GJ 523b: A colossal rocky world defying formation expectations

Astronomers have identified GJ 523b, an unusually dense exoplanet about 2.5 times Earth’s radius and roughly 23–24 times Earth’s mass, making it a so‑called mega‑Earth with a predominantly rocky composition and little atmosphere. Its high density challenges standard planet‑formation models that predict gas envelopes once cores reach ~20 Earth masses; explanations include atmospheric loss or a giant‑impact origin. The planet orbits its star every 17.75 days in a relatively young system (~170 million years old), with the discovery arising from NASA’s TESS data and ground‑based WIYN follow‑up observations.