As the largest planet in the solar system by a pretty wide margin, Jupiter has a lot of sway, but its history is still a bit of a mystery. Now astronomers have put forward a new theory, suggesting that the gas giant formed much farther away from the Sun and then migrated into its current position.

Modern-day Jupiter orbits the Sun at an average distance of 780 million km (485 million mi) or 5.2 Astronomical Units (AU). But that might not have always been the case – it's been suggested that Jupiter formed elsewhere in the solar system and migrated into its current orbit over time.

The new research isn't the first to put forward this idea, but it does have a different starting point from other theories. The prevailing idea, known as the "grand tack hypothesis," proposes that Jupiter formed at a distance of about 3.5 AU – where the asteroid belt now lies – before migrating towards the Sun. It may have gotten as close as 1.5 AU (where Mars orbits now) before reversing course and drifting out to its current position of 5.2 AU.

This journey – and the utter chaos it would have caused – could explain a few of the solar system's oddities, including why Mars is smaller than it "should" be and why there aren't any Super-Earths in the neighborhood.

The new study paints a different path for the gas giant. The team, led by astronomers at Lund University, used computer simulations to calculate that Jupiter formed four times farther away from the Sun, starting life as an icy asteroid. About two or three million years after it formed, gravitational forces from surrounding gases in the solar system pushed it inwards to its current orbit. This could have taken about 700,000 years to play out – a relatively short amount of time on the scale of solar system dynamics.

The smoking gun for the team's hypothesis lies in what are known as the Trojan asteroids, two sets of thousands of rocks that orbit the Sun along the same path as Jupiter. One clump sits in front of the planet and another behind, but the front group has an extra 50 percent more asteroids than the rear group, and nobody really knows why.

The team set out to find an answer, by running computer simulations of the early solar system. Sure enough, they found that when Jupiter migrates inwards, asteroids from the Hilda group are captured to form the same pattern that is currently seen, with more in front of the planet than behind.

"This is the first time we have proof that Jupiter was formed a long way from the Sun and then migrated to its current orbit," says Simon Pirani, lead author of the study. "We found evidence of the migration in the Trojan asteroids orbiting close to Jupiter."

The team says that Saturn, Uranus and Neptune may have also followed similar migration patterns in their history. And further abroad, it's generally accepted that "hot Jupiters" – gas giant exoplanets that orbit extremely closely to their host stars – would have formed farther out and migrated inwards.

Of course, this is just one more theory, and more evidence will be needed to back up the claim.

Interestingly, the simulations also suggest that the Trojan asteroids are probably made of the same stuff as Jupiter's rocky core. Further clues about the history and formation of Jupiter – and the solar system as a whole – could be gleaned from closer studies of these objects. And that's exactly what NASA's Lucy probe is due to do, after its launch in 2021.

The research was published in the journal Astronomy & Astrophysics.

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