Jupiter plows through dust where small worlds are trying to grow. Pebbles scatter. In a model called the Grand Tack, the giant heads toward the Sun and then turns back, like a sailing ship changing course.
You would need about 11 Earths in a row to span Jupiter. Its weight is about 318 Earths. It grew where water could freeze onto dust, far from the new Sun, and pulled in a huge store of gas.
But gas pulls on planets too. In this model, that pull draws Jupiter toward the path Mars takes today. The giant clears away much of the stuff Mars could have grown from. Mars ends up small.
Then Saturn catches up. Their trips around the Sun fall into a rhythm, and together they move back out. Jupiter's pull can also toss wet rocks toward the inner worlds, where Earth is growing.
Did it happen that way? We are not sure. The small size of Mars and the jumble of rocks in the asteroid belt fit the idea, but other models fit some clues too. Nobody has a map of Jupiter's whole route. We have the planets and the scraps it left behind.
