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 rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

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

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