'Ice giant' planets Uranus and Neptune might not be full of ice after all

Scientists may have missed the mark when they started referring to Uranus and Neptune as the "ice giant" planets of the solar system decades ago. 

Like giving a certain short-armed dinosaur a name that means "terrible lizard king," calling these planets "icy" hasn't aged well. New research from a team at the University of Zurich in Switzerland suggests the nickname was a misnomer, with the two blue worlds in distant space potentially being made of more rock than ice. 

The cold and remote planets originally earned their label of "ice giants" to contrast their interiors from those of Jupiter and Saturn, the so-called "gas giants" rich in hydrogen and helium. Uranus and Neptune are much smaller than their gassy counterparts but also bigger than the terrestrial worlds of Mercury, Venus, Earth, and Mars. 

But little is known about these two medium-size outer planets, which are the least-explored category of worlds in our solar system. NASA's Voyager 2 is the only spacecraft to have visited them, flying by Uranus in 1986 and Neptune in 1989. 

"This name is rather misleading since it implies that the planets are water-dominated in composition," Ravit Helled, an astrophysicist who initiated the study, told Mashable. "The name 'ice giants' also gives the impression that the planets are solid but in fact the materials in the deep interiors can be in liquid state."

The work has implications for the study of exoplanets — worlds that exist around stars other than the sun — and shows that more observations and theory are needed before jumping to conclusions about internal composition. The team's challenge to the "ice giants" category appears in the journal Astronomy & Astrophysics. 

To reach their findings, the researchers built a new way to model what could lie deep within Uranus and Neptune without relying on strict assumptions. They started with many random guesses about how dense each layer might be, according to the paper. Then, they used a step-by-step computer process to adjust those guesses until they matched real measurements of each planet’s gravity. Crucially, the guesses had to follow known rules for how materials behave under pressure and heat.

Their results show that both planets could have very different internal makeups. Some models present a water-dominated scenario, while others look rich in rock. There is no single clear answer about what these planets are mostly made of. 

If the planets were more rocky, that might mean they formed closer to the sun, then migrated farther away. Some scientists have suspected this to be the case, Helled said.

"Many studies on dynamics suggest that Uranus and Neptune formed closer to the sun," she said. 

All of the workable composition models include moving, churning layers made of electrically charged water, called "ionized water." These layers could help explain the strange, lopsided magnetic fields seen around both planets, Helled said. The temperatures inside could stay high enough that hydrogen, helium, and water remain mixed rather than separating.

The interior of Uranus could be more icy, left, or more rocky, right. Credit: Keck Institute for Space Studies / Chuck Carter illustration

The outer layers also differ. Uranus appears to have more hydrogen and helium near the surface than Neptune. The region that produces Uranus' magnetic field likely lies deeper inside the planet than the comparable region in Neptune.

But knowing their true nature will require dedicated missions to the planets, the researchers say. A spacecraft could measure their gravitational fields and atmospheric compositions. For now, it's safe to say the interior structures of medium-size planets are more complex than once thought, and it might be time to retire the "ice giant" moniker. 

"We could keep using this name," Helled said, "as long as people understand that this does not necessarily reflect the planetary composition and material state."