In our search for exoplanets, we’ve found that many of them fall into certain types or categories, such as Hot Jupiters, Super-Earths, and Ice Giants. While we don’t have any examples of the first two in our solar system, we do have two Ice Giants: Uranus and Neptune. They are mid-size gas planets formed in the cold outer regions of the solar system. Because of this, they are rich in water and other volatile compounds, and they are very different from large gas giants such as Jupiter. We still have a great deal to learn about these worlds, but what we’ve discovered so far has been surprising, such as the nature of their magnetic fields.
When the Voyager 2 spacecraft flew past Uranus and Neptune in the 1980s, it found that neither world had a strong dipolar magnetic field like Earth’s. Instead, each had a weaker and more chaotic magnetic field, similar to that of Mars. This was surprising given what we understand about planet formation.
In a planet’s youth, the interior becomes very hot due to gravitational compression. This would allow heavier material such as iron to sink to the core, while lighter material such as water would move toward the surface. For Earth, this created a nickel-iron core with a crust of silicates, water, and organics. The tremendous heat in the core would also allow for a convective region, where hot core material rises a bit before cooling and sinking, creating a circular flow of dense material. In Earth, this convective iron region generates our planet’s strong magnetic field. Since Uranus and Neptune likely have an Earth-sized metallic core, we would expect them to have a similar convection region generating a similar magnetic field. But that isn’t what we observe.
After the Voyager 2 discovery, it was thought that perhaps some mechanism prevented a convection region from forming. Perhaps the layers within a gas giant don’t mix, similar to the separation of oil and water. But the details remained unknown. Since we can’t create the tremendously high-density, high-pressure conditions of a gas giant’s core in the lab, we had no way to test various models. We also haven’t sent another probe to either planet, so we have no way to gather new data in situ.
One approach that could work to solve the mystery would be to use computer simulations. However, simulating the interactions of hundreds of molecules to calculate their bulk properties is extremely intensive. Too complex for computer systems of a decade ago. But a new study has simulated the bulk properties of more than 500 molecules, which is enough to calculate how an ice giant’s layers form.
The simulations show how water, methane, and ammonia in the middle region of Uranus and Neptune separate into two unmixable layers. This primarily occurs because hydrogen is squeezed out of the deep interior, which limits how mixing can occur. Without a convection zone in these layers, the planets cannot form a strong dipolar magnetic field. Uranus likely has a rocky core about the size of Mercury, while Neptune likely has a rocky core about the size of Mars.
Future lab experiments could confirm some of these bulk properties, and there is a proposed mission to Uranus that would gather data to confirm or disprove this model.
Reference: Militzer, Burkhard. “Phase separation of planetary ices explains nondipolar magnetic fields of Uranus and Neptune.” Proceedings of the National Academy of Sciences 121.49 (2024): e2403981121.