The Stripped Dense Core of Sub-Neptune Exoplanet TOI-849 b
In the vast diversity of sub-Neptunes , most worlds are defined by the thick, volatile shrouds of hydrogen and helium that envelop them. TOI-849 b serves as a stark, singular exception to this rule. Located approximately 730 light-years from our solar system, this exoplanet represents an extreme of planetary evolution . With a radius roughly 3.4 times that of Earth and a mass nearly 39 times greater, it occupies a position in the mass-radius diagram that is virtually empty. It is far too dense to be a standard gas-shrouded sub-Neptune, suggesting that it is not a world still in its infancy, but rather the exposed, skeletal remains of a gas giant that once possessed a far more bloated atmosphere. The physical composition of TOI-849 b is primarily iron, rock, and perhaps a small fraction of water ice at its deep interior. Astronomers classify this world as a " chthonian" planet —the exposed core of a larger gaseous entity whose outer layers were stripped awa...