The Dense Iron Core and Scorched Basalt Plains of Exoplanet GJ 367 b

Located approximately 31 light-years from Earth in the southern constellation of Vela, the exoplanet GJ 367 b represents one of the most structurally dense rocky worlds ever confirmed by modern astronomy. Discovered in 2021 through data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS) and subsequently characterized using ultra-precise radial-velocity measurements from the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph, this world belongs to an extraordinary class of planets known as ultra-short period (USP) sub-Earths. Orbiting its host star in just 7.7 hours, GJ 367 b offers planetary scientists a rare laboratory for studying planetary structure under conditions of extreme stellar proximity.

With a radius roughly 72 percent that of Earth and a mass equivalent to 63 percent of Earth's mass, GJ 367 b exhibits a calculated bulk density of approximately 10.2 grams per cubic centimeter. This density is significantly higher than Earth’s bulk density of 5.51 grams per cubic centimeter and even surpasses the density of pure iron at sea-level pressures. Planetary interior modeling indicates that GJ 367 b is dominated by a colossal, iron-rich core that constitutes between 86 and 91 percent of its total radius—a ratio far exceeding that of Mercury, the Solar System’s most iron-dense terrestrial planet. Consequently, GJ 367 b is frequently categorized as an extreme 'super-Mercury.'

The Dense Iron Core and Scorched Basalt Plains of Exoplanet GJ 367 b

Orbital Dynamics and Extreme Thermal Conditions

The orbital parameters of GJ 367 b put it in extreme physical conditions. The planet circles its host star—a cool, low-mass M-dwarf (red dwarf) designated GJ 367—at a distance of a mere 1.05 million kilometers, or approximately 0.0071 astronomical units (AU). For context, this is less than one-hundredth of the distance between Mercury and the Sun. At this short distance, gravitational tidal forces have locked the planet into a synchronous rotation, causing one side to face the host star eternally while the other remains in perpetual darkness.

Because of its extreme proximity, the day-side of GJ 367 b is blasted with stellar irradiation more than 500 times greater than the solar energy received by Earth. Equilibrium temperature calculations and mid-infrared thermal emission measurements reveal that the day-side surface reaches temperatures between 1,300 and 1,500 Kelvin (approximately 1,000 to 1,200 degrees Celsius). At these temperatures, volatile compounds cannot remain in a solid state, and silicate rocks at the sub-stellar point may softened into viscous, semi-molten sheets or undergo direct thermal erosion under vacuum conditions.

The Dense Iron Core and Scorched Basalt Plains of Exoplanet GJ 367 b - Section 2 overview

Internal Architecture and Mantle Stripping Hypotheses

The disproportionately large iron core of GJ 367 b presents a compelling mystery in planetary formation and evolution. Standard protoplanetary disk accretion models suggest that planets forming from solar-composition gas and dust should possess a metallic core comprising no more than 30 to 40 percent of their overall mass, surrounded by a thick silicate mantle. To account for a world where the metal core represents upwards of 70 percent of total mass, astrophysics researchers have proposed several formation scenarios.

The leading hypothesis involves catastrophic impact stripping. Early in the evolution of the planetary system, a protoplanet with a normal core-to-mantle ratio may have suffered one or more hypervelocity collisions with other large planetesimals. These energetic impacts could have blasted away the lighter outer silicate mantle, leaving behind the denser, iron-dominated core structure observed today. Alternatively, the young host star during its early, highly energetic phase may have generated intensely high-energy ultraviolet and X-ray radiation (XUV), driving photoevaporation that removed the planet's volatile layers, combined with localized high-temperature condensation processes in the innermost disk region during initial accretion.

The Dense Iron Core and Scorched Basalt Plains of Exoplanet GJ 367 b - Section 3 overview

Atmospheric Vacuum and Surface Environment

Observations conducted with thermal emission photometry—including thermal light curve analyses from space telescopes such as the James Webb Space Telescope (JWST)—indicate that GJ 367 b lacks a thick, volatile atmosphere. An atmosphere containing carbon dioxide, water vapor, or nitrogen would efficiently redistribute heat from the day-side to the night-side hemisphere. However, observed thermal profiles show a high day-to-night temperature contrast, characteristic of an unshielded, airless world in direct contact with space vacuum.

The physical surface of GJ 367 b is an unyielding, vacuum-exposed landscape composed primarily of dark basaltic minerals, iron-nickel grains, and oxidized metallic compounds. Without an atmospheric buffer, solar wind ions and cosmic radiation constantly bombard the dark regolith, causing space weathering and chemical reduction on the exposed rock faces. On the night-side, temperatures drop sharply toward cryogenic levels near absolute zero, creating a dramatic planetary contrast between a frozen, pitch-black midnight hemisphere and an intensely heated, crimson-glowing day-side facing the close-up red dwarf host star.

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