TOI-561 d
Astrophysical Synthesis & Habitability Characterization
TOI-561 d is a confirmed sub-neptune exoplanet orbiting the star TOI-561, located approximately 279.8 light-years (85.8 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 0.0809% across the stellar disk), this world measures 2.615 Earth radii and has a measured mass of 13.33 Earth masses. Combining its mass and radius yields a mean bulk density of 4.11 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 1.95 g⊕ (19.12 m/s²), and an escape velocity of 25.3 km/s.
In its orbital architecture around TOI-561, TOI-561 d completes one full revolution every 25.713 Earth days at a semi-major axis of 0.1587 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 67.1 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.111 carries the world between 0.1411 AU at periapsis and 0.1763 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 600 K (+327°C / +621°F) and receiving 21.20 S⊕ (~28,853 W/m²), TOI-561 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star TOI-561 is classified as a solar-type yellow star of spectral type G9 V with an effective photospheric temperature of 5,342 K, a radius of 0.843 R☉, a mass of 0.806 M☉, an estimated bolometric luminosity of 0.519 L☉. Stellar evolution modeling indicates an estimated stellar age of 16.1 Gyr and a metallicity of [Fe/H] = -0.41 dex (metal-poor relative to the Sun). From Earth, TOI-561 shines at apparent V-band magnitude 10.252 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The TOI-561 system is a multi-planet architecture hosting 4 confirmed worlds, with TOI-561 d sharing its stellar neighborhood alongside TOI-561 b, TOI-561 c, TOI-561 e.
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-561 |
| Spectral Type | G9 V |
| Effective Temp (Teff) | 5,342 K |
| Stellar Radius | 0.8430 R☉ |
| Stellar Mass | 0.8060 M☉ |
| Bolometric Luminosity | 0.519 L☉ |
| Metallicity [Fe/H] | -0.410 dex |
| Stellar Age | 16.100 Gyr |
| Apparent Magnitude | V: 10.252 · G: 10.013 |
| System Architecture | 1 Star, 4 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.158700 AU |
| Orbital Period | 25.713 days |
| Mean Orbital Velocity | 67.1 km/s |
| Orbital Eccentricity (e) | 0.11100 |
| Orbital Inclination (i) | 89.5100° |
| Mean Bulk Density | 4.11 g/cm³ (25.3 km/s vesc) |
| Insolation Flux | 21.2000 S⊕ (~28,853 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 279.800 LY (85.799 pc) |