On a rocky "mega-Earth" 40 times our planet's mass, gravity is so crushing that mountains collapse under their own weight, and pent-up heat periodically melts the entire surface.
Planetary scientists once believed worlds like this were impossible, assuming anything beyond a certain mass limit would inevitably sweep up massive amounts of gas to become a Neptune-like gas giant. Then came the discovery of these ultra-dense anomalies, composed entirely of solid rock and metal.
Because these worlds exist on the extreme outer edge of what physics allows for a terrestrial planet, their surfaces bear no resemblance to anything in our solar system. The defining feature of a mega-Earth is its intense gravitational pull. A world like TOI-849 b—a prime mega-Earth candidate weighing roughly 40 Earth masses—flattens its own topography. High-elevation crust simply cannot support its own weight, causing any jagged peaks to slump and flow back down. The landscape is likely characterized by sweeping, shallow rolling plains of ultra-compressed silicates and metals.
This immense mass also traps enormous amounts of internal radiogenic heat. On Earth, the crust fractures into tectonic plates, which drift and allow heat to escape. On a mega-Earth, the immense pressure of gravity likely fuses the mantle and crust into a highly viscous, rigid layer known as a "stagnant lid." Tectonic plates cannot form because the crust is simply too heavy to shift. Instead, heat builds up in the deep interior for millions of years until the pressure becomes unbearable. This triggers catastrophic, planet-wide volcanic eruptions that melt and recycle the entire surface before cooling into a solid crust again.
If a mega-Earth manages to retain any atmosphere after forming, or vents a secondary atmosphere of carbon dioxide and vaporized rock, the surface pressure could be hundreds of times higher than Earth's. In scenarios where a mega-Earth formed beyond the snow line and migrated inward, it might be a water world. However, the crushing gravity would force the water at the ocean floor into an exotic state called Ice VII. This high-pressure ice is solid at temperatures far above water’s normal boiling point, meaning the "surface" of the planet is a hot, rigid layer of strange ice rather than rock.