Barnard d
Astrophysical Synthesis & Habitability Characterization
Barnard d is a confirmed sub-earth exoplanet orbiting the star Barnard's star, located approximately 6 light-years (1.83 parsecs) from the Solar System. First announced in 2025 by Gemini Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 0.694 Earth radii and has a measured mass of 0.263 Earth masses. Combining its mass and radius yields a mean bulk density of 4.34 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 0.55 g⊕ (5.39 m/s²), and an escape velocity of 6.9 km/s.
In its orbital architecture around Barnard's star, Barnard d completes one full revolution every 2.34 Earth days at a semi-major axis of 0.0188 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 87.4 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.04 (ranging from 0.018 AU at periapsis to 0.0196 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 483 K (+210°C / +410°F) and receiving 10.04 S⊕ (~13,670 W/m²), Barnard d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Barnard's star is classified as a low-mass red dwarf of spectral type M3.5-4 V with an effective photospheric temperature of 3,195 K, a radius of 0.185 R☉, a mass of 0.162 M☉, an estimated bolometric luminosity of 0.00320 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.56 dex (metal-poor relative to the Sun). From Earth, Barnard's star shines at apparent V-band magnitude 9.54 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere. The Barnard's star system is a multi-planet architecture hosting 4 confirmed worlds, with Barnard d sharing its stellar neighborhood alongside Barnard b, Barnard c, Barnard e.
Methodology: Computed using the Schulze-Makuch et al. (2011) weighted metric comparing physical interior (radius, estimated density) and surface thermal regime (escape velocity, equilibrium temperature).
Discovery Facility: Gemini Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Barnard's star |
| Spectral Type | M3.5-4 V |
| Effective Temp (Teff) | 3,195 K |
| Stellar Radius | 0.1850 R☉ |
| Stellar Mass | 0.1620 M☉ |
| Bolometric Luminosity | 0.00320 L☉ |
| Metallicity [Fe/H] | -0.560 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 9.540 · G: 8.195 |
| System Architecture | 1 Star, 4 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.018800 AU |
| Orbital Period | 2.34 days |
| Mean Orbital Velocity | 87.4 km/s |
| Orbital Eccentricity (e) | 0.04000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 4.34 g/cm³ (6.9 km/s vesc) |
| Insolation Flux | 10.0441 S⊕ (~13,670 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 6.000 LY (1.827 pc) |