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A giant impact 4.5 billion years ago could be the reason Jupiter’s core is stranger than astronomers expected.

Astronomers thought that Jupiter began as a rocky and icy planetary embryo that later formed its massive gaseous envelope, drawing in hydrogen and helium from the solar nebula by virtue of its huge gravity. This would mean there was a relatively clear delineation between the solid core and the gas surrounding it.

However, that does not appear to be the case. Over the past few years, NASA’s Juno spacecraft has made measurements of Jupiter’s gravitational field and these suggest the solid core is mixed with hydrogen through a large part of its radius.

“Instead of a small compact core as we assumed, Jupiter seems to have a dilute, fuzzy core that extends to almost half of its radius,” says Shang-Fei Liu, from Sun Yat-sen University in Zhuhai, China.

Liu and his team modelled a scenario where, 4.5 billion years ago, a massive planetary embryo smacked into Jupiter. “After the impact, Jupiter’s primordial compact core was completely destroyed, and a dilute-core-like structure was formed,” says Liu. The simulations show the effects of this collision would remain within Jupiter until this day, fitting what has been observed by Juno.

The team’s simulations found there would be a 40 per cent chance of Jupiter swallowing a planetary embryo over a few million years.

These sorts of collisions were not rare during planet formation, says Vincent Eke at Durham University, UK. The effects are visible through out the solar system. “Mercury has almost no crust, Venus spins the wrong way around” he says. “Earth’s moon formed following a giant impact, Mars looks like it’s been scalped, and Uranus spins on its side.”

We don’t know enough about Saturn, Uranus or Neptune to say whether Jupiter’s core is unusual yet, says Eke.

However, Liu’s hypothesis will probably remain just that for the foreseeable future. “One probably cannot test this directly,” says Peter Read at the University of Oxford.