In the early years of the solar system, giant planets still in formation dodged around each other, did a do-si-do, and then swept one of their companions out of the sun’s gravitational grasp. Things settled down and our planetary system was in its final configuration.
It is unknown what triggered that planetary deck. Now, computer simulations suggest that hot radiation from the young sun evaporating its disk of planet-forming gas and dust led to scrambling of the orbits of the giant planets, researchers report in the April 28 Nature.
As a result, the four largest planets may have been in their final configuration within 10 million years of the birth of the solar system about 4.6 billion years ago. That’s much faster than the 500 million years that earlier work had suggested.
The planetary shuffling mechanism the team discovered in computer simulations is highly innovative, says Nelson Ndugu, an astrophysicist who studies planetary system formation at North-West University in Potchefstroom, South Africa, and Muni University in Arua, Uganda. . “It has enormous potential.”
Piles of evidence, including observations formation of extrasolar planetary systems (Serial number: 2/7/18), had already indicated that something in the early history of our solar system confounded the orbits of the giant planets, which scientists call the instability of giant planets (Serial number: 05/25/05).
“The evidence for the instability of giant planets is really strong,” says Seth Jacobson, a planetary scientist at Michigan State University in East Lansing. “It explains many features of the outer solar system,” he says, such as the large number of rocky objects beyond Neptune that form the Kuiper Belt (Serial number: 12/31/09).
To find out what triggered that 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 disk of planet-forming gas and dust surrounding the star. The team then tweaked parameters of the disk, such as its mass, density, and how fast it evolved.
The simulations also included the giant planets in formation – five of them, in fact. astronomers think of a third ice giantbesides Uranus and Neptune, it was originally a member of the solar system (Serial number: 04/20/12). Jupiter and Saturn round out the final count of these massive planets.
When the sun officially became a star, that is, at the moment it began to burn hydrogen in its core, approximately 4.6 billion years ago, its ultraviolet emission would have hit the gas in the disk, ionizing it and heating it to tens of thousands of degrees. “This is a very well-documented process,” says Jacobson. As the gas heats up, it expands and moves away from the star, starting from 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 on the new research.
In the team’s simulations, as the inner part of the disk dissolves, that area loses mass, so embedded planets that are still forming feel less gravity in that region, Jacobson says. But the planets still feel the same amount of pull from the outer region of the disk. This gravitational twist, as the team calls it, can trigger a rebound effect: “Originally, the planets migrated and reached the [inner] edge of this disk, and they reverse their migration,” says Liu.
Due to Jupiter’s large mass, it is mostly unaffected. However, Saturn is moving outward and into the region that, in the simulations, contains the three ice giant planets. That area becomes crowded, Liu says, and close planetary interactions follow. An ice giant is ejected from the solar system entirely, Uranus and Neptune drift a little further from the sun and “gradually form orbits close to our solar system configuration,” Liu says.
In their computer simulations, the researchers found that as solar radiation evaporates the disk, planetary reshuffling almost always occurs. “We can’t avoid this instability,” says Jacobson.
Now that the researchers have an idea of what might have caused this reorganization of the solar system, the next step is to simulate how the evaporation of the disk might affect other objects.
“We’ve focused a lot on the giant planets, because their orbits were the original motivation,” says Jacobson. “But now, we need to do follow-up work to show how this triggering mechanism relates to small bodies.”