Jupiter's Grand Attack on the Inner Solar System

Batygin, who first worked with Laughlin as an undergraduate at UC Santa Cruz and is now an assistant professor of planetary science at the California Institute of Technology, performed numerical calculations to see what would happen if a set of rocky planets with close-in orbits had formed prior to Jupiter’s inward migration.

At that time, it’s plausible that rocky planets with deep atmospheres would have been forming close to the sun from a dense disk of gas and dust, on their way to becoming typical “super-Earths” like so many of the exoplanets astronomers have found around other stars. As Jupiter moved inward, however, gravitational perturbations from the giant planet would have swept the inner planets (and smaller planetesimals and asteroids) into close-knit, overlapping orbits, setting off a series of collisions that smashed all the nascent planets into pieces.

“It’s the same thing we worry about if satellites were to be destroyed in low-Earth orbit. Their fragments would start smashing into other satellites and you’d risk a chain reaction of collisions. Our work indicates that Jupiter would have created just such a collisional cascade in the inner solar system,” Laughlin said.

The resulting debris would then have spiraled into the sun under the influence of a strong “headwind” from the dense gas still swirling around the sun. The ingoing avalanche would have destroyed any newly-formed super-Earths by driving them into the sun. A second generation of inner planets would have formed later from the depleted material that was left behind, consistent with evidence that our solar system’s inner planets are younger than the outer planets. The resulting inner planets–Mercury, Venus, Earth, and Mars–are also less massive and have much thinner atmospheres than would otherwise be expected, Laughlin said.

“One of the predictions of our theory is that truly Earth-like planets, with solid surfaces and modest atmospheric pressures, are rare,” he said.

Planet hunters have detected well over a thousand exoplanets orbiting stars in our galaxy, including nearly 500 systems with multiple planets. What has emerged from these observations as the “typical” planetary system is one consisting of a few planets with masses several times larger than the Earth’s (called super-Earths) orbiting much closer to their host star than Mercury is to the sun. In systems with giant planets similar to Jupiter, they also tend to be much closer to their host stars than the giant planets in our solar system. The rocky inner planets of our solar system, with relatively low masses and thin atmospheres, may turn out to be fairly anomalous.

According to Laughlin, the formation of giant planets like Jupiter is somewhat rare, but when it occurs the giant planet usually migrates inward and ends up at an orbital distance similar to Earth’s. Only the formation of Saturn in our own solar system pulled Jupiter back out and allowed Mercury, Venus, Earth, and Mars to form. Therefore, another prediction of the paper is that systems with giant planets at orbital periods of more than about 100 days would be unlikely to host multiple close-in planets, Laughlin said.

“This kind of theory, where first this happened and then that happened, is almost always wrong, so I was initially skeptical,” he said. “But it actually involves generic processes that have been extensively studied by other researchers. There is a lot of evidence that supports the idea of Jupiter’s inward and then outward migration. Our work looks at the consequences of that. Jupiter’s ‘Grand Tack’ may well have been a ‘Grand Attack’ on the original inner solar system.”

 

In a 2010 study, scientists say baby Jupiter was so strongly bashed by giant Earths that it lost part of its core. That would explain why Jupiter’s core is disproportionately smaller today than sibling planet Saturn, says astronomer Douglas Lin, with the University of California in Santa Cruz.  

 

Scientists suspect that gas giant planets like Jupiter and Saturn started off with rocky and/or icy bodies about 10 times the mass of Earth. At that point, their gravitational muscles would be strong enough to begin pulling gas from the surrounding region, causing them to balloon in size over time. 

 

“In principle, all gas giants need to get to a critical core mass before they can start to accrete gas,” Lin told Discovery News. “So the question is: Why is Jupiter’s core mass so much smaller than Saturn’s even though its total mass is so much bigger?” 

 

Working with computer models, astronomers Shu Lin Li of Peking University in China, Craig Agnor with Queen Mary University of London, and Lin came up with one possible answer: Perhaps Jupiter started off like Saturn, but lost part of its core as it evolved. The scientists calculated that if an object roughly five times the mass of Earth hit Jupiter, the impact would strike all the way to the planet’s core. Add another three or four crashes and Jupiter’s core would be eroded to roughly what it is today. 

 

“Jupiter has always had this problem because if we believe the numbers… the core seems to be too small — so small that it cannot accrete gas efficiently in the solar nebula. This scenario gets around the problem,” Lin said. 

 

Contacts and sources:

Tim Stephens

University of California in Santa Cruz