TOI-2257 b
Astrophysical Synthesis & Habitability Characterization
TOI-2257 b is a confirmed sub-neptune exoplanet orbiting the star TOI-2257, located approximately 188.5 light-years (57.79 parsecs) from the Solar System. First announced in 2021 by Transiting Exoplanet Survey Satellite (TESS) using the Transit detection technique (producing an estimated primary transit depth of 0.413% across the stellar disk), this world measures 2.194 Earth radii and has a measured mass of 5.45 Earth masses. Combining its mass and radius yields a mean bulk density of 2.85 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.13 g⊕ (11.08 m/s²), and an escape velocity of 17.6 km/s.
In its orbital architecture around TOI-2257, TOI-2257 b completes one full revolution every 35.189 Earth days at a semi-major axis of 0.145 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 44.8 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.496, swinging from 0.0731 AU at periapsis out to 0.2169 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
✓ Classified as a Temperate Habitable Zone Candidate.
With an estimated equilibrium temperature of 256 K (-17°C / +1°F) and an incident stellar irradiance of 0.67 S⊕ (~912 W/m²), TOI-2257 b orbits within the empirical circumstellar habitable zone where a rocky surface under sufficient atmospheric pressure can maintain liquid surface water.
The host star TOI-2257 is classified as a low-mass red dwarf of spectral type M3 with an effective photospheric temperature of 3,430 K, a radius of 0.313 R☉, a mass of 0.328 M☉, an estimated bolometric luminosity of 0.012 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.27 dex (metal-poor relative to the Sun). From Earth, TOI-2257 shines at apparent V-band magnitude 15.211 (faint and requiring research-grade or CCD telescopic photometry) 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-2257 |
| Spectral Type | M3 |
| Effective Temp (Teff) | 3,430 K |
| Stellar Radius | 0.3130 R☉ |
| Stellar Mass | 0.3280 M☉ |
| Bolometric Luminosity | 0.012 L☉ |
| Metallicity [Fe/H] | -0.270 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 15.211 · G: 14.162 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.145000 AU |
| Orbital Period | 35.189 days |
| Mean Orbital Velocity | 44.8 km/s |
| Orbital Eccentricity (e) | 0.49600 |
| Orbital Inclination (i) | 89.7860° |
| Mean Bulk Density | 2.85 g/cm³ (17.6 km/s vesc) |
| Insolation Flux | 0.6703 S⊕ (~912 W/m²) |
| Habitable Zone Status | ★ Temperate Zone |
| Celestial Distance | 188.500 LY (57.791 pc) |