Wandering Jupiter could have swept inner solar system clean

A wandering baby Jupiter could help explain why there are no planets closer to the sun than Mercury and why the innermost planet is so tiny, a new study suggests.

Jupiter’s core might have formed close to the sun and then meandered through the rocky planet construction zone. As the infant Jupiter moved, it would have absorbed some planet-building material while kicking out the rest. This would have starved the inner planets — Mercury, Venus, Earth and Mars — of raw materials, keeping them small and preventing any other planets from forming close to the sun, say planetary scientist Sean Raymond and colleagues online March 5 in Monthly Notices of the Royal Astronomical Society.

“When I first came up with it, I thought it was ridiculous,” says Raymond, of the Laboratory of Astrophysics of Bordeaux in Floirac, France. “This model is kind of crazy, but it holds up.”

Rocky planets snuggled up to their suns are common in our galaxy. Many systems discovered by NASA’s Kepler space telescope have multiple planets — several larger than Earth — crammed into orbits smaller than Mercury’s. Though Kepler is biased toward finding scrunched-up solar systems, researchers wonder why there is a large gap between the sun and Mercury.

Scientists suspect that the inner planets of our solar system formed 4.6 billion years ago from a belt of debris that stretched between the current orbits of Venus and Earth. Mercury and Mars were built out of material along the edges of this belt, which explains why they are relatively small. Jupiter, traditionally thought to have formed much farther out, gets the blame for creating the belt’s outer edge. What shaped the inner edge has remained difficult to explain (SN Online: 3/23/15).

Raymond and colleagues ran computer simulations to see what would happen to the inner solar system if a body with three times the mass of Earth started inside Mercury’s orbit and then migrated away from the sun. They found that if the interloper didn’t move too fast or too slow, it would sweep clean the innermost parts of the disk of gas and dust that encircled the young sun and leave just enough material to form the rocky planets.

Raymond and colleagues also discovered that young Jupiter could have corralled enough debris to form a second core — one that got nudged away from the sun as Jupiter migrated.  This second core could be the seed from which Saturn grew, the researchers suggest. Jupiter’s gravity could have dragged debris to the asteroid belt, too. Raymond says that might explain the origin of iron meteorites, which some researchers argue should have formed relatively close to the sun.

Jupiter plowing through the inner solar system sounds plausible, says Sourav Chatterjee, an astrophysicist at Northwestern University in Evanston, Ill. “But there are several ways this can go wrong.”

Building a giant planet core inside the orbit of Mercury is not hard, he says. Pebbles and boulders in the nascent solar system probably drifted inward. They could have piled up close to the sun where solar magnetic fields created turbulence that trapped infalling material. If just a fraction of this debris stuck together, a rocky orb a few times as massive as Earth could form.

Having proto-Jupiter wander to the outer solar system, however, is asking a lot, says Chatterjee. Gravitational interactions with spiral waves in the disk that surrounded the sun can propel a newborn planet either inward or outward. But how fast, how far and in which direction the planet travels depends on properties such as disk temperature and density, which Raymond and colleagues readily acknowledge. Their simulations assume and simplify disk characteristics to see if building the solar system inside-out is even plausible.

“We’re building up a logical chain that shows [this idea] is not completely crazy,” Raymond says. “We’re not saying it happened. Just if it happened, what would it do?”