A new take on simulating planet formation

An artist's conception of OTS 44, a brown dwarf star surrounded by a swirling disc of the sort of dust thought to be the building blocks of planets. OTS 44, which was found using the Spitzer Space Telescope, is a fifth the size of our sun, and only 1.5 % the mass of our sun. Image courtesy of NASA/JPL-Caltech.

Our Solar System formed about 4.5 billion years ago, so it is a little late to watch Earth, Mars, and their fellow planets coalescing out of the cloud of gas and dust surrounding the young sun.

Nonetheless, David Minton, a Purdue planetary scientist, is taking a look at the process — repeatedly — using Purdue’s Hansen community cluster supercomputer. His simulations may help modify the standard view of how the planets formed, as well as the outlook on planet formation in other solar systems.

The standard view is that the planets formed roughly in the position where we find them today. Earth formed out of material that came from the vicinity of Earth’s orbit, Mars likewise, Jupiter, Saturn, and so on.

“What we’re proposing is a little different,” Minton said. “There was a substantial amount of migration of these bodies when they were growing early in the Solar System’s history.”

The hypothesis from Minton, Hal Levison of the Southwest Research Institute in Boulder, Colorado, and their colleagues may help explain an enduring mystery in the standard view of the planets’ formation. Why is Mars, with substantially larger siblings Jupiter and Earth on either side of it, such a relative runt?

“Mars is about a tenth the mass of the Earth,” Minton said. “Most models of planet formation suggest that Mars ought to be closer to the mass of the Earth. We think we’re getting everything else about right and yet this problem has persisted.”

Levison and Minton believe the answer might be that the planets didn’t form in place, not fully anyway. Rather, they migrated away from the sun after reaching an embryonic stage, collecting additional mass as they traveled like a snowball rolling down a hill. This migration was driven by gravitational interactions with trillions of other small objects, called “planetesimals,” condensed from the solar gas and dust cloud.

Jupiter, Saturn, Uranus, and Neptune rolled along until they settled near their current positions. In their wake, Mars, last among the early forming planets, also came to a halt. It was stuck with no opportunity to add to its mass in further travels and out of position to get in on a later conglomeration process, closer to the sun, which yielded Earth and Venus.

Meanwhile, many planetesimals were coaxed into a position where they seeded the Solar System’s asteroid belt.

Among other things, Minton’s modeling consistently indicates that a migration process of this sort can produce the four giant planets and puny Mars.

Minton employs gravitational N-body and Monte Carlo simulations and uses planetary science community software, such as Mercury and Swift, along with his own codes. The number of objects he builds into his models (tens of millions) and the time scales (a billion years or more, sometimes in time steps of days) make his simulations computationally demanding.

A simulation he did previously looking at 140,000 asteroids over a billion years took nearly 10 months to run. His plans include repeating that study because, with the Hansen community cluster, he now has the computing power necessary not only to get results faster, but also to add in the gravitational impact of the larger asteroids. Gravity’s influence from those bodies individually is limited, but it may have a significant effect cumulatively.

A version of this article first appeared on the Purdue RCAC website.