Jupiter’s innards are teeming with the remains of baby planets that the gas giant gobbled up as it expanded to become the juggernaut we see today, scientists have found. The results come from the first clear view of the chemistry beneath the planet’s cloudy outer atmosphere.
Although it is the largest planet in the solar system, Jupiter disclosed very little of its inner workings. Telescopes have captured thousands of images of the swirling vortex clouds in the upper atmosphere of the gas giant, but these Van Gogh-style storms also act as a barrier blocking our view of what lies below.
“Jupiter was one of the first planets to form in our solar system“, during the first million years after the formation of the solar system about 4.5 billion years ago, lead researcher Yamila Miguel, an astrophysicist at Leiden University in the United States, told Live Science. “However, we know next to nothing for sure about how it formed,” she added.
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In the new study, researchers were finally able to see past Jupiter’s obscuring cloud cover using gravitational data collected by NASA’s Juno space probe. This data allowed the team to map the rocky material at the giant planet’s core, which revealed a surprisingly high abundance of heavy elements. The chemical makeup suggests that Jupiter devoured baby planets, or planetesimals, to fuel its expansive growth.
Cultivate a gas giant
Jupiter may be mostly a swirling ball of gas today, but it began life accreting rocky material, like all the other planets in the solar system. Like the planet gravity sucked in more and more rock, the rocky core became so dense that it began to suck in large amounts of gas from afar – mostly hydrogen and helium left over from the Sun‘s birth – to form its enormous gas-filled atmosphere.
There are two competing theories on how Jupiter managed to collect its initial rock material. One theory is that Jupiter has accumulated billions of smaller space rocks, which astronomers call pebbles (although these rocks are likely closer in size to boulders rather than actual pebbles).
The opposing theory, which is supported by the results of the new study, is that Jupiter’s core was formed from the absorption of numerous planetesimals – large space rocks spanning several kilometers, which, if they do not ‘had been undisturbed, would have potentially acted as seeds from which smaller rocky planets like Earth or Mars could develop.
However, so far it has not been possible to say with certainty which of these theories is correct. “Because we can’t directly observe how Jupiter formed, we have to put the pieces together with the information we have today,” Miguel said. “And it’s not an easy task.”
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Survey the planet
To try to settle the debate, the researchers had to construct an image of the interior of Jupiter. “Here on Earth, we use seismographs to study the interior of the planet using earthquakes,” Miguel said. But Jupiter has no surface on which to place such devices, 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 mostly collected by Juno, along with some data from its predecessor Galileo. The probes measured the gravitational field of the planet at various points around its orbit. The data showed that the 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 allowed the team to map slight variations in the planet’s gravity, which helped them see where the rocky material is on the planet.
“Juno provided very accurate gravity data that helped us limit the distribution of material inside Jupiter,” said Miguel. “This is very unique data that we can only get with a spacecraft orbiting the planet.”
The researcher’s models revealed that there is an Earth-mass equivalent of between 11 and 30 heavy elements within Jupiter (3-9% of Jupiter’s mass), which is far more than expected.
Pebbles versus planetesimals
The new models point to a planetesimal origin for Jupiter because the pebble accretion theory cannot explain such a high concentration of heavy elements, Miguel said. If Jupiter had originally formed from pebbles, the eventual start of the gas accretion process, once the planet was large enough, would have immediately ended the rock accretion stage. This is because the growing layer of gas would have created a pressure barrier that would prevent further pebbles from being driven into the planet’s interior, Miguel explained. This phase of reduced rock accretion would likely have given Jupiter a greatly reduced abundance of heavy metals, or metallicity, compared to what the researchers calculated.
However, planetesimals could have slipped onto Jupiter’s core even after the start of the gas accretion phase; this is 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 in Jupiter, the researchers said.
The study also revealed another interesting finding: Jupiter’s innards don’t mix well with its upper atmosphere, which goes against what scientists had previously predicted. The new model of Jupiter’s interior shows that the heavy elements the planet absorbed remained largely close to its core and lower atmosphere. The researchers had assumed that convection mixed Jupiter’s atmosphere, so that hotter gas near the planet’s core would rise into the outer atmosphere before cooling and falling back; if this were the case, the heavy elements would be mixed more evenly in the atmosphere.
However, it’s possible that some regions of Jupiter have a small convective effect, and more research is needed to determine exactly what’s going on in the gas giant’s atmosphere, Miguel said.
The researchers’ findings could also alter the origin stories of other planets in the solar system. “Jupiter was the most influential planet in the formation of the solar system,” Miguel said. Its gravitational pull has helped shape the sizes and orbits of its cosmic neighbors, and so determining how it arose has important ripple effects for other planets, she added. The results 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 could also have formed by engulfing planetesimals rather than pebbles, which means they may also have a higher metallicity than their appearance would suggest. It is therefore important that when we find these new worlds, which are being researched using NASA James Webb Telescopewe don’t judge them by their cloud cover, the researchers said.
The study was published online June 8 in the journal Astronomy and astrophysics (opens in a new tab).
Originally posted on Live Science.