🪐 The most common planets in the Galaxy are so... strange

When we talk about rocky planets, we often imagine Earth: a dense metallic core, a silicate mantle, and a thin atmosphere. Yet, this familiar structure might be an exception in the Universe, according to a new study submitted to the Astrophysical Journal.

Astronomers have long assumed that rocky exoplanets follow the same pattern as our Solar System. But the majority of planets discovered around other stars are sub-Neptunes or super-Earths, worlds larger than Earth but smaller than Neptune. Their formation should have been similar, with iron in the center, silicate above, and hydrogen on the surface. However, reality is quite different.


Inside these planets, extreme pressures and temperatures change the game. Above 4000 degrees Kelvin, hydrogen and molten silicate become completely miscible, like water and alcohol. They no longer form two separate layers, but a single homogeneous fluid. If a planet accumulates more than one percent of its mass in hydrogen, its interior becomes a unique mixture of iron, silicate, and hydrogen, with no distinct core or mantle.

This homogeneous structure has major consequences for the planet's evolution: it influences its cooling, its ability to retain its atmosphere, and how its radius changes over time. The authors of the study show that this miscibility model naturally explains intriguing observations, such as the "radius gap" separating super-Earths from sub-Neptunes, as well as the dependence of radius on orbital period.

A testable prediction emerges from this work: if hydrogen gradually escapes from the interior to join the atmosphere, young sub-Neptunes should appear more inflated than expected for their age. Current observations with the James Webb Space Telescope and future transit missions could soon confirm or refute this signature.

Of course, reservations remain. The model relies on theoretical extrapolations of the behavior of hydrogen, silicate, and iron under conditions still inaccessible in the laboratory, even though high-pressure experiments are progressing. Internal heat budgets remain uncertain, and the statistical approach used can only provide a probable picture, not a certainty.

Ultimately, the claim is bold: the most common planet in the Galaxy might not resemble Earth at all. The very idea of a planetary core, that small dense metallic heart we take for granted, could be the exception rather than the rule. In this scenario, it is our own planet that would be the oddity.