Carbon Monoxide Detected in Uranus, Points to Icy Core
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- Thibault Cavalié at the University of Bordeaux and colleagues detected significant amounts of carbon monoxide in Uranus's lower atmosphere using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile, observing the planet three times between 2022 and 2024.
- Modeling the measured carbon monoxide levels required ice-rich interior compositions; rock-heavy models could not reproduce the observations, leading Cavalié to state Uranus is "more on the ice-giant side than on the rock-giant side."
- Previously, Neptune showed abundant carbon monoxide consistent with an ice-rich core while Uranus did not, which led some astronomers to argue Uranus had a predominantly rocky interior — implying the two planets formed in very different ways.
- Carbon monoxide was also detected in Uranus's upper atmosphere, but Cavalié attributes this to a different source — likely a comet that struck the planet several centuries ago — rather than to the deep interior.
- Vanesa Ramirez at Leiden University cautioned that interpreting atmospheric abundances depends on assumptions about chemistry, mixing, and internal structure, and that multiple rock-to-ice ratios remain compatible with the current data, so the question is not fully settled.
Why it matters: If Uranus is indeed ice-rich rather than rocky, the planet and Neptune likely formed through similar processes — overturning a long-standing hypothesis that the two ice giants have fundamentally different origins and reshaping models of how the outer solar system's planets assembled.
