains into large agglomerates to accretion onto growing protoplanets.

The current work on exoplanets has three overarching goals: (1) to understand how planetary systems form and evolve, making it possible to characterize and explain the diversity of these systems; (2) to learn enough about the properties of exoplanets to be able to identify potentially habitable environments; and (3) to determine which sorts of systems are likely to host such habitable environments.

An important feature of this work is that it is complementary to the research being done to understand the solar system. By learning more about how the solar system formed, we accumulate information and understanding that will help us interpret the observations we are able to make of distant planetary systems, both those that are in the process of forming and

TABLE 6 Key Question Concerning Exoplanets

those that have already formed. Conversely, by gathering information about other planetary systems in the galaxy, we can begin to see patterns that will provide insight into the formation and structure of the solar system.

The synergy created by examining various issues from the perspective of planetary bodies in the solar system and in exoplanetary systems should help, for instance, in answering the question of whether the inner solar system—and Earth—is a typical outcome of planetary system formation or an outlier compared with most systems in the universe. The goal being that the promise of answering such questions will lead to a new era of collaborative research, with scientists in both arenas—that is, those studying solar system bodies and exoplanets—working together on such things as mission design and implementation, telescope observations, data analysis, and laboratory and experimental research.