A new model of the early solar system shows Jupiter rocketing about the place, moving toward the center of the solar system and back out again.
At one point, says NASA, it got about as close to the sun as Mars is now - and it's profoundly influenced the solar system, altering the asteroid belt and making Mars smaller than it would otherwise have been.
"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," says Kevin Walsh of the Southwest Research Institute.
"This change in direction is like the course that a sailboat takes when it tacks around a buoy."
According to the new model, Jupiter formed in a region of space about three-and-a-half times as far from the sun as Earth - but got caught in the currents of flowing gas around the Sun and started to get pulled towards it.
Jupiter spiraled slowly inward until it settled at a distance of about 1.5 astronomical units — about where Mars is now.
"We theorize that Jupiter stopped migrating toward the sun because of Saturn," says Avi Mandell, a planetary scientist at NASA Goddard.
Like Jupiter, Saturn got drawn toward the sun shortly after it formed.
Gradually, all the gas between the two planets got expelled, bringing their sun-bound spiral to a halt and eventually reversing the direction of their motion.
The two planets journeyed outward together until Jupiter reached its current position at 5.2 astronomical units and Saturn came to rest at about 7 astronomical units. Later, other forces pushed Saturn out to 9.5 astronomical units, where it is today.
"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," Mandell says.
Rather than having Jupiter destroy the asteroid belt as it moved toward the sun, the Grand Tack model has Jupiter perturbing the objects and pushing the whole zone farther out.
In the same way, as Jupiter moved away from the sun, the planet nudged the asteroid belt back inward and into its present location. And because Jupiter traveled much farther out than it had been before, it reached the region of space where icy objects are found, deflecting some towards 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," says Walsh. "Our model puts the right material in the right places, for what we see in the asteroid belt today."
The time that Jupiter spent in the inner solar system had another major effect: its presence made Mars smaller than it otherwise would have been.
If, as the Grand Tack model suggests, Jupiter spent some time parked in the inner solar system, it would have scattered some material available for making planets, leaving Mars with little planet-making material.
"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," says 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."