For years astronomers have debated whether Jupiter formed at its current distance from the Sun, or underwent some great journey from further out. The dominance of the Solar System's largest planet is so great the answer will determine how we envisage most other aspects of the formation of our planetary system – and others. Now a Swedish team of astronomers believes they have the answer, with the solution provided by a curious feature of a family of small asteroids.
Sixty degrees ahead and behind a planet in its orbit lie what are known as Lagrangian points. These provide havens of stability in the chaotic dynamics of a planetary system with many objects large enough to provide a powerful gravitational tug. Jupiter's Lagrangian points contain thousands of asteroids, collectively known as Trojans, having been assigned names of warriors from the Trojan War.
Lund University doctoral student Simona Pirani focused on an odd feature of the Trojans. Normally we would expect roughly equal numbers of asteroids to collect in the Lagrangian point before and behind a large planet. Instead, there are about 50 percent more Trojans in the lead position than trailing Jupiter, something researchers have tried unsuccessfully to explain for some time.
Pirani calculated that when planets move towards their star it creates a drag force that leads to more asteroids being trapped at the leading Lagrangian point than the trailing one.
In Astronomy and Astrophysics (paper available to read on arXiv), Pirani and team report only one scenario produced the pattern we see: Jupiter formed about 3-4 times further out, between where Uranus and Neptune now lie, and migrated inwards 2-3 million years later. They estimate the journey took 700,000 years, for much of which Jupiter was composed entirely of its core, only attracting its gas envelope towards the end.
“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," Pirani said in a statement. "We found evidence of the migration in the Trojan asteroids orbiting close to Jupiter.”
Pirani estimates there were once far more Trojans, in both locations, than we see today. She proposes more modeling to explain how the majority have been lost over 4 billion years. Confirmation of the theory came from the characteristics of the Hilda asteroids, a smaller group that orbit the Sun three times for every two orbits by Jupiter.
Many of the first planets we discovered around other stars are “hot Jupiters”, gas giants circling very close to their star. We know they can't have formed at their current locations, suggesting planetary migrations are common, but the details about why some giant planets move so much, while others are more stable, remain unclear.
Any planets that lay in the path of Jupiter's journey would have been destroyed or thrown wildly off course. On the other hand, Saturn, Uranus and Neptune followed Jupiter in, maintaining a roughly consistent ratio of distances from the Sun. Fortunately for us, the migration stopped where it did, rather than spiraling in further and disrupting the Earth in the process.