Jupiter and Saturn form the basis of a “grand unified theory” of the solar system, according to new computer simulations. The research traces three seemingly unrelated phenomena – the giant planets’ orbits, craters on the Moon, and the behaviour of certain asteroids – to the motions of the two gas giants nearly four billion years ago.
Astronomers believe objects in the solar system condensed from a rotating disc of gas and dust about 4.6 billion years ago. Initially, these bodies are thought to have travelled along circular orbits in the plane of the disc. But some later deviated from these paths through gravitational interactions with their neighbours. Jupiter, Saturn, and Uranus, for example, travel in slightly eccentric orbits.
Now, an international team of researchers has performed computer simulations that reproduce the orbits of the four giant planets – Jupiter, Saturn, Uranus, and Neptune – in exquisite detail. The team has published a trio of papers about their findings in Nature.
In the model, the four planets form in 10 million years within the current orbit of Uranus. Surrounding them in a ring are several thousand rocky objects called planetesimals, left over from the formation of the planets.
Gravity pulls the two types of object towards each other, so planetesimals begin to “leak” into the giant planet zone and the orbits of the giant planets gradually change. After 700 million years, Saturn has migrated outward and Jupiter inward to the extent that they reach a “resonance” point. This means they begin to march in lockstep with each other, with Jupiter completing two orbits around the Sun for every one of Saturn’s. The resonance allows the pair to greatly disturb the orbits of the other planets.
“Hell breaks loose”
“Basically, everything sits around for 700 million years and then boom – all hell breaks loose,” says Hal Levison, a team member at the Southwest Research Institute in Boulder, Colorado, US. He says that in the model, Jupiter and Saturn hurl Uranus and Neptune outwards like bowling balls into a sea of planetesimals, which scatter like pins.
This scattering allows Neptune to double the size of its own orbit, bringing it approximately to the distance it is today. But the scattering also sends planetesimals falling towards the Sun, which may explain another persistent mystery, say the researchers.
They believe impacts from these scattered planetesimals can account for large, dark basins on the Moon. Samples of lunar rocks collected by astronauts had dated the impacts at about 650 million years after the formation of the solar system, but astronomers were not sure what caused the so-called “Late Heavy Bombardment” (LHB), which occurred millions of years after the Moon and planets had formed.
And the simulations show the scattering even explains a third phenomenon. Asteroids called Trojans both precede and trail after Jupiter’s orbit at angles of 60 degrees. Astronomers had suggested they became stuck there when the giant planet built up its own mass. But that and other models could not explain all of the orbital characteristics of the asteroids.
Now, the simulations reveal some scattered planetesimals can bounce off Saturn and rebound “in the right place on Jupiter to get trapped,” says Levison, who says the simulations replicated both the number and orbits of the asteroids. “I didn’t become a believer that we were right until we got the Trojans right,” he told New Scientist. “This is a comprehensive model which explains all of these things.”
But Joe Hahn, an astronomer at Saint Mary’s University in Halifax, Canada, who wrote an accompanying article about the new work, urges caution in interpreting the model. “The fact that a simulation of planet formation produces an end state in good agreement with the observed solar system does not prove that the simulated events actually happened,” he writes.
He says other scenarios, including objects the size of Earth that wandered through the early solar system, might explain the orbits of the giant planets. Such an object could have collided with Uranus, for example, explaining why that planet is tilted by 98 degrees to the plane in which it orbits.
Journal reference: Nature (vol 435, p 432, 459, 462, 466)