Gas from Uranus reveals it has an icy centre, settling a long-standing planetary debate

For decades, Uranus has harbored a fundamental identity crisis. While textbooks confidently label the seventh planet from the Sun an "ice giant," a persistent faction of planetary scientists has argued that it might actually be a "rock giant."[4][5]

The debate stems from a frustrating lack of data. Because Uranus is wrapped in a thick, opaque blanket of hydrogen and helium gas, peering into its deep interior is virtually impossible.[2][6]

To understand what lies beneath, astronomers must sniff the upper layers of the atmosphere for chemical clues dredging up from the abyss.[2][4]

For years, the key missing ingredient on Uranus was carbon monoxide. On its sister planet, Neptune, carbon monoxide is abundant in the deep atmosphere, strongly indicating a vast interior reservoir of oxygen and water ice.[4]

Uranus, however, appeared stubbornly devoid of deep-atmosphere carbon monoxide. This absence led to a controversial hypothesis: perhaps Uranus was fundamentally different from Neptune, built primarily of dry rock rather than volatile ices.[2][4]

Now, that controversy appears to be settled. A team of astronomers led by Thibault Cavalié at the University of Bordeaux has successfully detected carbon monoxide in Uranus's deep troposphere for the very first time.[1][2][3]

"We find that Uranus is more on the ice-giant side than on the rock-giant side," Cavalié noted following the discovery. "It tells us that this controversy is over now."[2]

The breakthrough relied on the Atacama Large Millimeter/submillimeter Array (ALMA), a sprawling network of radio antennas situated high in the Chilean desert.[2][3]

Between 2022 and 2024, the research team pointed ALMA at Uranus, conducting high-resolution spectral mapping to search for the faint rotational signatures of specific molecules.[3][7]

ALMA's unprecedented sensitivity allowed the team to peer deeper into the Uranian atmosphere than previous infrared or radio observatories.[3]

The data revealed an unambiguous signature of carbon monoxide residing in the troposphere—the lowest, densest layer of the planet's atmosphere where deep internal gases mix.[3]

To understand the implications of this gas, the researchers ran the ALMA data through complex thermochemical and hydrodynamic computer simulations.[1][3]

Carbon monoxide requires a massive supply of oxygen to form. The simulations demonstrated that the only way Uranus could sustain this specific concentration of carbon monoxide is if its deep interior is enriched with oxygen by at least a factor of 11 compared to the primordial solar nebula.[3]

In planetary chemistry, abundant oxygen equates to abundant water ice. The presence of the gas confirms that Uranus's core and mantle are absolutely stuffed with frozen volatiles, firmly cementing its status as a true ice giant.[1][2]

Interestingly, the ALMA observations also solved a secondary mystery regarding gases in the planet's upper atmosphere.[3]

The researchers detected a separate, distinct layer of carbon monoxide, along with hydrogen cyanide, floating high up in the Uranian stratosphere.[3][7]

Because the cold "tropopause" layer acts as a barrier preventing deep gases from rising that high, this stratospheric carbon monoxide had to come from an external source.[7]

The team's models indicate that this upper-atmosphere gas was likely deposited by a massive, rogue comet that smashed into Uranus several centuries ago.[2][3]

By confirming that Uranus and Neptune share similar icy interiors, the findings streamline our understanding of how the outer solar system formed.[1][2]

It suggests both planets coalesced in the same distant, freezing region of the protoplanetary disk—likely along the "carbon monoxide ice line"—before migrating to their current orbits.[4]

While ALMA has provided the crucial remote evidence, planetary scientists emphasize that definitively mapping the interior of Uranus will ultimately require a dedicated spacecraft.[4]

Until a future probe physically plunges into the cyan clouds to sample the gases directly, this submillimeter spectral data stands as the strongest proof yet of the ice giant's true nature.[3][4]