The current generation of planetologists has discovered more about the Solar System than in all of prior recorded history. Like earlier adventurers of Renaissance times, we’re now living in another golden age of exploration—in this case, exploration of alien worlds and unearthly environments quite foreign to our own. Much like the pathfinders who ventured forth in the great sailing ships to map the New World on planet Earth several centuries ago, today’s scientists vicariously crew robotic probes that trek around the Solar System. The effort is still a work-in-progress, but the results thus far have revolutionized our knowledge of both our present cosmic neighborhood and our planet’s natural history.
Our Solar System harbors a vast array of material objects beyond the central Sun. Planets, moons, comets, and asteroids are all well-known, if not completely understood, inhabitants of our minuscule niche in the suburbs of the Milky Way. The wide range of physical and chemical properties among the peculiar planets and their motley moons can give the impression that our Solar System is full of debris, or at least great diversity—the remains of a more violent, yet simultaneously more formative, era in the history of our local interplanetary environment. Can we realistically expect to identify all the pieces of this celestial puzzle and thereby decipher the full mosaic of our planetary origins? The answer, we think, is yes.
Each planet contributes knowledge that widens our appreciation for planetary evolution, much as diverse stars add to our understanding of the life cycle of stars. Most planets and moons are now in different stages of development, much as red giants and white dwarfs represent distinct phases of stellar evolution. The Jovian Planets are galactic fragments frozen in time, not massive enough to have become stars, yet too massive to have condensed rocky surfaces. To varying degrees, these gassy worlds, having originated mainly via gravitational instabilities, preserve the pristine properties of the early Solar System. By contrast, the less massive Terrestrial Planets, formed mostly via accretion, have evolved a great deal, cooling and crystallizing hard rocky surfaces while outgassing atmospheres and sometimes oceans. At least one of these small planets has spawned life.
Regard the Solar System, then, as not just a collection of planetary refuse. Every planet and its family of moons have something to tell us, something about their origin and evolution. Each time a new space probe reconnoiters a planet—and more robots are on their way right now—we glean a little more about bizarre landscapes, alien atmospheres, and comparative planetology. We learn how each planet fits into the overall architecture and general history of the whole Solar System, thereby helping us frame a planetary heritage of which humankind can be proud.
Planet Earth, in particular, is large enough to have remained warm inside and thus continues to experience some surface activity. Yet its outside has cooled enough to allow gravity to bind air and water to its surface. Earth thereby still evolves, though slowly and subtly. Our distance from the Sun and our atmospheric blanket combine to keep Earth’s surface temperature suitable for water to remain liquefied—an apparently vital factor in any environment hospitable to life as we know it. To be sure, many of the key attributes of our planet—and they are key, if not special or unique, such as tectonic action, liquid water, free oxygen, and life itself—depend on a planet’s size, location, and timing in its planetary system. Without any anthropocentrism meant or implied, Earth seems to be the right body at the right place at the right time in cosmic history for intelligent life to decipher the grand narrative of cosmic evolution.
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