From a cloud of gas and debris to a system of ordered planets with regular orbits, much has changed since the solar system first began to coalesce about 4.5 billion years ago. Fortunately, the solar system is now full of clues to its past, and astronomers, with the help of computer models, are finding new ways to link together previously unconnected observations to explain how the planetary system came to resemble what it is today.
For many years, astronomers have known that the largest of the outermost planets Jupiter, Saturn, Uranus and Neptune orbit along a path that deviates from a near-perfect circle and do not share the same orbital plane. They also have known that several hundred asteroids either precede or follow Jupiter in its journey around the sun. And in a third and separate observation, astronomers have known that about 600 million years after Earth's moon formed, it experienced a period of heavy bombardment by space debris, as evidenced by its cratered surface. Only now have astronomers been able to connect these three mysterious observations through a series of cosmic chain reactions.
Reporting in a series of papers in the May 26 Nature, Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice, France, and colleagues say that they have created a model that confirms that a sudden shuffling of orbits among the giant planets, several hundred million years after their formation, may be responsible for the series of events. "This work is unique in the sense [that] we explain in a level of detail no one's ever seen before," Morbidelli says.
A dramatic increase in the rate of surface cratering occured 600 million years after the moon formed. A new hypothesis may explain why this event, among others, took place. Image courtesy of NASA.
As the giant planets tossed around the planetesimals debris left over from planet formation their orbits shifted. According to Joe Hahn, a planetary scientist at St. Mary's University in Halifax, Nova Scotia, who wrote an accompanying comment on the group's research in the same issue of Nature, "the key feature is planet scattering. All the orbits get very quickly disturbed."
Eventually the orbits of Jupiter and Saturn fell into a resonance such that Jupiter revolved around the sun exactly twice per single revolution of Saturn. It was this ratio, Morbidelli says, that allowed the orbital system to "come to an abrupt change."
The researchers' model shows that the strong gravitational tugs from the orbital resonance of Jupiter and Saturn could account for the eccentric and inclined orbits of the large planets. And the tugs could also account for the energy needed to kick Neptune out to a distance of twice its previous orbit, where it remains today.
"It's just gravity," says Harold Levison of the Southwest Research Institute in Boulder, Colo., who is a co-author of the study. "There was a significant torque that allowed you to change eccentricities."
Neptune's ejection also sent the planet crashing into a belt of planetesimals that had gathered at the edge of the solar system. The impact sent some of the debris hurtling back toward the sun, an action that Morbidelli's team says could explain the period of heavy bombardment of the inner solar system, including Earth's moon.
But perhaps the most surprising find for Morbidelli and colleagues was their model's ability to explain the presence of Jupiter's Trojans, the asteroids known to join Jupiter in its orbit around the sun. "In the beginning we weren't thinking about Trojans at all," Morbidelli says. Early works showed that if the Trojans were "priomodial" captured into orbit as Jupiter formed then the Trojans would have been lost during the orbital shuffle. Thus, these early models must be wrong, Morbidelli says; "because we have the Trojans."
Additional modeling revealed that some planetesimals knocked loose by Neptune's ejection may have been captured by Jupiter's large gravitational presence. With the increased orbital distance between Jupiter and Saturn, tugs of gravity would not be enough to dislodge the debris from orbit. So, the model allows the Trojans to remain in a gravitational balance, preceding or trailing Jupiter in its orbit. "It was a gorgeous result," Morbidelli says. "We have turned what seemed to be a weakness of our model into a strength."
This series of still images represent three main stages in the orbital shift model: left, just before the shift; middle, debris scatters after Neptune is ejected, sending rubble towards the moon; right, 200 million years later when the planets settle down into their final orbits. AU in these graphs stands for astronautical units. One AU is equal to the distance from the sun to Earth. Images from an animation courtesy of Morbidelli and colleagues.
"I would not have expected the same model to explain both the planetary orbits and the Trojans," Hahn says. "All these things are big plusses in their favor. It's a compelling story."
But Hahn is not entirely convinced that the orbital rearrangement actually took place in the early solar system. About the proposed theory, Hahn says: "Does that mean that's the way the solar system behaved? Or is it the luck of the model?"
Hahn says that it is difficult to prove a model of the early history of the solar system because the evolution is not directly observable. "All we see is the end state," he says. "But you can make your model compelling by showing it agrees and is consistent with what we see in the solar system."
Because Morbidelli's team's model seems compatible with the constraints of
the three previously unconnected cosmic events, they remain hopeful that their
model accurately describes the processes of the early solar system. The team
continues to try to add support to their model by adding new parameters, this
time focusing on the mechanics of the debris in the disk-shaped region of planetesimals
beyond Neptune known as the Kuiper Belt. "I'm working on it as we speak,"
Alessandro Morbidelli's home page
Animation of orbital shift [download AVI file, 4 MB]
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