The High Density Ice Crust of Exoplanet LHS 1140 b
The physical parameters of LHS 1140 b present a dense, high-gravity environment. The planet's bulk density is estimated at roughly 5.9 grams per cubic centimeter, comparable to or slightly exceeding Earth's mean density. Initial modeling suggested an iron-dominated metallic core encased within a thick silicate mantle. However, recent space-based spectroscopic observations from instruments such as the James Webb Space Telescope (JWST) have refined this internal picture. Rather than a pure rock-and-iron composition, the planet's bulk density combined with its transit depth profile strongly points toward a world possessing a massive liquid or solid water envelope comprising up to 10 to 20 percent of its total mass, capped by an extensive glacial crust.
Internal Stratigraphy and Mass-Radius Dynamics
The internal architecture of LHS 1140 b is shaped by intense gravitational compression and tidal stresses. At its center lies a differentiated iron-nickel core estimated to account for roughly 30 percent of the total planetary mass. Surrounding this metallic center is a high-density mantle composed of compressed magnesium-silicate minerals, including high-pressure phases like bridgmanite and post-perovskite. Above the silicate mantle, extreme pressure forces water into exotic high-pressure crystalline ice phases, such as Ice VI and Ice VII, forming a rigid inner ice shell tens of kilometers thick.
Surface gravity on LHS 1140 b is estimated at approximately 1.8 times that of Earth ($1.8\,g$), significantly compressing any overlying atmospheric or volatile layers. This high surface gravity increases the atmospheric scale height scale factor, keeping the gases close to the planetary crust and resisting thermal escape despite the radiation environment imposed by its parent M-dwarf star. Unlike more active red dwarfs, LHS 1140 is a photometrically quiet star with low magnetic activity and minimal flare events, allowing LHS 1140 b to retain its volatile inventory over billions of years without devastating atmospheric erosion.
Orbital Configuration and Thermal Dynamics
LHS 1140 b completes a single orbit around its host star in just 24.7 days at a mean orbital radius of approximately 0.0936 Astronomical Units (AU). Because of its close proximity to the M-dwarf host, tidal friction over hundreds of millions of years has locked the planet into a synchronous, 1:1 spin-orbit resonance. Consequently, LHS 1140 b presents the same hemispheric face to its sun at all times, establishing a permanent day-night thermal divide across its surface.
This tidal locking creates extreme climate zoning. The substellar point—the region directly beneath the host star—receives continuous, low-intensity stellar irradiation. Thermal equilibrium calculations indicate that while the perpetual night side remains frozen beneath thick global ice sheets at temperatures far below freezing, the substellar zone absorbs sufficient stellar flux to melt surface volatiles. Models suggest this produces an "eyeball world" morphology: a global frozen surface interrupted by a localized ocean basin at the day-side pole, measuring hundreds of kilometers across, surrounded by fractured ice margins.
Atmospheric Composition and Photochemical Processing
Transmission spectroscopy of LHS 1140 b has revealed key insights into its atmospheric chemistry. Data indicates the absence of a primary, low-density hydrogen-helium envelope, which was likely stripped away during the system's early formation phase. Instead, the planet possesses a secondary atmosphere characterized by a high mean molecular weight, dominated principally by molecular nitrogen ($N_2$), with secondary trace fractions of carbon dioxide ($CO_2$) and water vapor ($H_2O$).
The high atmospheric pressure at the base of the nitrogen atmosphere creates substantial Rayleigh scattering, giving the sky a deep, dense scatter profile when viewed through light cast by the crimson host star. Photochemical interactions between stellar ultraviolet radiation and trace carbon dioxide induce chemical reactions in the upper stratosphere, forming thin layers of hydrocarbon haze and carbon monoxide. Heat redistribution via atmospheric advective currents carries thermal energy from the substellar day-side ocean toward the cold night-side ice cap, driving violent, continuous global winds along the terminator boundary where the frozen crust transitions into liquid seas.
