When you buy through links on our articles, Future and its syndication partners may earn a commission.Credit: forplayday/iStock/Getty ImagesWe have learned a lot about the planets in our own backyard, and for a long time we assumed the rest of the galaxy looked roughly the same. A rocky planet meant a clear-cut structure: a dense metallic core, a silicate mantle, and a thin atmosphere on top. That picture works fine for Earth.But according to a new paper submitted to the Astrophysical Journal, it might not work for most of the rocky planets in the universe. By far the most common type of planet we have found around other stars is about a class of worlds called sub-Neptunes: planets larger than Earth but smaller than Neptune. Their close cousins, the super-Earths, are slightly smaller and likely lost most of their hydrogen long ago. The textbook story has these planets forming in essentially the same way Earth did, just with different amounts of leftover gas piled on top. Iron sinks to the middle, silicate rock floats above it, hydrogen sits on top of that.AdvertisementAdvertisementBut here is the wrinkle. At the pressures and temperatures inside a sub-Neptune, hydrogen, silicate, and iron don’t actually behave like they do near the surface of Earth. Above about 4,000 degrees Kelvin, hydrogen and molten silicate become fully miscible. They stop being oil and water. They become one fluid. The authors behind a new study submitted to the Astrophysical Journal and currently available on arXiv worked out what that means for the structure of these planets, and the answer is surprising.If a planet accretes less than about one percent of its mass in hydrogen, it follows the familiar script and forms a discrete metallic core just like Earth. But if it picks up more hydrogen than that, the whole inside of the planet becomes a single, mixed, churning fluid of iron, silicate, and hydrogen. No core. No mantle. Just a homogeneous blend all the way down to within a few thousand kilometers of the center.That is a significant departure from how we usually draw these worlds in cross-section. The internal structure determines how a planet cools, how it holds onto its atmosphere, and how its radius evolves over time. The authors find that this miscibility framework ca …
