What is it about?
Most rocky planets are assumed to have an Earth-like layered structure, with a dense metallic core surrounded by a rocky mantle. Our study shows that this assumption may break down for large rocky exoplanets. Under the extremely high pressures and temperatures inside these planets, metallic iron and molten rock can become increasingly soluble in each other and may eventually mix into a single liquid. As a result, sufficiently massive rocky planets could avoid forming, or lose, a distinct metallic core. This provides a new physical pathway for producing “coreless” planets that does not require unusually oxidized or water-rich starting materials.
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Why is it important?
The conventional picture of rocky planets is largely based on the Solar System, where planets such as Earth, Venus and Mars are separated into metallic cores and silicate mantles. Our results suggest that this familiar architecture may not apply to more massive rocky worlds. The key difference is pressure: conditions inside super-Earths can reach regimes that have no counterpart in the terrestrial planets of our Solar System and can fundamentally change how metal and rock interact. This finding expands the range of possible exoplanet interior structures and could affect how planetary mass and radius are interpreted, as well as our understanding of magnetic fields, thermal evolution, mantle chemistry and atmosphere–interior interactions.
Perspectives
What excites me most about this work is that it shows how studying exoplanets can challenge ideas that seem almost self-evident from our experience with the Solar System. We normally think of a rocky planet as having a metallic core and a silicate mantle, because that is what we see in the terrestrial planets around us. But super-Earths explore pressures and temperatures that nature does not provide inside Earth, and the basic distinction between “metal” and “rock” may begin to disappear. To me, this is a good example of why exoplanets are more than distant versions of familiar planets: they allow us to test planetary physics in entirely new regimes and, in turn, rethink what a rocky planet can be.
Haiyang Luo
Nanjing University
Read the Original
This page is a summary of: Coreless exoplanets induced by metal–silicate miscibility, Proceedings of the National Academy of Sciences, August 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2615884123.
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