Jupiter is an (ancient) pillar of the solar system, according to scientists
Jupiter came together in what was just a blink in geologic time. The stormy planet's rocky core formed 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 or 3 million years, that core grew to 50 times the size of Earth.
Thomas Kruijer, a researcher at the Lawrence Livermore National Laboratory in California, was a co-author of the new article about Jupiter's beginnings. Scientists have previously made computer models of the birth of Jupiter. The study, though, "is the first time that we can say something about Jupiter based on measurements done in the lab," said Kruijer.
The solar system began as a disk of dust and gas 4.6 billion years ago. Of the planets, first came the gas giants, who were then followed by such rock-and-metal planets, including Earth. Jupiter is the biggest of the brood, and despite being mostly gas by bulk, it is more than 300 times the mass of Earth. For that reason astronomers suspect the planet is the oldest. Jupiter was probably 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.
Protecting The Inner Solar System
The planet also protected the inner solar system from stray meteorites, or rocks flying through space. Jupiter's gravity was strong enough to pull passing rocks into its orbit. It cut the inner solar system off from the outer solar system, according to Brandon Johnson, a scientist at Brown University.
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 are jumbled together in a single belt between Jupiter and Mars. Rocks from this mixture sometimes land on Earth, where scientists such as Kruijer can study them.
The idea then is that Jupiter temporarily split the meteorites in the solar system into two categories. There were the meteorites between Jupiter and the sun and the ones beyond Jupiter, or inner and outer space rocks.
Kruijer and other scientists can now tell the difference between these rocks due to their specific chemical makeups, 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, Kruijer said.
Keeping Meteorite Groups Apart
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 studies. Crucially, the two populations existed simultaneously for a few million years.
The rocks did not come from different times, but from different locations. Something must have kept them apart, and the most likely culprit, the authors of the study say, is a young Jupiter. "It's hard to think of any other possibility," Krujier 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."
Planetary scientists are like detectives, though, scouring a scene for hints about what went on. Batygin said that as with any mystery, "there's always room for doubt with these types of problems."
The Reason Earth Is Small With A Thin Atmosphere
It might be that the structure of the early disk kept the meteorite groups isolated, said Kevin Walsh, an astronomer at Southwest Research Institute in Colorado. He was not involved with this work. "The key point the authors make is that Jupiter must form to keep these asteroid reservoirs separate while they form," he said. "It is possible that we have a naive understanding of the way asteroid building blocks could move in an early solar system, and that a Jupiter mass planet isn't necessary."
However, an early Jupiter like the one described works well with other ideas about the early solar system. One concept, called the grand tack hypothesis, casts Jupiter as a wanderer. In the grand tack hypothesis, first proposed by Walsh and other scientists in 2011, Jupiter began to barrel toward the center of the solar system until Saturn formed, pulling Jupiter backward. This motion could be responsible for, among other things, the mixing of the meteorite groups into one belt.
It is also likely that the young and massive Jupiter is responsible for a small Earth with a thin atmosphere. "We occupy a somewhat strange world, galactically speaking," Batygin said. Earth, which formed about 100 million years after the beginning of the solar system, lacked the gravity for a thick "nasty hydrogen helium atmosphere" found on other worlds, and Jupiter probably sucked up most of that material.
The Importance Of Being Jupiter
Scientists looking for planets in other star systems have found several super-Earths, planets larger than Earth but smaller than gas giants like Neptune. Few exoplanets are as small as two Earths and exist in the livable zone of their star. Kruijer speculated that the young Jupiter is the reason our solar system does not have any super-Earths close to our star.
In this light, Jupiter is a pillar of the solar system. "Even in its infancy, Jupiter really controlled the dynamics and evolution of the solar system," Johnson said. "It's the biggest thing there is. Even at a million years it's changing the way that our solar system looked."