Planetary disks start out as a mixture of dust and gas, but the gas does not last that long. When the star in their center ignites, the radiation that it emits begins to drive away the gas, eventually leaving a counter with nothing but dust behind it. It creates a narrow window for the formation of gas giants, which must grow large enough to begin to wrap gas before the star drives everything away.
Our current models suggest that the best way to do this is to start with a large solid body, about 1
The Juno mission was intended, among other things, to test this idea by studying the gravitational field of the giant planet. But the information it sent back suggests that something strange is going on in Jupiter, with more heavy material outside the immediate core area than we expect. Now, an international team of scientists is giving a possible explanation: Jupiter's core was blown by an opposite collision with a massive protoplanet.
What lies beneath
We obviously cannot directly imagine what is happening inside Jupiter. Instead, we need to find out what's there based on conclusions made by the planet's gravitational field. And Juno was the first probe specifically designed to improve our understanding of the gravitational field. While additional data is still coming in, a preliminary analysis suggests that one explanation for what we see is that the planet has a core that the new article describes as "diluted". Instead of concentrating the heavier solid in the core, some of the heavier elements appear to spread white across the interior of the planet, reaching up to about halfway to the surface of the planet.
How it happened is not at all clear, given that we believe that the only way for a planet like Jupiter to happen is to start with a solid core. It is possible that further Juno data will indicate that a diffuse core is unlikely. Alternatively, our models for planet formation may be wrong. But the researchers start with the premise that everything is correct and that there is something unexpected in Jupiter's interior.
An alternative is that the metallic hydrogen layer in Jupiter has gradually eroded the core, but we do not know if metallic hydrogen can or how heavier elements would be mixed in it. Instead, the authors consider the possibility that Jupiter's core was disturbed by a collision, much like the one that formed the Earth-Moon system – even though it was quite different in scale.
Collisions could be driven by Jupiter's formation itself. A core of 10 soils constitutes only about 5% of Jupiter's final mass, and the decaying process that surrounded it with gas would have improved its gravitational force by a factor of 30 in less than a million years. All other bodies nearby could be pulled into a collision. And since Jupiter's core is believed to have been formed through a series of collisions between smaller bodies, there is a reasonable chance that there was something nearby that could undergo a collision.
To test this idea, the researchers ran a large number of early solar system simulations, varying the exact configuration of Jupiter and all nearby orbits. They found that in many of these simulations, the growth of Jupiter caused everything nearby to cross paths, often resulting in collisions. Due to Jupiter's enormous features, most of the collisions ended with sending the protoplanet directly to Jupiter's core.
They then turned to another set of simulations and looked at what happened to Jupiter's core as a result. The exact details depend on the size of what hits Jupiter and the size of the giant planet at impact. The simulation that they ran in detail means that Jupiter encounters an eight-earth-mass core surrounded by two masses of gas. Smaller objects, including Earth-sized protoplanets, would disintegrate in the atmosphere before reaching the core.
Despite the astonishing scale of this collision, it adds only a small amount to the total energy delivered to Jupiter during its formation. But it changes the energy of the core itself, which begins to swing. And convection begins to bring the products of these oscillations higher up into the planet's envelope. Within a few days, Jupiter settles in a state where its core is diffuse and extends almost halfway to the surface of the planet.
Of course, this event occurred over four billion years ago, and it must have been stable during the intervening period discovered by Juno. The researchers found that this was possible if Jupiter's internal temperature stabilized at 30,000 Kelvin. Each heater and convection becomes high enough to eliminate the slope between the core and its surroundings, which stabilizes the presence of heavier materials above the core. Not all coolers and convections are strong enough and heavier materials settle back into the core.
Since most planets are considered to have been built by multiple collisions between protoplanets and smaller bodies, the authors think it is worth investigating whether diffuse nuclei can be a common feature of gas giants. There have been a number of giant exoplanets that appear to have high metal content in their atmospheres, which may be a product of similar events.
There is no obvious way to test these things right now, and there is still a chance that further data from Juno will suggest alternative explanations. But if the idea holds up, planetary scientists will undoubtedly begin to consider the consequences of these collisions and may come up with some open indication of the marks they leave on gas giants.
Nature 2019. DOI: 10.1038 / s41586-019-1470-2 (About DOI).