Kepler-362 b: Core Structure of a Sub-Earth World
Kepler-362 b is a sub-Earth rocky exoplanet whose dense silicate crust and 838 kelvin surface temperature challenge interior structure models. Discovered by NASA's Kepler space telescope through transit photometry, this compact world orbits its solar-type host star once every 4.02 days at a semi-major axis of approximately 0.048 astronomical units. Measuring just 0.88 Earth radii — equivalent to a physical diameter of roughly 11,200 kilometers, Kepler-362 b belongs to a distinct planetary class smaller than Earth that endures intense stellar illumination. Located roughly 1,600 light-years away in the constellation Cygnus, the host star Kepler-362 shines with a G-type spectrum. At a distance of less than five percent of an astronomical unit, Kepler-362 b absorbs roughly six times the solar flux that Mercury receives in our Solar System. Stellar radiation drives surface temperatures to an equilibrium value of 838 kelvin (565 degrees Celsius), stripping volatile compounds and leaving behind a bare, airless sphere of iron and rock.
Compressional Physics of a Sub-Earth Iron Core
Determining the internal architecture of Kepler-362 b requires modeling the physical behavior of silicates and metallic alloys compressed under planetary gravity. While smaller in bulk volume than Earth, sub-Earth exoplanets still maintain substantial central hydrostatic pressures. At the center of Kepler-362 b, pressures exceed 150 gigapascals, forcing iron-nickel compounds into dense crystalline metallic phases.
Gravity drives this interior compression. Theoretical mass-radius relationship models, anchored by Birch-Murnaghan equations of state for high-pressure solids, estimate the planetary mass of Kepler-362 b between 0.55 and 0.75 Earth masses. This yields a mean bulk density ranging between 4.8 and 6.2 grams per cubic centimeter, indicating a solid terrestrial world with an uncompressed density higher than Mars. The metallic core accounts for approximately 30 to 40 percent of the planet's total mass. This iron-rich core extends across roughly 55 percent of the planet's physical radius, creating a dense central anchor beneath a compressed mantle.
Silicate Mantle Dynamics and Thermal Convection
Surrounding the metallic core lies a dense silicate mantle dominated by ultramafic minerals. Magnesium-rich olivine and pyroxene form the primary mineral constituents within this convective region. Under internal pressures exceeding 100 gigapascals near the core-mantle boundary, olivine transitions into high-pressure phase polymorphs, primarily wadsleyite, ringwoodite, and silicate perovskite (bridgmanite).
Heat transfer within the mantle proceeds via solid-state thermal convection driven by internal radioisotope decay. Decay of uranium-238, thorium-232, and potassium-40 supplies continuous thermal energy, which combines with residual heat trapped during initial accretion. Because Kepler-362 b orbits in close proximity to its host star, gravitational tidal forces exert a continuous mechanical drag on the planet's interior. This tidal flexing generates additional viscous dissipation heat within the lower mantle, keeping deeper rock ductile despite the small size of the planet.
Surface Volatile Stripping and Crustal Evolution
The equilibrium surface temperature of 838 kelvin severely limits the mineralogical stability of the planetary surface. Volatile compounds including water, carbon dioxide, nitrogen, and methane cannot condense or remain trapped on the surface. During the host star's active juvenile phase, extreme ultraviolet radiation and stellar winds drove rapid photoevaporation, stripping away any initial hydrogen-helium gas envelope.
What remains on the surface is a bare igneous crust composed of dark basalt and magnesium-rich peridotite. Mantle convection drives thermal plumes toward the upper lithosphere, fracturing the surface and feeding magmatic eruptions across the dayside. Without an insulating atmosphere to retain heat, surface thermal energy radiates directly into space. Severe temperature gradients between the illuminated and shadowed hemispheres induce mechanical stress, cracking basaltic rock faces into extensive rubble fields of sharp silicate debris.MQaczT1nMuH8GR78jfxth4hPUfQYyH
Photometric Transit Precision and Mass Limits
Astronomers confirmed Kepler-362 b by measuring the subtle reduction in stellar light produced when the planet transited the disk of its star. The observed transit depth, measuring less than 0.01 percent of the star's total flux, provided an exact physical measurement of the planetary radius at 0.88 Earth radii over a transit duration of approximately 2.1 hours. High photometric stability confirms that the orbital inclination sits near 89 degrees relative to our line of sight.
The central measurement challenge remaining for astronomers is refining the absolute mass measurement of Kepler-362 b. Because the host star shines at an optical magnitude of 14.5, measuring the tiny radial velocity wobble, estimated at less than 0.4 meters per second, requires extended observational campaigns on giant telescopes. Precise radial velocity measurements from spectrographs like ESPRESSO on the Very Large Telescope will narrow the error margin on the planetary mass, allowing planetary geophysicists to pin down the exact iron-to-silicate ratio inside this sub-Earth world.
