Jupiter formed in a geologic blink. Its rocky core coalesced less than a million years after the beginning of our solar system, scientists reported Monday in the Proceedings of the National Academy of Sciences. Within another 2 million or 3 million years, that core grew to 50 times the mass of Earth.
Scientists have previously built computer models of the birth of Jupiter. But this study “is the first time that we can say something about Jupiter based on measurements done in the lab,” said study co-author Thomas Kruijer, a researcher at the Lawrence Livermore National Laboratory in California.
To probe the planet’s creation, experts sampled extraterrestrial material that happens to land on Earth — ancient meteorites.
Our solar system began as a disk of dust and gas 4.6 billion years ago. Of the planets, first came the gas giants, followed by such rock-and-metal terrestrial worlds as Earth. Jupiter is the biggest of the brood. Despite being mostly gas by bulk, it’s more than 300 times the mass of Earth. For that reason astronomers suspect the planet was the oldest, able to scoop up more material out of the disk before its younger siblings appeared.
The new study supports the idea of a firstborn Jupiter. When Jupiter formed, the growing planet swept up a great swath of gas and dust as it circled the sun.
What’s more, it acted as a barrier to shield the inner solar system from wayward meteorites. When the solar system was about 1 million years old, Jupiter’s gravity was strong enough to prevent rocks from crossing beyond its orbit.
“At about 1 million years you have Jupiter getting big enough to cut off the inner solar system from the outer solar system,” said Brown University’s Brandon Johnson, a planetary scientist who was not involved with the new research.
Then, when the solar system was around 4 million years old, Jupiter grew to about 50 Earth masses and headed toward the sun. This allowed the outer asteroids to mix with the inner rocks. Today, they’re jumbled together in a single belt, which exists between Jupiter and Mars. Rocks from this mixture land on Earth, where scientists such as Kruijer can study them.
Population split
The new study adds evidence to the idea that Jupiter temporarily split the population of meteorites in the solar system in two: those between Jupiter and the sun, and those beyond Jupiter. If a pair of inner and outer space rocks landed in your front yard, and you picked them up after they cooled down and the dust settled, you wouldn’t be able to spot a difference.
But Kruijer and his colleagues can measure specific chemical signatures in meteorites — which reveal not only the rocks’ age but which of the two groups they once belonged to. It was only recently that technological advances allowed scientists to measure the differences in the two, Kruijer said.
The meteorite groups separated around 1 million years after the solar system formed, and stayed apart until about 4 million years post-formation, according to the new analysis. Crucially, the two populations existed simultaneously for a few million years.
“It cannot be a temporal change. It must be a spatial separation,” Kruijer said. Something must have kept them apart. The most likely culprit, the authors of the study say, is a young Jupiter. “It’s hard to think of any other possibility,” he said.
“This is interesting work and presents an interesting result, which conforms well with our existing understanding,” said Konstantin Batygin, a planetary astrophysicist at the California Institute of Technology who was not involved with the research. “It may very well be what had happened.”
And it’s likely that this young and massive Jupiter is responsible for a small Earth with a thin atmosphere. Earth, which formed about 100 million years after the solar nebula, lacked the gravity for a thick “nasty hydrogen helium atmosphere” found on other worlds. Thank Jupiter for sucking up most of that material.
