Jupiter's innards are full of the remains of baby planets that the gas giant gobbled up as it expanded to become the behemoth we see today, scientists have found. The findings come from the first clear view of the chemistry beneath the planet's cloudy outer atmosphere.

Despite being the largest planet in the solar system, Jupiter has divulged very little about its inner workings. Telescopes have captured thousands of images of the swirling vortex clouds in the gas giant's upper atmosphere, but these Van Gogh-esque storms also act as a barrier blocking our view of what's below.

"Jupiter was one of the first planets to form in our solar system," in the first few million years after the solar system took shape around 4.5 billion years ago, lead researcher Yamila Miguel, an astrophysicist at Leiden University in The Netherlands, told Live Science. However, we know almost nothing for certain about how it formed, she added.

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In the new study, researchers were finally able to peer past Jupiter's obscuring cloud cover using gravitational data collected by NASA's Juno space probe. This data enabled the team to map out the rocky material at the core of the giant planet, which revealed a surprisingly high abundance of heavy elements. The chemical make-up suggests Jupiter devoured baby planets, or planetesimals, to fuel its expansive growth.

Growing a gas giant

Jupiter may predominantly be a ball of swirling gas today, but it started its life by accreting rocky material — just like every other planet in the solar system. As the planet's gravity pulled in more and more rocks, the rocky core became so dense that it started pulling in large amounts of gas from far distances — predominantly hydrogen and helium left over from the sun's birth — to form its enormous gas-filled atmosphere.

There are two competing theories about how Jupiter managed to collect its initial rocky material. One theory is that Jupiter accumulated billions of smaller space rocks, which astronomers nickname pebbles (although these rocks are likely closer in size to boulders rather than actual pebbles).

The opposing theory, which is supported by the findings from the new study, is that Jupiter's core was formed from the absorption of many planetesimals — large space rocks spanning several miles, which if left undisturbed could have potentially acted as seeds from which smaller rocky planets like Earth or Mars could develop.

However, until now it has not been possible to definitively say which of these theories is correct. "Because we cannot directly observe how Jupiter was formed we have to put the pieces together with the information we have today," Miguel said. "And this is not an easy task."

Probing the planet

To try to settle the debate, researchers needed to build a picture of Jupiter's insides. "Here on Earth, we use seismographs to study the interior of the planet using earthquakes," Miguel said. But Jupiter has no surface to put such devices onto, and Jupiter's core is unlikely to have much tectonic activity anyway, she added.

Instead, the researchers built computer models of Jupiter's innards by combining data, which was predominantly collected by Juno, as well as some data from its predecessor Galileo. The probes measured the planet's gravitational field at different points around its orbit. The data showed that rocky material accreted by Jupiter has a high concentration of heavy elements, which form dense solids and, therefore, have a stronger gravitational effect than the gaseous atmosphere. This data enabled the team to map out slight variations in the planet's gravity, which helped them to see where the rocky material is located within the planet.

"Juno provided very accurate gravity data that helped us to constrain the distribution of the material in Jupiter's interior," Miguel said. "It is very unique data that we can only get with a spacecraft orbiting around the planet."

The researcher's models revealed that there is an equivalent of between 11 and 30 Earth masses of heavy elements within Jupiter (3% to 9% of Jupiter's mass), which is much more than expected.

Pebbles vs. planetesimals

The new models point to a planetesimal-gobbling origin for Jupiter because the pebble-accretion theory cannot explain such a high concentration of heavy elements, Miguel said. If Jupiter had initially formed from pebbles, the eventual onset of the gas accretion process, once the planet was large enough, would have immediately ended the rocky accretion stage. This is because the growing layer of gas would have created a pressure barrier that stopped additional pebbles from being pulled inside the planet, Miguel explained. This curtailed rocky accretion phase would likely have given Jupiter a greatly reduced heavy metal abundance, or metallicity, than what the researchers calculated.

However, planetesimals could have glommed onto Jupiter's core even after the gas accretion phase had begun; that's because the gravitational pull on the rocks would have been greater than the pressure exerted by the gas. This simultaneous accretion of rocky material and gas proposed by the planetesimal theory is the only explanation for the high levels of heavy elements within Jupiter, the researchers said.

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The study also revealed another interesting finding: Jupiter's insides do not mix well into its upper atmosphere, which goes against what scientists had previously expected. The new model of Jupiter's insides shows that the heavy elements the planet has absorbed have remained largely close to its core and the lower atmosphere. Researchers had assumed that convection mixed up Jupiter's atmosphere, so that hotter gas near the planet's core would rise to the outer atmosphere before cooling and falling back down; if this were the case, the heavy elements would be more evenly mixed throughout the atmosphere.

However, it is possible that certain regions of Jupiter may have a small convection effect, and more research is needed to determine exactly what is going on inside the gas giant's atmosphere, Miguel said.

The researchers' findings could also change the origin stories for other planets in the solar system. "Jupiter was the most influential planet in the formation of the solar system," Miguel said. Its gravitational pull helped to shape the size and orbits of its cosmic neighbors, and so determining how it came to be has important knock-on effects for other planets, she added. The findings also suggest a potential planetesimal origin for the other gas giants in the solar system: Saturn, Uranus and Neptune.

Other gaseous worlds in other star systems might also have formed by gobbling up planetesimals rather than pebbles, which means they may also have higher metallicity than their appearance would suggest. It is, therefore, important that when we find these new worlds, which are being searched for using NASA's James Webb Telescope, we don't judge them by their cloudy covers, the researchers said.

The study was published online June 8 in the journal Astronomy and Astrophysics.

Originally published on Live Science.