HD 40307 g
Astrophysical Synthesis & Habitability Characterization
HD 40307 g is a confirmed sub-neptune exoplanet orbiting the star HD 40307, located approximately 42.2 light-years (12.94 parsecs) from the Solar System. First announced in 2013 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 2.56 Earth radii and has a measured mass of 7.1 Earth masses. Combining its mass and radius yields a mean bulk density of 2.33 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.08 g⊕ (10.59 m/s²), and an escape velocity of 18.6 km/s.
In its orbital architecture around HD 40307, HD 40307 g completes one full revolution every 197.8 Earth days at a semi-major axis of 0.6 AU (situated at an orbital separation between Mercury's 0.39 AU and Venus's 0.72 AU). Its mean orbital velocity is approximately 33 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.29, swinging from 0.426 AU at periapsis out to 0.774 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 2.560 R⊕, HD 40307 g exceeds the empirical ~1.8–2.2 R⊕ Fulton radius valley threshold, indicating a substantial hydrogen-helium or supercritical volatile envelope without a conventional solid terrestrial surface.
The host star HD 40307 is classified as an orange K-type star of spectral type K2.5 V with an effective photospheric temperature of 4,956 K, a radius of 0.7167 R☉, a mass of 0.77 M☉, an estimated bolometric luminosity of 0.278 L☉. Stellar evolution modeling indicates an estimated stellar age of 4.5 Gyr and a metallicity of [Fe/H] = -0.31 dex (metal-poor relative to the Sun). From Earth, HD 40307 shines at apparent V-band magnitude 7.17 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HD 40307 system is a multi-planet architecture hosting 5 confirmed worlds, with HD 40307 g sharing its stellar neighborhood alongside HD 40307 b, HD 40307 c, HD 40307 d, HD 40307 f.
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: La Silla Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 40307 |
| Spectral Type | K2.5 V |
| Effective Temp (Teff) | 4,956 K |
| Stellar Radius | 0.7167 R☉ |
| Stellar Mass | 0.7700 M☉ |
| Bolometric Luminosity | 0.278 L☉ |
| Metallicity [Fe/H] | -0.310 dex |
| Stellar Age | 4.500 Gyr |
| Apparent Magnitude | V: 7.170 · G: 6.844 |
| System Architecture | 1 Star, 5 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.600000 AU |
| Orbital Period | 197.8 days |
| Mean Orbital Velocity | 33 km/s |
| Orbital Eccentricity (e) | 0.29000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.33 g/cm³ (18.6 km/s vesc) |
| Insolation Flux | 0.6378 S⊕ (~868 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 42.200 LY (12.936 pc) |