Which planet in our solar system formed first? The question has long been one of the great unsolved mysteries of planetary science. The first planet was almost certainly one of the gas or ice giants, but with no samples from anything beyond the asteroid belt, determining which planet is in fact the oldest has eluded researchers for decades.

But now, an international team of scientists from the Lawrence Livermore National Laboratory (LLNL) thinks it's finally solved the mystery. According to a new paper in the Proceedings of the National Academy of Sciences, the oldest planet in the solar system is Jupiter.

"We do not have any samples from Jupiter, in contrast to other bodies like the Earth, Mars, the moon and asteroids," said Thomas Kruijer, lead author of the paper and a researcher at LLNL, in a press release. "In our study, we use isotope signatures of meteorites (which are derived from asteroids) to infer Jupiter's age."

It might seem odd that scientists used meteorites to determine the age of Jupiter, but the findings present a clear line of deduction that identifies Jupiter as the oldest planet. Meteorite samples analyzed fall into two distinct groups with different isotope signatures. The differences in composition indicate that the meteorite groups formed in two distinct clouds of gas and dust, both surrounding the sun but separated from each other. Through models, the LLNL team has demonstrated that an explanation for these two distinct sources of meteorites is that Jupiter formed, and then the new planet cleared a path through the dust and debris surrounding the sun, known as an accretion disk, creating two distinct areas of planet and asteroid formation.

A timeline for the evolution of the solar system can be inferred. Our sun ignited into a star about 4.6 billion years ago, and at the time it was surrounded by an accretion disk of gas and ice and rock. Jupiter's rocky core formed as the first planet, only about one million years after the sun's first light, and it cut a gap in the accretion disk following its orbit. As Jupiter grew and the other planets started to form, asteroids also coalesced in both the inner and outer part of the accretion disk, separated by Jupiter. These asteroids have different isotope compositions, according to which area they formed in.

Fast forward some 4.5 billion years, and the asteroids that formed beyond Jupiter have all been shoved into the asteroid belt by the gas giants. The asteroids collide with each other, bits break off, and the bits reach us on Earth as meteorites, where we analyze them.

"Jupiter is the oldest planet in the solar system, and its solid core formed well before the solar nebula gas dissipated, consistent with the core accretion model for giant planet formation," said Kruijer.

The findings indicate that Jupiter formed only one million years after the beginning of the solar system as a rocky core, and the planet grew to about 20 Earth masses over the next million years. Over the course of an additional three to four million years, Jupiter's core grew to 50 Earth masses. The new study aligns with the prevailing theory that Jupiter formed as a rocky core and then accumulated large amounts of gas to become the giant planet we have today.

"Our measurements show that the growth of Jupiter can be dated using the distinct genetic heritage and formation times of meteorites," Kruijer said.

There is still work to be done to confirm, without a doubt, that Jupiter is the oldest planet in the solar system. That said, many have long suspected that the Jovian beast formed first, and the new findings present a convincing argument that this the case.

Jupiter is still a jerk, though.

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