The Dense Iron Core and Scorched Crust of Exoplanet

The Dense Iron Core and Scorched Crust of Exoplanet Gliese 367 b

In the constellation Vela, approximately 31 light-years from Earth, orbits one of the most physically extreme terrestrial worlds discovered to date: Gliese 367 b (cataloged as GJ 367 b). Classified as an ultra-short period (USP) sub-Earth exoplanet, GJ 367 b completes a full revolution around its parent M-dwarf star in just 7.7 hours. This hyper-velocity orbit places the world mere 1.05 million kilometers from the stellar surface, exposing its bare, rocky surface to relentless bombardment from stellar radiation, severe tidal forces, and intense thermal energy.

First detected by NASA’s Transiting Exoplanet Survey Satellite (TESS) and subsequently characterized through high-precision radial velocity measurements using the HARPS spectrograph, GJ 367 b presents an uncommon physical baseline. Despite having a radius only 70 percent that of Earth, its mass remains exceptionally high at roughly 55 percent of Earth's mass. This combination translates to an extraordinary bulk density of approximately 10.4 grams per cubic centimeter—a figure far exceeding that of Earth or Mercury and approaching the density of solid iron.

Internal Structure and Mantle Depletion

The remarkably high density of GJ 367 b indicates a severely altered internal architecture. Standard planetary structure models indicate that the planet is dominated by an oversized metallic iron-nickel core that accounts for roughly 86 percent of its total radius. This vast core is encased in only a thin, depleted shell of silicate rocks and mantle minerals, creating a structural ratio that far exceeds even Mercury’s iron-rich internal proportions.

Astrophysical simulations suggest two primary formation pathways for this metallic concentration. The planet may have initially coalesced as a larger terrestrial world with a conventional mantle-to-core ratio, which was subsequently stripped away by giant impacts during the early, chaotic phase of the planetary system's assembly. Alternatively, intense photoevaporation driven by the host star's volatile high-energy radiation during its active youth may have vaporized and eroded the planet's outer silicate mantle over millions of years, leaving behind a dense, heavy core fragment dominated by iron alloys.

Thermal Profile and Tidal Dynamics

Because GJ 367 b orbits so close to its parent M-dwarf star, its orbital period and rotational period have synchronized through gravitational interactions, resulting in full tidal locking. One hemisphere permanently faces the stellar furnace, while the other faces the absolute cold of deep space. Without a thick atmosphere to redistribute thermal energy across the globe, the temperature differential between the dayside and nightside is extreme.

The equilibrium temperature on the dayside of GJ 367 b is estimated to reach approximately 1,500 Kelvin (1,226 degrees Celsius). At these temperatures, volatile compounds, water ice, and light gaseous elements cannot exist in any state. Silicate minerals on the stellar-facing hemisphere reside near or above their melting points, causing surface rocks to breakdown, crack, or transform into localized sheets of liquid basalt and iron slurry. Conversely, the nightside, perpetually shielded from stellar radiation, plunges to temperatures near absolute zero, leaving any solid rock encased in permanent darkness.

Atmospheric Absence and Surface Vapor Dynamics

Spectroscopic evaluations and physical models confirm that GJ 367 b lacks a substantial secondary atmosphere. Any primordial envelope of hydrogen and helium, as well as outgassed carbon dioxide or nitrogen, would have been swept away by stellar wind pressure and thermal escape long ago due to the planet's weak gravitational escape velocity relative to its intense surface thermal energy.

However, the scorching conditions on the dayside generate a tenuous, transient micro-atmosphere composed entirely of vaporized rock and metal. As temperatures peak at the sub-stellar point, elements such as silicon, iron, magnesium, and sodium sublime directly from the surface crust into a low-density gas vapor. As planetary rotation and local thermal expansion drive these mineral gases toward the cooler twilight boundary (the terminator line), the vaporized metals and silicates cool rapidly, condensing and raining back onto the dark, frozen crust as fine mineral dust or metallic grains.

Scientific Impact on Exoplanetary Evolution

GJ 367 b serves as an irreplaceable natural laboratory for planetary physics. Its existence demonstrates that sub-Earth worlds can survive in close proximity to low-mass M-dwarf stars, surviving intense radiation regimes while retaining high-density metallic cores. The structural parameters of GJ 367 b continue to refine theoretical models regarding protoplanetary disk chemistry, mantle stripping mechanics, and the ultimate environmental limit of rocky exoplanets orbiting near host stars.

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