Jupiter's hike is riddled

Giant Planet drifted inward even before forming his gas shell

Even as an unfinished planetary nucleus, the gas giant Jupiter wandered far from its place of formation. © NASA / JPL / Space Science Institute
Read out

Early drift: Jupiter has already changed its position in the solar system as an unfinished planet. Even before the formation of its gas envelope, the planetary nucleus moved from the outer edge of the primeval cloud into its current orbit. Astronomers have discovered this from the distribution of thousands of Trojans in the Jupiter orbit - asteroids that accompany the gas giant on its orbit. They come from the region of origin of the gas giant and are thus "little siblings" of the Jupiter's core.

Jupiter is the largest and oldest planet of our solar system and has influenced its development decisively. The core of the gas giant was probably created a million years after the formation of our home system and tore a hole in the original cloud. For some time, there has been evidence that Jupiter was born far beyond his current position - and then even temporarily migrated to the inner solar system. Only this migration could Earth and Mars owe their existence.

The Jupiter Trojans (green) circle in the Jupiter orbit 60 degrees and 60 degrees behind the planet. © Mdf / Public domain

The puzzle of the Jupiter Trojans

But how exactly the migration of Jupiter looked and when it took place, is still unclear. To clarify this, Simona Pirani from the University of Lund in Sweden and her team have now used some "cosmic helpers": the Trojans of Jupiter. These thousands of dark boulders orbit around the sun in the same orbit as the gas giant, but remain at a constant 60 degrees in front of or behind the planet.

The interesting thing about these Trojans: "The leading group is significantly larger than the rear, " report Pirani and her team. According to estimates, there are about 50 percent more asteroids in the trojan group ahead than in the following. "This asymmetry has always given us riddles, " says Pirani's colleague Anders Johansen. One of the possible causes, however, was the migration of Jupiter.

Jupiter emerged four times further out

To verify this, the scientists performed a simulation using a model of the early solar system - including the young planet, the primeval cloud, and the 44, 000 pieces of rock flying around. They used ten different scenarios to find out if and how Jupiter's migration can explain the existence and distribution of Trojans. display

And in fact: As the simulations showed, the asymmetry of the Jupiter Trojans can only be explained by a Jupiter migration. According to this, the planet must have emerged four times farther from the sun than its current orbit suggests. Only then did he drift inward on a spiraling path. "This is the first clear evidence that Jupiter was formed far from the sun and then moved to its current orbit, " says Pirani.

Ancient planetary building blocks

The simulation also misses when the migration of the planet took place and where the Trojans come from. Accordingly, the still largely gas-free planetary nucleus began to drift inwards two to three million years after its formation. "Jupiter experiences this migration and continues to grow at the same time, " the researchers report. Through his migration, the young Jupiter destabilized the tracks of numerous smaller chunks in its area of ​​origin and pulled them with him.

"Almost all Trojan asteroids have been trapped in the origin zone of Jupiter's planetary core, about 18 astronomical units from the Sun, " report Pirani and her team. "They therefore contain valuable information about the core building blocks of our gas giants." Part of this information could be decoded by NASA's "Lucy" spacecraft, which is scheduled to launch in 2021 on a voyage to the Jupiter Trojans.

Drift shaped Trojan tracks

How quickly Jupiter and its small companions drifted inward, the researchers determined using the asymmetry of the Trojans. Because it originated because the inner migration of Jupiter did not influence their paths evenly. Instead, gravitational effects ensured that the asteroid's orbital parts of the asteroid, which are located in front of the planet, became more stable, but the underlying sections of the track were shortened.

Over time, this has resulted in a larger portion of the Trojans entering an asymmetric "tadpole" orbit around the Lagrange point L4, the location in Jupiter orbit that precedes the planet by 60 degrees. However, as the researchers discovered, this only works if Jupiter migrated fast enough in its growth phase. According to their calculations, the planet only needed around 700, 000 years for the one-way drift.

"We can learn a lot about the core of Jupiter and its growth through the Trojans, " says Johansen. (Astronomy & Astrophysics, 2019; doi: 10.1051 / 0004-6361 / 201833713)

Source: Lund University

- Nadja Podbregar