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 away by intense tidal forces or runaway evaporation. The orbit is exceptionally tight, with the planet completing a full revolution around its host star in less than 19 hours. This proximity forces the body into a state of permanent thermal extremity, where the surface temperature exceeds 1,500 degrees Celsius, ensuring that any volatile material remaining on the surface remains in a state of high-pressure vapor.

The density of TOI-849 b rivals that of Earth, an extraordinary feat for a world so massive. While a typical Neptune-sized planet would hold onto a massive gaseous envelope, this body is remarkably compact. The gravity is immense, crushing the underlying silicate mantle and metallic core into a highly compressed state. The geologic history of the planet is one of catastrophic loss; the core likely formed early in the protoplanetary disk, accumulating a massive atmosphere that was eventually ablated by the host star's intense radiation or through gravitational interaction during migration toward the center of its system.

Observation of the planet’s transit reveals little in the way of significant gaseous outflow, confirming that it is largely a solid or molten world. Unlike its counterparts in the sub-Neptune regime, there is no evidence of a thick layer of primordial hydrogen. Instead, the spectrum suggests an atmosphere composed of heavy metal vapors and ionized minerals boiled directly off the molten surface. It is a world of stagnant, high-pressure equilibrium, where the distinction between the atmosphere and the molten crust is blurred by the sheer intensity of the radiative environment.

Ultimately, TOI-849 b acts as a laboratory for understanding the limits of planetary mass. It defies standard formation models, forcing a reconsideration of how cores are constructed in the turbulent environment surrounding young stars. It is a lonely, scorched remnant, a testament to the violent forces that can render a once-mighty gas giant into a bare, stripped core of metal and stone.