The solar system’s early days were chaotic. Our celestial neighborhood began as a rotating cloud of gas and dust. Some 4.6 billion years ago, the massive center of this cloud condensed into the sun. Leftover gas and dust, spurred by local gravitational attraction, coalesced to form planetoids. Over the course of the solar system’s first few million years, these planetary precursors wandered chaotic paths. Some merged into the first planets, while others were exiled to the solar system’s frigid outback. But, after 100 million years of wanderlust, this gravitational ballet concluded with eight clear winners: four rocky planets and four gas giants, separated by the Asteroid Belt, and interspersed with comets and dwarf planets.
This general model for the solar system’s formation rests on key assumptions about the geologic history of the planets over the past 4.6 billion years. Many of these assumptions are testable, thanks to humankind’s thirst for space exploration. In fact, this model has withstood the findings of countless missions to outer space. But, with chaos reigning in those first 100 million years, today’s solar system still sports peculiarities in need of explanation, and today’s astrophysicists are diving into the details.
Several of these puzzles involve our Moon. Earth is the only terrestrial planet to harbor a prominent satellite (Mars’s diminutive moons were purloined from the Asteroid Belt). Its large size (fifth largest out of over 170 known moons) and skewed orbit led to the theory that Earth and the Moon were once the same celestial body. By this widely-accepted theory, a Mars-sized body struck a primitive Earth, scattering debris into orbit that rapidly coalesced into the Moon. This theory explains the Moon’s unusual orbit, and the similar chemical composition of both bodies.
The impact-based theory of the Moon’s formation is decades old, and explains many puzzles of the Earth-Moon duet. But, recent studies of Moon mineral samples show that this theory needs tweaking. The Moon, like the Earth, harbors a good deal of water molecules in its interior, and those water molecules are ancient in origin. On a celestial body with such a violent and heated birth, they are also unexpected. These water molecules must have been present when the Moon was formed over 4.5 billion years ago, and isotope analyses of these water molecules indicate that they share the same chemical signature as water molecules from Earth’s interior. Thus, the Moon’s water came from ancient Earth and somehow survived the hypothesized heat and carnage of the Moon’s creation. This may indicate that significant portions of the future Moon remained relatively cool and inert as they were dislodged from the primordial Earth.
These new findings are not the only chemical or geological mysteries that have prompted modifications to the model of our solar system’s formation. Mars is smaller than it should be. The Asteroid Belt harbors many rocks from the outer solar system. Mercury’s surface harbors ice. Those pesky water molecules within Earth and the Moon are also chemically identical to water molecules from carbonaceous chondrites, an ancient class of meteorites. But all of these mysteries are roughly consistent with a new and radical proposal of planetary movement early in the solar system’s history. Known as the Great Tack, this early planetary dance centers on an unconventional celestial do-si-do: the intrusion of Jupiter into the inner solar system. The Great Tack proposes that, when the inner planets were forming, Jupiter drew as close to the sun as Mars’s current orbit. This migration scattered debris and planetoids, depriving Mars of solid material that could have added to its mass. Jupiter also brought frozen asteroids and planetoids, accounting for the outer solar system-derived members of today’s Asteroid Belt and the water-bearing carbonaceous chondrites incorporated into Earth’s interior (and later, the Moon).
The Great Tack theory postulates that, a few million years after its intrusion, Jupiter retreated to the outer solar system. This hypothesis accounts for many peculiarities that aren’t easily explained by the prevailing model for solar system formation. It is also consistent with Jupiter’s pattern of interference in the solar system (another theory charges that Jupiter and Saturn pushed Uranus and Neptune to the outer reaches of the solar system). Some astrophysicists believe that the Great Tack may be a typical step in solar system formation, as evident by the many young stars with young, massive “hot Jupiters” in close orbit.
If Jupiter did undergo a Great Tack, astronomers cannot yet explain why the massive gas giant later reversed course and left the inner solar system. Whatever the reason, Earth would not be in its present, life-bearing disposition had Jupiter stayed. Future missions must collect evidence to test the Great Tack theory definitively. But for now, this proposal adds new weight to the old term planet, which (from ancient Greek planētēs) means “wanderer.”