LP 791-18 d
Astrophysical Synthesis & Habitability Characterization
LP 791-18 d is a confirmed terrestrial exoplanet orbiting the star LP 791-18, located approximately 86.4 light-years (26.49 parsecs) from the Solar System. First announced in 2023 by Spitzer Space Telescope using the Transit detection technique (producing an estimated primary transit depth of 0.2702% across the stellar disk), this world measures 1.032 Earth radii and has a measured mass of 0.91 Earth masses. Combining its mass and radius yields a mean bulk density of 4.57 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 0.85 g⊕ (8.34 m/s²), and an escape velocity of 10.5 km/s.
In its orbital architecture around LP 791-18, LP 791-18 d completes one full revolution every 2.755 Earth days at a semi-major axis of 0.01992 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 78.7 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.0011 (ranging from 0.0199 AU at periapsis to 0.0199 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 396 K (+123°C / +253°F) and receiving 5.83 S⊕ (~7,935 W/m²), LP 791-18 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star LP 791-18 is classified as a low-mass red dwarf of spectral type M(6.1 +/- 0.7) V with an effective photospheric temperature of 2,960 K, a radius of 0.182 R☉, a mass of 0.139 M☉, an estimated bolometric luminosity of 0.00228 L☉. Stellar evolution modeling indicates an estimated stellar age of 0.5 Gyr and a metallicity of [Fe/H] = -0.09 dex (near-solar heavy-element abundance). From Earth, LP 791-18 shines at apparent V-band magnitude 16.91 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The LP 791-18 system is a multi-planet architecture hosting 3 confirmed worlds, with LP 791-18 d sharing its stellar neighborhood alongside LP 791-18 b, LP 791-18 c.
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: Spitzer Space Telescope · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | LP 791-18 |
| Spectral Type | M(6.1 +/- 0.7) V |
| Effective Temp (Teff) | 2,960 K |
| Stellar Radius | 0.1820 R☉ |
| Stellar Mass | 0.1390 M☉ |
| Bolometric Luminosity | 0.00228 L☉ |
| Metallicity [Fe/H] | -0.090 dex |
| Stellar Age | 0.500 Gyr |
| Apparent Magnitude | V: 16.910 · G: 15.072 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.019920 AU |
| Orbital Period | 2.755 days |
| Mean Orbital Velocity | 78.7 km/s |
| Orbital Eccentricity (e) | 0.00110 |
| Orbital Inclination (i) | 89.3400° |
| Mean Bulk Density | 4.57 g/cm³ (10.5 km/s vesc) |
| Insolation Flux | 5.8300 S⊕ (~7,935 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 86.400 LY (26.493 pc) |