Jupiter is a gigantic ball of churning gas clouds, and that makes it hard to see what's in the middle. It's believed to be a relatively small, rocky core, but data from the Juno probe found that it's less dense and more spread out than expected. Now, astronomers believe they have an answer – a huge ancient planet, with 10 times the mass of Earth, crashed into the gas giant in the early days of the solar system.
Juno has been circling Jupiter for more than three years, conducting a number of flybys and studying the gas giant with various instruments. Along with snapping photos of the clouds with unprecedented clarity, the craft also managed to probe below them, taking gravitational readings to get an idea of the core. And surprisingly, it was found to be less dense and more extended than scientists thought.
But researchers on the new study, involving astronomers from Rice University in the US and Sun Yat-sen University in China, have a new theory to explain it. The team says that a colossal collision in the distant past could have churned up the core, mixing the dense materials in the middle with the gaseous or liquid layers above it. That could have the effect of making it both larger and less dense, as Juno suggested.
The idea is more likely than you might think. These kinds of collisions happened all the time in the early days of the solar system, before the planets settled into their current stable orbits. It's been theorized that Earth was hit by a Mars-sized planetoid which created the Moon and gifted us most of our water. Mars' two moons are thought to have been born the same way, and some of Uranus' oddities could be explained by yet another cosmic collision.
To test the idea for Jupiter, the team ran thousands of simulations of the early solar system. They found that overall, there was a 40 percent chance of ancient Jupiter swallowing a whole world within its first few million years. And because of its sheer size, Jupiter's gravity tended to focus incoming planetoids so that they were more likely to smash head-on, rather than graze the gas giant.
So the odds of it happening may be relatively high, but would such an event produce the kind of core Juno's data hinted at? To find out, the researchers then ran 3D computer models that demonstrated what would happen to the planet in these kinds of collisions. And they found that it lines up pretty well with observations.
"Because it's dense, and it comes in with a lot of energy, the impactor would be like a bullet that goes through the atmosphere and hits the core head-on," says Andrea Isella, co-author of the study. "Before impact, you have a very dense core, surrounded by atmosphere. The head-on impact spreads things out, diluting the core."
Interestingly, it would have taken a particularly huge planetoid to do this kind of damage.
"The only scenario that resulted in a core-density profile similar to what Juno measures today is a head-on impact with a planetary embryo about 10 times more massive than Earth," says Shang-Fei Liu, co-author of the study.
The team also points out that even if this collision happened near the beginning of the solar system, some 4.5 billion years ago, it still might not have settled back down into a dense core. In fact, it might not for several billions of years yet.
Further studies by Juno will most likely help fill in our understanding of the solar system's largest planet.
The research was published in the journal Nature. The team describes the work in the video below.
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