Planetary disks are initially a mix of dust and gas, but the gas does not adhere for long. As the star ignites in its center, the radiation it emits begins to repel the gas, eventually leaving behind a disk that contains only dust. This creates a narrow window for the formation of gas giants, which must be large enough to begin to suck in gas before the star displaces everything, large solid body, about 1
Among other things, this was the mission Juno wanted to test this idea by examining the gravitational field of the giant planet. But the data that has been returned indicates that something strange is going on in Jupiter, with more heavy material outside the immediate core than we would expect. Now, an international research team is providing a possible explanation: Jupiter's core has been destroyed by a frontal collision with a massive protoplanets.
What's Under It
Obviously, we can not directly imagine what's going on in Jupiter. Instead, we need to figure out what's in there based on the conclusions from the planet's gravitational field. And Juno was the first probe specifically designed for a better understanding of this gravitational field. As more data comes in, a preliminary analysis suggests that one explanation for what we are seeing is that the planet has a core that the new paper describes as "diluted". Instead of concentrating the heavier solid material in the nucleus, some of the heavier elements appear to be widely distributed over the interior of the planet, reaching to about half the planetary surface.
How that happened is not clear at all. Considering that we think that the only way for a planet like Jupiter is to start with a solid core. It is possible that additional Juno data indicates that a diffuse kernel is unlikely. Alternatively, our models of planet formation might be wrong. The researchers, however, assume that everything is fine inside Jupiter and something unexpected happens.
One possibility is that the metallic hydrogen layer of Jupiter has gradually eroded the core, but we do not know whether metallic hydrogen is present or how heavier elements would mix in it. Instead, the authors consider the possibility that Jupiter's core was destroyed by a collision similar to that formed by the Earth-Moon system, even though it is totally different in size.
Collisions could be triggered by Jupiter's formation itself. A 10-mass core accounts for only about 5% of Jupiter's final mass, and the out-of-control process that surrounded it with gas would have increased its appeal by a factor of 30 in less than a million years. All other bodies in the vicinity could be involved in a collision. And since it is assumed that Jupiter's nucleus has been formed by a series of collisions between smaller bodies, there is a reasonable likelihood that there is something in the vicinity that could trigger a collision.
To test this idea, researchers conducted a large number of simulations of the early solar system, which varies the exact configuration of Jupiter and all nearby orbital bodies. They found that in many of these simulations, the growth of Jupiter caused everything to intersect nearby, often resulting in collisions. Due to Jupiter's immense appeal, most of the collisions ended up frontally, sending the protoplanets straight into the core of Jupiter.
Then they turned to another set of simulations and investigated what happened to the core of Jupiter as a result. The exact details depend on the size of the Jupiter impact and the size of the giant planet at the time of the impact. The simulation, which they carried out in detail, provides that Jupiter is hit by a core of eight earth masses, which is surrounded by two natural gas masses. Smaller objects, including earth-sized protoplanets, would dissolve in the atmosphere before they reach the nucleus.
Despite the shocking magnitude of this collision, it contributes little to the total energy supplied to Jupiter during its formation. But it changes the energy of the core itself, which starts to vibrate. And the convection brings the products of these vibrations higher into the shell of the planet. Within a few days, Jupiter is in a state where its nucleus is diffuse and extends almost halfway across the surface of the planet.
Of course, this event occurred over four billion years ago, and it would have to remain stable for the intervening time to be discovered by Juno. The researchers found that this was possible when the internal temperature of Jupiter stabilized at 30,000 Kelvin. Any heating and convection will be high enough to clear the gap between the core and its surroundings, stabilizing the presence of heavier material over the core. Any cooling and convection is not strong enough, and heavy material settles back into the core.
Since most planets are thought to have been formed by multiple collisions between protoplanets and smaller bodies, the authors suggest that investigating whether diffused nuclei are possible may be a common feature of gas giants. There were a number of giant exoplanets that seem to have high metal content in their atmosphere, which could be due to similar events.
There is currently no obvious way to test these things, and there is still a possibility that this will continue. Juno data will suggest alternative explanations. But if the idea is upheld, planetary scientists will no doubt begin to investigate the effects of these collisions, and they may receive an obvious indication of the traces they leave on the gas giant.
Nature 2019. DOI: 10.1038 / s41586-019-1470-2 (Information on DOIs).