At 23.5 times Earth’s mass, GJ 523b is massive enough that standard planet-formation models struggle to explain why it didn’t accumulate a huge hydrogen-helium envelope — yet the planet is extraordinarily dense and apparently mostly rock, and astronomers still don’t know how it ended up that way.

GJ 523b packs 23.5 Earth masses into a world only 2.55 Earth radii wide, challenging models that expect such a massive young planet to retain a deep hydrogen-helium envelope. The post At 23.5 times Earth’s mass, GJ 523b is massive enough that standard planet-…