Our usual picture of Uranus and Neptune as “ice giant” planets may not be entirely correct. According to new work by scientists at the University of Zürich (UZH), Switzerland, the outermost planets in our solar system may in fact be rock-rich worlds with complex internal structures – something that could have major implications for our understanding of how these planets formed and evolved.
Within our solar system, planets fall into three categories based on their internal composition. Mercury, Venus, Earth and Mars are deemed terrestrial rocky planets; Jupiter and Saturn are gas giants; and Uranus and Neptune are ice giants.
An agnostic approach
The new work, which was led by PhD student Luca Morf in UZH’s astrophysics department, challenges this last categorization by numerically simulating the two planets’ interiors as a mixture of rock, water, hydrogen and helium. Morf explains that this modelling framework is initially “agnostic” – meaning unbiased – about what the density profiles of the planets’ interiors should be. “We then calculate the gravitational fields of the planets so that they match with observational measurements to infer a possible composition,” he says.
This process, Morf continues, is then repeated and refined to ensure that each model satisfies several criteria. The first criteria is that the planet should be in hydrostatic equilibrium, meaning that its internal pressure is enough to counteract its gravity and keep it stable. The second is that the planet should have the gravitational moments observed in spacecraft data. These moments describe the gravitational field of a planet, which is complex because planets are not perfect spheres.
The final criteria is that the modelled planets need to be thermodynamically and compositionally consistent with known physics. “For example, a simulation of the planets’ interiors must obey equations of state, which dictate how materials behave under given pressure and temperature conditions,” Morf explains.
After each iteration, the researchers adjust the density profile of each planet and test it to ensure that the model continues to adhere to the three criteria. “We wanted to bridge the gap between existing physics-based models that are overly constrained and empirical approaches that are too simplified,” Morf explains. Avoiding strict initial assumptions about composition, he says, “lets the physics and data guide the solution [and] allows us to probe a larger parameter space.”
A wide range of possible structures
Based on their models, the UZH astrophysicists concluded that the interiors of Uranus and Neptune could have a wide range of possible structures, encompassing both water-rich and rock-rich configurations. More specifically, their calculations yield rock-to-water ratios of between 0.04-3.92 for Uranus and 0.20-1.78 for Neptune.
The models, which are detailed in Astronomy and Astrophysics, also contain convective regions with ionic water pockets. The presence of such pockets could explain the fact that Uranus and Neptune, unlike Earth, have more than two magnetic poles, as the pockets would generate their own local magnetic dynamos.
Traditional “ice giant” label may be too simple
Overall, the new findings suggest that the traditional “ice giant” label may oversimplify the true nature of Uranus of Neptune, Morf tells Physics World. Instead, these planets could have complex internal structures with compositional gradients and different heat transport mechanisms. Though much uncertainty remains, Morf stresses that Uranus and Neptune – and, by extension, similar intermediate-class planets that may exist in other solar systems – are so poorly understood that any new information about their internal structure is valuable.
Things we don’t know about Uranus (and Neptune)
A dedicated space mission to these outer planets would yield more accurate measurements of the planets’ gravitational and magnetic fields, enabling scientists to refine the limited existing observational data. In the meantime, the UZH researchers are looking for more solutions for the possible interiors of Uranus and Neptune and improving their models to account for additional constraints, such as atmospheric conditions. “Our work will also guide laboratory and theoretical studies on the way materials behave in general at high temperatures and pressures,” Morf says.
