K2-157 d
Astrophysical Synthesis & Habitability Characterization
K2-157 d is a confirmed sub-neptune exoplanet orbiting the star K2-157, located approximately 973.2 light-years (298.37 parsecs) from the Solar System. First announced in 2025 by Paranal Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 5.16 Earth radii (0.46 Jupiter radii) and has a measured mass of 23.3 Earth masses. Combining its mass and radius yields a mean bulk density of 0.94 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.88 g⊕ (8.63 m/s²), and an escape velocity of 23.8 km/s.
In its orbital architecture around K2-157, K2-157 d completes one full revolution every 66.5 Earth days at a semi-major axis of 0.309 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 50.6 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.5, swinging from 0.1545 AU at periapsis out to 0.4635 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 429 K (+156°C / +313°F) and receiving 6.46 S⊕ (~8,792 W/m²), K2-157 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-157 is classified as a solar-type yellow star of spectral type G9 V with an effective photospheric temperature of 5,334 K, a radius of 0.86 R☉, a mass of 0.89 M☉, an estimated bolometric luminosity of 0.537 L☉. Stellar evolution modeling indicates an estimated stellar age of 8.8 Gyr and a metallicity of [Fe/H] = 0.02 dex (near-solar heavy-element abundance). From Earth, K2-157 shines at apparent V-band magnitude 12.942 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The K2-157 system is a multi-planet architecture hosting 3 confirmed worlds, with K2-157 d sharing its stellar neighborhood alongside K2-157 b, K2-157 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: Paranal Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | K2-157 |
| Spectral Type | G9 V |
| Effective Temp (Teff) | 5,334 K |
| Stellar Radius | 0.8600 R☉ |
| Stellar Mass | 0.8900 M☉ |
| Bolometric Luminosity | 0.537 L☉ |
| Metallicity [Fe/H] | 0.020 dex |
| Stellar Age | 8.800 Gyr |
| Apparent Magnitude | V: 12.942 · G: 12.642 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.309000 AU |
| Orbital Period | 66.5 days |
| Mean Orbital Velocity | 50.6 km/s |
| Orbital Eccentricity (e) | 0.50000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.94 g/cm³ (23.8 km/s vesc) |
| Insolation Flux | 6.4600 S⊕ (~8,792 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 973.200 LY (298.373 pc) |