HD 21520 b
Astrophysical Synthesis & Habitability Characterization
HD 21520 b is a confirmed sub-neptune exoplanet orbiting the star HD 21520, located approximately 258.3 light-years (79.21 parsecs) from the Solar System. First announced in 2024 by Transiting Exoplanet Survey Satellite (TESS) using the Transit detection technique (producing an estimated primary transit depth of 0.0561% across the stellar disk), this world measures 2.697 Earth radii and has a measured mass of 17.7 Earth masses. Combining its mass and radius yields a mean bulk density of 4.98 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 2.43 g⊕ (23.83 m/s²), and an escape velocity of 28.7 km/s.
In its orbital architecture around HD 21520, HD 21520 b completes one full revolution every 25.129 Earth days at a semi-major axis of 0.1726 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 74.7 km/s (compared to Earth's 29.8 km/s). The orbit is modeled as circular (e = 0.00), maintaining a constant stellar distance throughout its year.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 637 K (+364°C / +687°F) and receiving 25.74 S⊕ (~35,036 W/m²), HD 21520 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 21520 is classified as a stellar host with an effective photospheric temperature of 5,871 K, a radius of 1.044 R☉, a mass of 1.09 M☉, an estimated bolometric luminosity of 1.162 L☉. Stellar evolution modeling indicates an estimated stellar age of 5.3 Gyr and a metallicity of [Fe/H] = 0.049 dex (near-solar heavy-element abundance). From Earth, HD 21520 shines at apparent V-band magnitude 9.17 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere.
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: Transiting Exoplanet Survey Satellite (TESS) · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 21520 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,871 K |
| Stellar Radius | 1.0440 R☉ |
| Stellar Mass | 1.0900 M☉ |
| Bolometric Luminosity | 1.162 L☉ |
| Metallicity [Fe/H] | 0.049 dex |
| Stellar Age | 5.300 Gyr |
| Apparent Magnitude | V: 9.170 · G: 9.018 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.172600 AU |
| Orbital Period | 25.129 days |
| Mean Orbital Velocity | 74.7 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.2300° |
| Mean Bulk Density | 4.98 g/cm³ (28.7 km/s vesc) |
| Insolation Flux | 25.7431 S⊕ (~35,036 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 258.300 LY (79.208 pc) |