Uranus and Neptune are the two furthest planets in the Solar System and have been visited only once by human spacecraft – by Voyager 2 over 30 years ago – so there is a lot about them that we do not know. One thing we thought we knew, however, was what type of planet they are. Now, a new study wants to challenge something quite crucial about how we classify them: these worlds, it argues, are not ice giants.

The four "rocky" planets of the Solar System – Earth, Mars, Venus, and Mercury – are small terrestrial planets made of solid rock and metal. The four giant planets – Jupiter, Saturn, Uranus, and Neptune – are divided into two categories because, though large, they are not the same. The first two are "gas giants" because they are overwhelmingly composed of hydrogen and helium, over 90 percent by mass. The latter two are known as "ice giants". Hydrogen and helium comprise less than 20 percent of the planets' masses. Uranus and Neptune, instead, are rich in numerous molecules like water and ammonia that were present in solid ice when the planets formed billions of years ago, as models tell us.

For decades, our understanding of Uranus and Neptune's interiors has been based on what we can glean from their surface features, the behaviors of their moons, their magnetic fields, and other indirect means. Which has occasionally led us astray. 

If I was one of these billionaires… just floating around with all my money, I would fund two missions: I'd fund an orbiter to Uranus and an orbiter to Neptune.
— Prof Brian Cox

This new work suggests a different way of looking at these outer planets. Instead of modeling the interior of these two worlds based on the potentially flawed information we have, they created random models and then compared those to observational data, building a catalog of models that fit. They looked at both water-dominated and rock-dominated scenarios and concluded that while the planets have this mixture of molecules, a rockier internal structure makes more sense with the current observations. 

“Overall, our findings challenge the conventional classification of Uranus and Neptune as ’ice giants’ and underscore the need for improved observational data or formation constraints to break compositional degeneracy,” the authors write in their paper.