"Unusually dense" exoplanet the size of Neptune observed

Neptune has more than three times the diameter of the Earth but is much less dense. A planet orbiting a star 726.5 light years away is just as big, but even denser.

Using NASA's Transiting Exoplanet Survey Satellite (TESS), an international team of astronomers has discovered a new massive and dense exoplanet. The newfound alien world, designated TOI-332 b, is one of the densest Neptune-sized planets known to date. The finding is reported in a paper published August 23 on the pre-print server arXiv. … TOI-332 b has a radius of about 3.2 Earth radii and an unusually large mass of 57.2 Earth masses, what yields a very high density—at a level of 9.6 g/cm3. The newfound planet orbits its host every 18.65 hours, at a distance of 0.016 AU from it. The equilibrium temperature of TOI-332 b was calculated to be around 1,871 K.

Earth's density is 5.51 g/cm³. TOI-332 b is therefore "potentially more similar to that of terrestrial planets" than gas giants, which is to say it is a big-ass hot rock that just crushed the table where they keep the planetary formation hypotheses. "How such a giant core exists without a gaseous envelope remains an unanswered question," write the paper's authors.

A Cthonian planet, perhaps?