GJ 523b, about 2.5 times Earth's size and 23 times its mass, appears to have only a thin atmosphere despite being large enough to be expected to retain a thick gas layer.
James Webb Space Telescope data, combined with TESS detection and WIYN telescope follow-up, showed the 170-million-year-old planet is unusually dense—roughly 60% Neptune's size but mostly rock around a massive core.
Researchers say that defies standard planet-formation expectations, because worlds reaching around 20 Earth masses typically pull in hydrogen-rich atmospheres, as Jupiter and Saturn did.
Two leading explanations are that intense heat near its star stripped the atmosphere away or that a collision between two planets left behind a hot, rocky remnant with little gas.
Thomas Beatty said GJ 523b could finally define the long-used but loosely applied 'mega-Earth' category and may require geologists and atmospheric scientists to explain how such a world formed.