The Hydrogen Envelope and Supercritical Mantle of Sub-Neptune HD 110067 c
Discovered through high-precision transit photometry by space-based observatories, HD 110067 c orbits its host star at a distance of approximately 0.09 astronomical units, completing a full revolution every 13.6 days. With a radius roughly 2.38 times that of Earth and a bulk mass measuring approximately 5.3 Earth masses, the planet exhibits a mean density of roughly 2.1 grams per cubic centimeter. This density profile immediately excludes a purely terrestrial, silicate-iron composition, pointing instead to a substantial volatile shell wrapping a dense rocky core.
Atmospheric Dynamics and Thermal Structure
The upper atmosphere of HD 110067 c is dominated by molecular hydrogen and helium, punctuated by trace quantities of water vapor, methane, and carbon monoxide. Driven by an equilibrium temperature estimated between 500 and 600 Kelvin, the planetary atmosphere exists in a perpetual chemical equilibrium shaped by photochemical reactions. Solar irradiation from the parent star drives high-altitude hydrocarbon disassociation, producing a fine, microscopic photolytic haze layer that coats the upper stratosphere in a pale, muted grey-cyan cast.
Because the planet is tidally influenced by its proximity to HD 110067, a strong thermal contrast exists between the stellar-facing hemisphere and the anti-stellar nightside. Global circulation models suggest that this heat imbalance drives powerful atmospheric jet streams operating at speeds exceeding several hundred meters per second. These zonal winds transport thermal energy from the sub-stellar point toward the dark hemisphere, smoothing out localized atmospheric collapse and maintaining smooth, parallel atmospheric bands across the planetary disk.
Interior Architecture and Volatile Reservoirs
Beneath the diffuse, low-pressure hydrogen-helium cloud tops lies a rapid transition into high-pressure regimes where atmospheric gas compresses into exotic fluid phases. As altitude decreases, temperatures and pressures cross the critical point of water, transforming lower-mantle volatiles into a dense, supercritical fluid ocean. In this layer, water lacks a distinct boundary between liquid and vapor, existing as a high-density thermal fluid under gigapascal-scale pressures.
At the center of HD 110067 c sits a differentiated core composed primarily of iron-nickel compounds and magnesium silicate minerals. Measuring roughly 1.2 to 1.4 Earth radii, this solid-to-semifluid core accounts for the bulk of the planet's gravitational anchor. The immense weight of the overlying water and gas layers exerts sufficient overburden pressure to squeeze silicates at the mantle-core boundary into high-density mineral phases like bridgmanite and post-perovskite, generating internal frictional heating as the core slowly cools.
Resonances and Orbital Architecture
HD 110067 c is an integral link in one of the most mechanically pristine orbital structures ever detected. The entire six-planet system exists in a chain of unbroken orbital resonances, with planet c locked in a precise 3:2 resonance with inner neighbor HD 110067 b and a 3:2 resonance with outer neighbor HD 110067 d. For every three orbits completed by planet b, planet c completes exactly two orbits, creating periodic gravitational impulses that maintain system stability.
This resonant chain indicates that HD 110067 c has experienced virtually zero orbital migration or dynamical violent scattering since its formation within the protoplanetary disk over four billion years ago. Free from major impacts or catastrophic gravitational disruptions, the planet has retained its primeval hydrogen envelope, serving as a pristine remnant of planetary accretion in volatile-rich circumstellar environments.
