It is becoming increasingly obvious that our Solar System — with its inner collection of small rocky planets and an outer region buffeted by gas planets — is quite uncommon. According to a remarkable new study, the reason may have to do with Jupiter and an ancient migratory journey that kickstarted the destruction of our solar system's earliest planets.
Top image: An artist's depiction of Kepler 62f, a so-called super-Earth. According to a new theory, our sun may have once hosted planets like these. (Credit: NASA Ames/JPL-Caltech.)
Our Solar System is atypical in that lacks planets that orbit close to the Sun; aside from small bits of debris, there is nothing substantive within the orbit of Mercury.
The default mode of star systems, as it were, tends to involve the formation of planets featuring tight orbital periods (<100 days) and masses much greater than Earth's. So not only does our Solar System feature an uncharacteristic planetary configuration, it's also missing a hell of a lot of mass.
Orbital distribution of sub-Jovian extrasolar planets. Batygin & Laughlin, 2015/PNAS.
According to Caltech astronomer Konstantin Batygin and UC Santa Cruz astronomer Greg Laughlin, it's possible that our early Solar System did in fact contain this mass, possibly in the form of planetisimals — or maybe even as a set of super-Earths.
Regardless, something happened to them. And that something was Jupiter.
The Grand Tack Scenario
Soon after its formation, Jupiter is theorized to have migrated inward from a distance of 5 AU (where 1 AU = average distance of Earth from the Sun) to a distance of ~1.5 AU before reversing direction and settling into its current orbital position. This journey, which happened billions of years ago, set off of a chain of events that caused the first generation of inner planets to break up and fall into the Sun. What's more, this process set the stage for the formation of small-mass terrestrial planets, like Earth.
The reason for the sudden change in Jupiter's direction has to do with a process called resonant locking. This happens when the gravitational effects of two convergently migrating planets leads to a reversal in direction. In the case of our Solar System, the second object was — you guessed it — Saturn.
A depiction of a time early in the solar system's history when Jupiter made a grand inward migration. As it moved inward, Jupiter (white) picked up primitive planetary building blocks, or planetesimals, and drove them into eccentric orbits (turquoise) that overlapped the unperturbed part of the planetary disk (yellow), setting off a cascade of collisions that would have ushered any interior planets into the sun. (Caption and image credit: K.Batygin/Caltech/Kimm Fesenmaier.)
Back in 2001, researchers at Queen Mary University of London proposed a scenario in which a newly formed Jupiter cleared a gap in the protoplanetary disk. Then, as the Sun pulled the disk's gas inwards, Jupiter also began to drift inward. Astronomers liken it to a conveyor belt. As noted in a Caltech release:
"Jupiter would have continued on that belt, eventually being dumped onto the sun if not for Saturn," explains Batygin. Saturn formed after Jupiter but got pulled toward the sun at a faster rate, allowing it to catch up. Once the two massive planets got close enough, they locked into a special kind of relationship called an orbital resonance, where their orbital periods were rational—that is, expressible as a ratio of whole numbers. In a 2:1 orbital resonance, for example, Saturn would complete two orbits around the sun in the same amount of time that it took Jupiter to make a single orbit. In such a relationship, the two bodies would begin to exert a gravitational influence on one another.
But that's when all hell broke loose in the inner solar system.
According to Batygin and Laughlin's simulations, Jupiter would have thrown all the planetisimals it encountered into orbital resonances. As they got closer to the sun, their orbits would have gotten more elliptical. Traveling in these new and elongated orbits, these objects — some of them measuring as much as 100 km wide — swept through previously unperturbed regions of the disk. This facilitated a cascade of collisions among the debris. Calculations show that, during this phase, every planetisimal would have hit another object at least once every 200 years, shredding them apart and throwing them into the sun at an increasing rate.
Fascinatingly, the researchers performed another simulation involving a population of super-Earths in the inner solar system (six to be exact, a la Kepler-11). Their models show that these super-Earths, which collectively contained a combined mass 40 times that of Earth, would be shepherded into the sun by a swarming flurry of decaying planetismals over a period of — get this — a mere 20,000 years.
"It's a very effective physical process," noted Batygin in Caltech. "You only need a few Earth masses worth of material to drive tens of Earth masses worth of planets into the sun."
Room for (Habitable) Earth
As things calmed down, the remaining material eventually congealed to form the terrestrial planets we know today, namely Mercury, Venus, Earth, and Mars. Importantly, because much of the hydrogen and helium of the protoplanetary disk was gone by this point, this scenario helps to explain why the atmospheres of the second-gen planets are subsequently low in volatiles. It's probably the reason why Earth lacks a hydrogen atmosphere — and why we should expect super-Earths to be rich in hydrogen.
Also, this scenario suggests that Solar Systems with an architecture like ours may be extremely uncommon. Having two gas giants perform a migratory dance like the one experienced in the early stages of our Solar System's history is something that probably doesn't happen very often.
Taken together, these factors demonstrate the exceptional status of Earth and the intricate steps required for a star system to become potentially habitable.
Read the entire study at PNAS: "Jupiter's decisive role in the inner Solar System's early evolution."