HR 858 b
Astrophysical Synthesis & Habitability Characterization
HR 858 b is a confirmed sub-neptune exoplanet orbiting the star HR 858, located approximately 104.3 light-years (31.96 parsecs) from the Solar System. First announced in 2019 by Transiting Exoplanet Survey Satellite (TESS) using the Transit detection technique (producing an estimated primary transit depth of 0.0213% across the stellar disk), this world measures 2.004 Earth radii and has a measured mass of 3.55 Earth masses. Combining its mass and radius yields a mean bulk density of 2.43 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 0.88 g⊕ (8.63 m/s²), and an escape velocity of 14.9 km/s.
In its orbital architecture around HR 858, HR 858 b completes one full revolution every 3.585 Earth days at a semi-major axis of 0.04888 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 148.3 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 1552 K (+1279°C / +2334°F) and receiving 989.70 S⊕ (~1,346,982 W/m²), HR 858 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HR 858 is classified as a warm F-type star of spectral type F6 V with an effective photospheric temperature of 6,354 K, a radius of 1.258 R☉, a mass of 1.204 M☉, an estimated bolometric luminosity of 2.314 L☉. Stellar evolution modeling indicates an estimated stellar age of 2 Gyr and a metallicity of [Fe/H] = 0.025 dex (near-solar heavy-element abundance). From Earth, HR 858 shines at apparent V-band magnitude 6.38 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HR 858 system is a multi-planet architecture hosting 3 confirmed worlds, with HR 858 b sharing its stellar neighborhood alongside HR 858 c, HR 858 d.
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 | HR 858 |
| Spectral Type | F6 V |
| Effective Temp (Teff) | 6,354 K |
| Stellar Radius | 1.2580 R☉ |
| Stellar Mass | 1.2040 M☉ |
| Bolometric Luminosity | 2.314 L☉ |
| Metallicity [Fe/H] | 0.025 dex |
| Stellar Age | 2.000 Gyr |
| Apparent Magnitude | V: 6.380 · G: 6.253 |
| System Architecture | 2 Stars, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.048880 AU |
| Orbital Period | 3.585 days |
| Mean Orbital Velocity | 148.3 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 85.9800° |
| Mean Bulk Density | 2.43 g/cm³ (14.9 km/s vesc) |
| Insolation Flux | 989.7000 S⊕ (~1,346,982 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 104.300 LY (31.964 pc) |