HD 101581 b
Astrophysical Synthesis & Habitability Characterization
HD 101581 b is a confirmed terrestrial exoplanet orbiting the star HIP 56998, located approximately 41.7 light-years (12.78 parsecs) from the Solar System. First announced in 2025 by Transiting Exoplanet Survey Satellite (TESS) using the Transit detection technique (producing an estimated primary transit depth of 0.0194% across the stellar disk), this world measures 0.956 Earth radii and has a measured mass of 0.827 Earth masses. Combining its mass and radius yields a mean bulk density of 5.22 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 0.9 g⊕ (8.83 m/s²), and an escape velocity of 10.4 km/s.
In its orbital architecture around HIP 56998, HD 101581 b completes one full revolution every 4.466 Earth days at a semi-major axis of 0.046 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 112.1 km/s (compared to Earth's 29.8 km/s).
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 834 K (+561°C / +1042°F) and receiving 80.00 S⊕ (~108,880 W/m²), HD 101581 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HIP 56998 is classified as an orange K-type star of spectral type K4/5V with an effective photospheric temperature of 4,675 K, a radius of 0.63 R☉, a mass of 0.653 M☉, an estimated bolometric luminosity of 0.17 L☉. Stellar evolution modeling indicates an estimated stellar age of 6.88 Gyr and a metallicity of [Fe/H] = -0.343 dex (metal-poor relative to the Sun). From Earth, HIP 56998 shines at apparent V-band magnitude 7.77 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HIP 56998 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 101581 b sharing its stellar neighborhood alongside HD 101581 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: Transiting Exoplanet Survey Satellite (TESS) · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HIP 56998 |
| Spectral Type | K4/5V |
| Effective Temp (Teff) | 4,675 K |
| Stellar Radius | 0.6300 R☉ |
| Stellar Mass | 0.6530 M☉ |
| Bolometric Luminosity | 0.17 L☉ |
| Metallicity [Fe/H] | -0.343 dex |
| Stellar Age | 6.880 Gyr |
| Apparent Magnitude | V: 7.770 · G: 7.384 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.046000 AU |
| Orbital Period | 4.466 days |
| Mean Orbital Velocity | 112.1 km/s |
| Orbital Eccentricity (e) | Circular (0.00) |
| Orbital Inclination (i) | 87.7800° |
| Mean Bulk Density | 5.22 g/cm³ (10.4 km/s vesc) |
| Insolation Flux | 80.0000 S⊕ (~108,880 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 41.700 LY (12.778 pc) |