A new study, published March 23 in the Proceedings of the National Academy of Sciences, suggests that Jupiter’s inward-outward migration early in the Solar System’s history could have destroyed a first generation of giant terrestrial planets, called super-Earths, and set the stage for the formation of Earth, Mars and other planets that the inner Solar System has today.
This image shows a super-Earth exoplanet and its moons. Image credit: Luciano Mendez / CC BY-SA 3.0.
Thanks to recent surveys of extrasolar planets scientists know that about half of Sun-like stars in our galactic neighborhood have orbiting planets.
Yet those systems look nothing like our own. In our Solar System, very little lies within Mercury’s orbit; there is only a little debris, but certainly no planets.
That is in sharp contrast with what astronomers see in most planetary systems. These systems typically have one or more super-Earths orbiting closer to their parent stars than Mercury does, but very few objects at distances beyond.
“Indeed, it appears that the Solar System today is not the common representative of the galactic planetary census. Instead we are something of an outlier,” said Dr Konstantin Batygin of California Institute of Technology in Pasadena, the lead author on the study.
Jupiter is critical to understanding how the Solar System came to be the way it is today.
The model developed by Dr Batygin and his colleague, Dr Gregory Laughlin of the University of California, Santa Cruz, incorporates something known as the Grand Tack scenario, which was first posed in 2001 by British astronomers.
That scenario says that during the first few million years of the Solar System’s lifetime, when planetary bodies were still embedded in a disk of gas and dust around a relatively young Sun, Jupiter became so massive and gravitationally influential that it was able to clear a gap in the disk. And as the Sun pulled the disk’s gas in toward itself, Jupiter also began drifting inward, as though carried on a giant conveyor belt.
In an earlier model, the terrestrial planets conveniently end up in their current orbits with their current masses under a particular set of circumstances – one in which all of the inner Solar System’s planetary building blocks (planetesimals) happen to populate a narrow ring stretching from 0.7 to 1 AU approximately 10 million years after the Sun’s formation.
According to the Grand Tack scenario, the outer edge of that ring would have been delineated by Jupiter as it moved toward the Sun on its conveyor belt and cleared a gap in the disk all the way to Earth’s current orbit.
“But what about the inner edge? Why should the planetesimals be limited to the ring on the inside? That point had not been addressed,” Dr Batygin noted.
“The answer could lie in primordial super-Earths. The empty hole of the inner Solar System corresponds almost exactly to the orbital neighborhood where super-Earths are typically found around other stars.”
It is therefore reasonable to speculate that this region was cleared out in the primordial Solar System by a group of first-generation planets that did not survive.
The new study shows that as Jupiter moved inward, it pulled all the planetesimals it encountered along the way into orbital resonances and carried them toward the Sun. But as those planetesimals got closer to the Sun, their orbits also became elliptical.
“You cannot reduce the size of your orbit without paying a price, and that turns out to be increased ellipticity. Those new, more elongated orbits caused the planetesimals, mostly on the order of 100 km in radius, to sweep through previously unpenetrated regions of the disk, setting off a cascade of collisions among the debris,” Dr Batygin said.
In fact, the study show that during this period, every planetesimal would have collided with another object at least once every 200 years, violently breaking them apart and sending them decaying into the Sun at an increased rate.
The researchers did one final simulation to see what would happen to a population of super-Earths in the inner Solar System if they were around when this cascade of collisions started.
They ran the simulation on a well-known extrasolar system known as Kepler-11, which features six super-Earths with a combined mass 40 times that of Earth.
“The model predicts that the super-Earths would be shepherded into the Sun by a decaying avalanche of planetesimals over a period of 20,000 years. It’s a very effective physical process. You only need a few Earth masses worth of material to drive tens of Earth masses worth of planets into the Sun,” Dr Batygin explained.
“When Jupiter tacked around, some fraction of the planetesimals it was carrying with it would have calmed back down into circular orbits. Only about 10 percent of the material Jupiter swept up would need to be left behind to account for the mass that now makes up Mercury, Venus, Earth, and Mars.”
“From that point, it would take millions of years for those planetesimals to clump together and eventually form the terrestrial planets – a scenario that fits nicely with measurements that suggest that Earth formed 100-200 million years after the birth of the Sun.”
“Since the primordial disk of hydrogen and helium gas would have been long gone by that time, this could also explain why Earth lacks a hydrogen atmosphere. We formed from this volatile-depleted debris,” he said.
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Konstantin Batygin & Greg Laughlin. Jupiter’s decisive role in the inner Solar System’s early evolution. PNAS, published online March 23, 2015; doi: 10.1073/pnas.1423252112
