Uranus and Neptune may be more rocky than icy, new study reveals

Researchers from the University of Zurich and the NCCR PlanetS are challenging what we know about the planets of our solar system by suggesting that Uranus and Neptune may be more rocky than icy. While their study, published in the journal Astronomy & Astrophysics (A&A), does not confirm that the planets are one or the other, it challenges the idea that they are ice-rich alone. This aligns with the discovery that Pluto, the dwarf planet, is predominantly rocky.

“The ice giant classification is oversimplified as Uranus and Neptune are still poorly understood,” said Luca Morf, PhD student at the University of Zurich and the lead author of the study, per UZH News. “Models based on physics were too assumption-heavy, while empirical models are too simplistic.” So the team combined both methods to develop unbiased, yet physically consistent models.

To this end, they randomly assigned a density profile for the planetary interior, after which they calculated the planetary gravitational field, which aligned with the observational data. Once a possible composition was inferred, the process was then repeated to acquire the best match between the observational data and models. Their model revealed that the potential interior composition of the planets is not limited to ice. “It is something that we first suggested nearly 15 years ago, and now we have the numerical framework to demonstrate it,” stated Ravit Helled, a professor at the University of Zurich.

The model’s new range of internal composition reveals that both planets are capable of being either water-rich or rock-rich. The results can also be an addition to the perspectives on the magnetic fields of Uranus and Neptune. They are unique, as compared to the North and South magnetic poles of Earth. These planets have more complex fields with more than two poles. “Our models have so-called 'ionic water' layers which generate magnetic dynamos in locations that explain the observed non-dipolar magnetic fields,” explained Helled. The study also observed Uranus’ magnetic field to originate much deeper compared to Neptune. 

Much like a lot of studies, the results, though promising, are prone to certain uncertainties. The primary reason is that the physicists are not entirely clear on how materials behave under the pressure and temperature found at the planet’s core. Despite the limitations, the results also open the doors to considering the possibility of a new potential interior composition. This challenges the age-old theories on the giants, thereby aiding future research on planetary conditions.

Morf expressed his plans to expand the models in the future to address the uncertainties that exist. Helled also expressed how, depending on the assumptions made by the model, both Uranus and Neptune can be rock giants or ice giants. “Current data are currently insufficient to distinguish the two, and we therefore need dedicated missions to Uranus and Neptune that can reveal their true nature,” Helled explained. 

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