How Jupiter split the asteroid belt in two shows its great age

An analysis of meteorites shows that Jupiter divided the rocks of the asteroid belt into two families within the first million years of the solar system

It’s one hell of a bowling ball. A comparison of meteorites has revealed that, in its youth, Jupiter carved a path through the solar system that separated space rocks into two separate families. And it did it within a million years of the dawn of the solar system, so Jupiter was already huge at least 4 billion years ago.

Meteorites found on Earth have different proportions of isotopes of the elements in them, depending on whether they are from a rock that originated inside or outside Jupiter’s orbit. Differences between the rocks are seen even in those chipped off planetesimals that were formed within a million years of the solar system’s formation, so they must have separated before that.

To have been big enough to carve such a gap in the protoplanetary disk – a cloud of dense gas and dust orbiting the newly formed sun – Jupiter’s core would have to have been about 20 times the mass of Earth. This new meteorite analysis, by Thomas Kruijer at the University of Münster in Germany and his colleagues, indicates the Jupiter must have reached that size within the solar system’s first million years.

Jupiter is thought to have formed like other gas giants: its rocky core came first, then it gradually accreted more material from the dust and gas around the sun, and later acquired its gassy envelope.

Kruijer’s team analysed 19 iron meteorites found on Earth that originated from small bodies that formed alongside Jupiter. The isotopes of elements in the rocks that formed inside Jupiter’s orbit had more elements of the type that are slowly built up in stars, whereas the ones from farther out in the solar system were enriched in the heavy elements that form faster in high-energy situations.

“These isotopic labels get carried around by very small dust grains,” says Francis Nimmo at the University of California, Santa Cruz. “Somehow, you have to set up a situation in which these little dust grains get mixed around one part of the solar system but not the other.”

Some of the isotopes in the meteorites from outside Jupiter’s orbit are over half a million years younger, which means after Jupiter divided the disc, they were unable to hop the gap into the inner area.

“We need more evidence that says this is where those two meteorite classes form – one inward and one outward,” says Jonathan Lunine at Cornell University in New York. “But it’s a very nice measurement.”

The work supports one of the leading theories of the early solar system – the grand tack hypothesis. This supposes that Jupiter was born a few million years before Saturn and its enormous mass was tugged towards the sun. Once Saturn formed, however, its gravity pulled Jupiter back from the brink of stellar destruction and towards the outer solar system.

These two drifts across the solar system meant Jupiter crashed through the asteroid belt twice, mixing the two types of objects in the asteroid belt, leaving behind the well-mixed objects we see today.

“The really striking thing is that the grand tack was proposed before anyone knew about this isotopic labeling, but the isotopes really seem to agree well with the grand tack,” says Nimmo. “That’s kind of powerful.”

“It’s a really neat story,” says Nimmo, “and I think it’s probably right.”

Read more: Mars is so small because Jupiter shook up its formation

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