In the early years of the solar system, the still-forming giant planets dodged, did a do-si-do, and then pulled one of their partners away from the sun’s gravitational grip. Things worked out, and our planetary system was in its final configuration.
What triggered this planetary reshuffle is unknown. Now, computer simulations suggest that hot radiation from the young sun evaporating its disc of planet-forming gas and dust has led to the orbits of giant planets being scrambled, researchers report in the April 28 Nature.
As a result, the four largest planets could have been in their final configuration within 10 million years of the birth of the solar system around 4.6 billion years ago. This is much faster than the 500 million years suggested by previous work.
The planetary churning mechanism the team discovered in computer simulations is highly innovative, says Nelson Ndugu, an astrophysicist who studies the formation of planetary systems at North West University in Potchefstroom, South Africa, and Muni University in Arua, Uganda. “He has enormous potential.”
Plenty of evidence, including sightings of extrasolar planetary systems forming (SN: 02/07/18), had previously indicated that something in the early history of our solar system was confusing the orbits of the giant planets, what scientists call giant planetary instability (SN: 05/25/05).
“The evidence for giant planet instability is really strong,” says Seth Jacobson, a planetary scientist at Michigan State University in East Lansing. “This explains many features of the outer solar system,” he says, such as the large number of rocky objects beyond Neptune that make up the Kuiper Belt (SN: 12/31/09).
To figure out what triggered this instability, Jacobson and his colleagues ran computer simulations of the thousands of ways the early solar system could have developed. It all started with a young star and a disc of gas and dust forming a planet surrounding the star. The team then modified the disc’s parameters, such as its mass, density and rate of evolution.
The simulations also included the giant planets still forming – five of them, in fact. Astronomers believe that a third ice giant, in addition to Uranus and Neptune, was originally a member of the solar system (SN: 04/20/12). Jupiter and Saturn complete the final tally of these massive planets.
When the sun officially became a star, that is to say when it began to burn hydrogen at its core – around 4.6 billion years ago – its ultraviolet emission would have affected the disc gas, ionizing it and heating it to tens of thousands of degrees. “It’s a very well-documented process,” says Jacobson. As the gas heats up, it expands and moves away from the star, starting at the inner part of the disk.
“The disk disperses its gas from the inside out,” says Beibei Liu, an astrophysicist at Zhejiang University in Hangzhou, China. He and Jacobson collaborated with astronomer Sean Raymond of the Laboratoire d’Astrophysique de Bordeaux in France in the new research.
In the team’s simulations, as the inner part of the disk dissolves, this area loses mass, so buried and still-forming planets feel less gravity from this region, Jacobson says. But the planets still feel the same amount of pull from the outer disc region. This gravitational torque, as the team calls it, can trigger a rebound effect: “Originally, the planets migrate inward, and they reach the [inner] edge of that disk, and they reverse their migration,” Liu explains.
Due to the large mass of Jupiter, it is practically unaffected. Saturn, however, is moving outward and into the region which, in the simulations, contains the three ice giant planets. This area becomes overpopulated, Liu says, and close planetary interactions ensue. An ice giant is entirely expelled from the solar system, Uranus and Neptune move a little further from the sun and “they gradually form orbits close to the configuration of our solar system”, explains Liu.
In their computer simulations, the researchers found that when solar radiation evaporates the disk, planetary shuffling almost always follows. “We can’t avoid this instability,” says Jacobson.
Now that the researchers have an idea of what may have caused this reshuffling of the solar system, the next step is to simulate how evaporation from the disk could affect other objects.
“We really focused on the giant planets, because their orbits were the initial motivation,” says Jacobson. “But now we need to do the follow-up work to show how this trigger mechanism relates to small bodies.”