HD 40307 d

Astrophysical Synthesis & Habitability Characterization

HD 40307 d is a confirmed gas giant exoplanet orbiting the star HD 40307, located approximately 42.2 light-years (12.94 parsecs) from the Solar System. First announced in 2009 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world has a minimum or true mass of 1775.389 Earth masses (5.59 M♃), while its physical transit radius has not yet been geometrically observed.

In its orbital architecture around HD 40307, HD 40307 d completes one full revolution every 20.432 Earth days at a semi-major axis of 0.1321 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 70.3 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.07 carries the world between 0.1229 AU at periapsis and 0.1413 AU at apoapsis, driving measurable seasonal insolation cycles.

⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 541 K (+268°C / +514°F) and receiving 13.16 S⊕ (~17,908 W/m²), HD 40307 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

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 d sharing its stellar neighborhood alongside HD 40307 b, HD 40307 c, HD 40307 f, HD 40307 g.

Discovery Facility: La Silla Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).