Kepler-262 b: Desiccated Super-Earth Architecture Inside the Fulton Gap

Kepler-262 b is a scorched rocky super-Earth orbiting its host star every 13.2 days at an equilibrium temperature of 734 kelvin. NASA's Kepler space telescope identified this exoplanet by measuring the tiny drop in starlight caused as the world transited its G-type parent star. High-precision photometry confirmed a planetary radius of 1.36 Earth radii. That dimension places Kepler-262 b squarely within a critical physical regime of exoplanet structural evolution. Worlds of this specific scale sit right at the boundary separating high-density, rocky terrestrial planets from inflated, low-density sub-Neptunes wrapped in thick gaseous envelopes.

Evaluating a World Below the Radius Valley

Planetary scientists map exoplanet sizes across the galaxy to understand how planetary systems assemble. Statistical distribution plots reveal a clear dip in planet occurrence between 1.5 and 2.0 Earth radii. Researchers call this feature the Fulton gap, or the photoevaporation radius valley. Worlds larger than 2.0 Earth radii generally retain massive envelopes of hydrogen and helium gas. Worlds smaller than 1.5 Earth radii consist primarily of dense rock and metal. Kepler-262 b sits clearly on the terrestrial side of this divide at 1.36 Earth radii.

Kepler-262 b orbits its host star at a distance of approximately 0.108 astronomical units. At this close proximity, intense stellar irradiation warms the planetary surface day after day. Radiative equilibrium calculations yield a theoretical surface temperature near 734 kelvin (461 degrees Celsius). That heat exceeds the melting point of lead and tin. Surface volatiles like water, ammonia, and methane cannot condense into stable liquids or ice sheets under these physical conditions.

Mass constraints derived from planet mass-radius relations place Kepler-262 b between 2.5 and 3.8 Earth masses. That mass range generates a surface gravity between 1.35 and 1.85 times Earth standard gravity. Dense silicate minerals and iron dominate the planetary interior. Iron and nickel form a high-density metallic core, surrounded by a thick mantle composed of magnesium- and iron-rich silicates.

Photoevaporation and the Loss of Primordial Gas

Reconstructing the birth of Kepler-262 b requires examining conditions during the early history of the system. Protoplanetary cores that grow to several Earth masses inside a gas-rich circumstellar disk accrete solid planetesimals alongside lightweight hydrogen and helium gas. A core reaching three Earth masses typically captures a primordial atmosphere accounting for one to five percent of the planet's total mass. That small percentage doubles the observable radius of the planet.

That primordial gas envelope did not survive on Kepler-262 b. During the first hundred million years after host star formation, young G-type stars emit strong bursts of extreme-ultraviolet (EUV) and X-ray radiation. High-energy XUV photons bombarded the upper atmosphere of Kepler-262 b, heating gas particles to temperatures exceeding ten thousand kelvin. This intense thermal energy drove hydrodynamic escape.

Atmospheric gas expanded rapidly, reaching thermal velocities higher than the planet's gravitational escape velocity. Stellar winds dragged the escaping hydrogen into interstellar space over a period of tens to hundreds of millions of years. Planetary gravity held the solid iron-silicate core together while radiation stripped away the volatile gaseous shell. What remains today is a bare core stripped of its primordial atmosphere.

Crustal Volatiles and High-Temperature Silicates

Without a primordial hydrogen shell, any remaining gaseous envelope on Kepler-262 b must originate from secondary planetary processes. Interior mantle outgassing and continuous volcanic activity can release carbon dioxide, sulfur dioxide, or heavy nitrogen compounds trapped in mantle minerals during accretion. Gravity wins that atmospheric retention battle for heavier molecular gases far more easily than for lightweight hydrogen.

Tidal forces operating over billions of years have circularized the orbit of Kepler-262 b and forced its rotation into a tidally locked state. Synchronous rotation means one hemisphere faces the host star in permanent daylight, while the opposite hemisphere faces dark space. The dayside experiences relentless stellar heating, producing strong thermal gradients across the planetary surface.

At 734 kelvin, secondary atmospheres face relentless solar wind erosion and thermal escape. Lightweight outgassed molecules break down under stellar ultraviolet light. If a secondary atmosphere exists on Kepler-262 b today, surface pressure likely measures a fraction of a bar, maintained by heavy carbon or sulfur volatiles off-gassed from the interior lithosphere.

The crust itself bears the physical marks of extreme thermal loading. High-temperature silicate minerals like pyroxene, olivine, and calcium-rich plagioclase feldspar form the primary surface lithosphere. Continuous thermal expansion and compression crack the dry rock surface, forming vast expanses of dark basaltic plains, heat-stressed fracturing networks, and dense silicate ridges.

Tidal Gravitational Drag and Surface Constraints

Disambiguating the exact surface conditions on Kepler-262 b requires precise thermal phase curve observations. Photometric transits establish planetary radius relative to the host star, but thermal emission curves track heat transport across the dayside and nightside hemispheres. A thick secondary atmosphere would transfer heat around to the nightside through atmospheric circulation, lowering the dayside thermal peak. A bare, airless rocky surface radiates heat directly back into space, producing a sharp thermal contrast between hemispheres.

Current physical boundary conditions constrain the semi-major axis of Kepler-262 b to 0.108 AU, with an orbital eccentricity near zero driven by long-term tidal drag. The planetary mass-radius profile confirms a high bulk density consistent with iron-silicate rock. Resolving whether Kepler-262 b retains a thin carbon-rich atmosphere or exists as a dry, airless slab of baked basalt depends on upcoming space-based infrared phase curve measurements that isolate the planet's thermal light from its host star.