TOI-421 b
Astrophysical Synthesis & Habitability Characterization
TOI-421 b is a confirmed sub-neptune exoplanet orbiting the star TOI-421, located approximately 244 light-years (74.8 parsecs) from the Solar System. First announced in 2020 by Transiting Exoplanet Survey Satellite (TESS) using the Transit detection technique (producing an estimated primary transit depth of 0.0781% across the stellar disk), this world measures 2.64 Earth radii and has a measured mass of 6.7 Earth masses. Combining its mass and radius yields a mean bulk density of 2.01 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 0.96 g⊕ (9.41 m/s²), and an escape velocity of 17.8 km/s.
In its orbital architecture around TOI-421, TOI-421 b completes one full revolution every 5.198 Earth days at a semi-major axis of 0.0554 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 116 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.13 carries the world between 0.0482 AU at periapsis and 0.0626 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 922 K (+649°C / +1200°F) and receiving 154.57 S⊕ (~210,370 W/m²), TOI-421 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star TOI-421 is classified as a solar-type yellow star of spectral type G9 V with an effective photospheric temperature of 5,291 K, a radius of 0.866 R☉, a mass of 0.833 M☉, an estimated bolometric luminosity of 0.527 L☉. Stellar evolution modeling indicates an estimated stellar age of 10.9 Gyr and a metallicity of [Fe/H] = -0.044 dex (near-solar heavy-element abundance). From Earth, TOI-421 shines at apparent V-band magnitude 9.931 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The TOI-421 system is a multi-planet architecture hosting 2 confirmed worlds, with TOI-421 b sharing its stellar neighborhood alongside TOI-421 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 | TOI-421 |
| Spectral Type | G9 V |
| Effective Temp (Teff) | 5,291 K |
| Stellar Radius | 0.8660 R☉ |
| Stellar Mass | 0.8330 M☉ |
| Bolometric Luminosity | 0.527 L☉ |
| Metallicity [Fe/H] | -0.044 dex |
| Stellar Age | 10.900 Gyr |
| Apparent Magnitude | V: 9.931 · G: 9.778 |
| System Architecture | 2 Stars, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.055400 AU |
| Orbital Period | 5.198 days |
| Mean Orbital Velocity | 116 km/s |
| Orbital Eccentricity (e) | 0.13000 |
| Orbital Inclination (i) | 85.6800° |
| Mean Bulk Density | 2.01 g/cm³ (17.8 km/s vesc) |
| Insolation Flux | 154.5700 S⊕ (~210,370 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 244.000 LY (74.797 pc) |