June 7, 2011

Youthful Jupiter Could Not Sit Still

In a new model of the early solar system, Jupiter is found to have been a youthful traveler, roaming as close to the Sun as where Mars is now and back out again before settling in its position as the fifth planet from our Sun.

Scientists note that the planet's travels greatly influenced the solar system, altering the asteroid belt and making Mars smaller than it should have been. The model was developed by an international team of scientists and was published in the journal Nature on June 5th.

Among the international consortium are scientists from NASA's Goddard Space Flight Center in Greenbelt, Maryland.

According to the new model, Jupiter formed in a region of space about 3.5 times further away from the Sun than Earth is. Because a huge amount of gas still enveloped the Sun back then, Jupiter got caught in the currents of the flowing gas and was pulled toward the sun. The gas giant slowly spiraled inward until it settled at about where Mars is now, but was not then.

"We refer to Jupiter's path as the Grand Tack, because the big theme in this work is Jupiter migrating toward the sun and then stopping, turning around, and migrating back outward," said the paper's lead author, Kevin Walsh of the Southwest Research Institute in Boulder, Colorado. "This change in direction is like the course that a sailboat takes when it tacks around a buoy."

"We theorize that Jupiter stopped migrating toward the sun because of Saturn," said Avi Mandell, a planetary scientist at NASA Goddard and a co-author on the paper.

Like Jupiter, Saturn was drawn toward the Sun shortly after it formed, and the model shows that once the two massive planets came close enough to each other, their fates became eternally linked. Gradually, all the gas between the two planets got expelled, bringing their death spiral into oblivion to a halt and eventually reversing the direction of their motion. Both planets journeyed back outward together until Jupiter reached its current position at 5.2 astronomical units (AU) and Saturn came to a rest at about 7 AU -- but then due to other forces, it was pushed out to about 9.5 AU, where it is today.

These extraordinary movements took place over hundreds of thousands to millions of years.

"Jupiter migrating in and then all the way back out again can solve the long-standing mystery of why the asteroid belt is made up of both dry, rocky objects and icy objects," said Mandell.

Astronomers believe the asteroid belt exists because Jupiter's gravity prevented the rocky material there from coming together to form a new planet. Instead, the region of space remained a loose collection of objects.

Some scientists previously thought that Jupiter possibly could have moved closer to the Sun at some point in its cycle, but the theory had faults: Jupiter was expected to scatter the material in the asteroid belt so much that the belt would no longer exist.

"For a long time, that idea limited what we imagined Jupiter could have done," said Walsh.

The Grand tack model, however, suggests Jupiter only disturbing the objects enough to push the whole region farther out. "Jupiter's migration process was slow," explained Mandell, "so when it neared the asteroid belt, it was not a violent collision but more of a do-si-do, with Jupiter deflecting the objects and essentially switching places with the asteroid belt."

Then, as Jupiter moved back out away from the Sun, the planet nudged the asteroid belt back inward and into its current location between the modern orbits of Mars and Jupiter. And because Jupiter traveled much farther out than it had before, it reached a region of space where icy objects are found. The gas giants deflected some of these icy objects toward the Sun and into the asteroid belt.

"The end result is that the asteroid belt has rocky objects from the inner solar system and icy objects from the outer solar system," said Walsh. "Our model puts the right material in the right places, for what we see in the asteroid belt today."

Jupiter's time in the inner solar system made Mars a smaller planet as well. "Why Mars is so small has been the unsolvable problem in the formation of our solar system," said Mandell. "It was the team's initial motivation for developing a new model of the formation of the solar system."

Mars should have been much larger than Earth and Venus because the region where it formed had more raw materials to draw on, but because of Jupiter's travels, it is smaller. "For planetary scientists, this never made sense," Mandell added.

The Grand Tack model also suggests that if Jupiter spent some time hovering in the inner solar system, it would have scattered some material available for making planets. Much of the material past about 1 AU would have been dispersed, leaving Mars out at 1.5 AU with little to grow on. Earth and Venus, however, would have formed in the region richest in planetary ingredients.

"With the Grand Tack model, we actually set out to explain the formation of a small Mars, and in doing so, we had to account for the asteroid belt," said Walsh. "To our surprise, the model's explanation of the asteroid belt became one of the nicest results and helps us understand that region better than we did before."

The new model places Jupiter, Saturn and the other giants in positions that concur with the "Nice model," a relatively new theory that explains the movements of the large planets later in the solar system's history.

The Grand tack model also makes our solar system be much like other planetary systems that have been found in the universe. In many of those cases, enormous gassy planets called "hot Jupiters" sit extremely close to their Suns, much closer than Mercury is to our Sun.

"Knowing that our own planets moved around a lot in the past makes our solar system much more like our neighbors than we previously thought," Walsh said. "We're not an outlier anymore."

The other co-authors of the study are Alessandro Morbidelli at the Observatoire de la Cote d'Azur in Nice, France; Sean Raymond at the Observatoire de Bordeaux in France; and David O'Brien at the Planetary Science Institute in Tucson, Arizona.


Image 1: Not long after Jupiter formed, it got pulled slowly toward the sun, carried on currents of swirling gas. Saturn also got pulled in, and when the two giant planets came close enough to each other, their fates became linked. Their sun-bound death spiral came to a halt when Jupiter was about where Mars is now; then the pair turned and moved away from the sun. The researchers who developed this model of the early solar system call it the "Grand Tack," a reference to the sailing maneuver. Credit: NASA/GSFC

Image 2: In this artist's conception, gas and dust"”the raw materials for making planets"”swirl around a young star. The planets in our solar system formed from a similar disk of gas and dust captured by our sun. Credit: NASA/JPL-Caltech


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