The Swollen Hydrogen Atmosphere and Volatile Mantle of Kepler-36c

Located approximately 1,530 light-years from Earth in the constellation Cygnus, Kepler-36c represents one of the most physically instructive sub-Neptune exoplanets discovered by NASA's Kepler Space Telescope. Orbiting a subgiant host star slightly more evolved than our Sun, Kepler-36c belongs to a class of intermediate-mass worlds that have no direct analogue within our solar system. Radius and mass determinations reveal a planetary envelope characterized by an exceptionally low mean bulk density, standing in stark contrast to its immediate orbital companion. The physical parameters of Kepler-36c provide vital empirical constraints on models of volatile accretion, atmospheric retention, and gravitational dynamics in close-in planetary systems.

Physical Properties and Bulk Composition

Kepler-36c possesses a mass equivalent to approximately 8.1 Earth masses ($8.1 M_\oplus$) and a inflated radius measuring roughly 3.67 Earth radii ($3.67 R_\oplus$). These dimensions yield a mean density of just 0.89 grams per cubic centimeter—considerably less than the density of liquid water and less than one-eighth the density of its inner planetary neighbor, Kepler-36b. This striking disparity in bulk composition between two bodies orbiting the same star at nearly identical semi-major axes remains one of the premier case studies in planetary formation theory.

Spectroscopic and interior modeling indicates that Kepler-36c is composed of a dense, heavy-element core surrounded by a vast, fluid-like mantle rich in volatile compounds such as water, ammonia, and methane. Enveloping this volatile interior is a deep, low-density gaseous atmosphere composed primarily of molecular hydrogen ($H_2$) and helium ($He$). Although the hydrogen-helium envelope accounts for only an estimated 10 percent of the planet's total mass, its vast vertical extent contributes to more than half of the planet’s overall physical radius.

The Swollen Hydrogen Atmosphere and Volatile Mantle of Kepler-36c

Atmospheric Thermal Structure and Dynamics

Positioned at a semi-major axis of approximately 0.128 astronomical units (AU) from its host star, Kepler-36c absorbs a intense flux of stellar radiation. With an equilibrium temperature estimated between 900 K and 1,050 K (roughly 625°C to 775°C), the planet’s upper atmosphere exists in a highly thermalized, expanded state. The extreme thermal heating drives vigorous vertical convection within the troposphere, carrying trace volatile molecules from the high-pressure interior upward into regions of lower pressure.

Photochemical processes active within the upper layers of the atmosphere are inferred to break down species such as methane ($CH_4$) and ammonia ($NH_3$), yielding complex hydrocarbon hazes and secondary compounds. These high-altitude aerosols act as efficient absorbers and scatterers of incoming optical light, generating significant thermal inversions in the upper stratosphere. Theoretical climate models suggest that high-speed zonal winds, driven by the intense day-night radiative imbalance inherent to tidally influenced short-period orbits, transport thermal energy from the star-facing hemisphere across the planet's limb to the dark nightside.


Interior Mechanics: From Supercritical Fluid to Core

Descending through the extended gaseous envelope of Kepler-36c, the boundary between atmosphere and interior becomes poorly defined. As ambient pressure escalates past thousands of atmospheres, the hydrogen-helium phase smoothly transitions from a gaseous state into a dense, supercritical fluid without a discrete liquid-gas surface interface. Farther beneath this convective gas layer lies a deep volatile mantle subjected to gigapascal pressures and temperatures exceeding several thousand Kelvin.

Under these extreme physical conditions, water and other volatiles behave as high-density ionic fluids—often referred to as superionic ice—where oxygen lattices remain fixed while hydrogen ions move freely through the matrix, generating significant electric conductivity. At the planetary center rests a silicate-iron solid core estimated at 1 to 3 Earth masses. The immense heat retained from primordial accretion, supplemented by ongoing tidal dissipation, drives active hydrodynamic convection within the conducting volatile fluids, potentially generating internal magnetic fields capable of shaping the planet's magnetosphere.

Orbital Dynamics and Near-Resonance Gravitational Interactions

Kepler-36c completes one revolution around its host star in approximately 16.2 days. Its planetary neighbor, the rocky super-Earth Kepler-36b, completes an orbit every 13.8 days. This arrangement places the two planets near a 7:6 orbital resonance, with semi-major axes differing by less than 0.013 AU—a distance corresponding to only five times the distance between the Earth and the Moon at their closest approach (conjunction).

Every 97 Earth days, Kepler-36c and Kepler-36b reach conjunction, bringing the two worlds into extreme gravitational proximity. During these encounters, mutual gravitational torques induce significant perturbations in their respective orbital velocities and orbital eccentricities. These perturbations manifest as pronounced transit timing variations (TTVs) exceeding tens of hours, which allowed astronomers to precisely determine the mass of Kepler-36c. The gravitational interactions also induce periodic tidal flexing within the interior of Kepler-36c, converting orbital energy into internal heat and fueling continuous volatile convection within its superheated mantle.


Atmospheric Mass Loss and Long-Term Evolution

Given the intense extreme-ultraviolet (EUV) and X-ray irradiation received from its subgiant host star, Kepler-36c is subject to ongoing hydrodynamic atmospheric escape. High-energy photons ionize molecular hydrogen in the upper thermosphere, heating the gas to temperatures where the thermal velocity exceeds the planet’s local escape velocity. This process produces a steady planetary wind that carries light gases into interplanetary space.

Despite this continuous photoevaporative erosion, the total mass of Kepler-36c ($8.1 M_\oplus$) provides a deep enough gravitational potential well to retain its thick volatile mantle and a substantial fraction of its primordial hydrogen-helium envelope throughout its multi-billion-year lifespan. As the host star continues its stellar evolution along the subgiant branch, increasing stellar luminosity will further heat the atmosphere of Kepler-36c, accelerating mass-loss rates and driving further expansion of its outer gaseous boundaries.

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