TOI-5817 b
Astrophysical Synthesis & Habitability Characterization
TOI-5817 b is a confirmed sub-neptune exoplanet orbiting the star TOI-5817, located approximately 263.4 light-years (80.75 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 0.0392% across the stellar disk), this world measures 3.08 Earth radii (0.27 Jupiter radii) and has a measured mass of 10.3 Earth masses. Combining its mass and radius yields a mean bulk density of 1.94 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.09 g⊕ (10.69 m/s²), and an escape velocity of 20.5 km/s.
In its orbital architecture around TOI-5817, TOI-5817 b completes one full revolution every 15.61 Earth days at a semi-major axis of 0.1223 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 85.2 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.2 carries the world between 0.0978 AU at periapsis and 0.1468 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 950 K (+677°C / +1251°F) and receiving 126.86 S⊕ (~172,661 W/m²), TOI-5817 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star TOI-5817 is classified as a solar-type yellow star of spectral type G2 IV-V with an effective photospheric temperature of 5,770 K, a radius of 1.427 R☉, a mass of 0.97 M☉, an estimated bolometric luminosity of 2.025 L☉. Stellar evolution modeling indicates an estimated stellar age of 9.7 Gyr and a metallicity of [Fe/H] = -0.24 dex (metal-poor relative to the Sun). From Earth, TOI-5817 shines at apparent V-band magnitude 8.64 (accessible to binoculars and small amateur telescopes) in the Northern 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 | TOI-5817 |
| Spectral Type | G2 IV-V |
| Effective Temp (Teff) | 5,770 K |
| Stellar Radius | 1.4270 R☉ |
| Stellar Mass | 0.9700 M☉ |
| Bolometric Luminosity | 2.025 L☉ |
| Metallicity [Fe/H] | -0.240 dex |
| Stellar Age | 9.700 Gyr |
| Apparent Magnitude | V: 8.640 · G: 8.483 |
| System Architecture | 2 Stars, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.122300 AU |
| Orbital Period | 15.61 days |
| Mean Orbital Velocity | 85.2 km/s |
| Orbital Eccentricity (e) | 0.20000 |
| Orbital Inclination (i) | 88.7200° |
| Mean Bulk Density | 1.94 g/cm³ (20.5 km/s vesc) |
| Insolation Flux | 126.8632 S⊕ (~172,661 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 263.400 LY (80.754 pc) |