TOI-178 d
Astrophysical Synthesis & Habitability Characterization
TOI-178 d is a confirmed sub-neptune exoplanet orbiting the star TOI-178, located approximately 204.5 light-years (62.7 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 0.1393% across the stellar disk), this world measures 2.695 Earth radii and has a measured mass of 5.2 Earth masses. Combining its mass and radius yields a mean bulk density of 1.46 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.72 g⊕ (7.06 m/s²), and an escape velocity of 15.5 km/s.
In its orbital architecture around TOI-178, TOI-178 d completes one full revolution every 6.558 Earth days at a semi-major axis of 0.0592 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 98.2 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.0068 (ranging from 0.0588 AU at periapsis to 0.0596 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 690 K (+417°C / +783°F) and receiving 24.91 S⊕ (~33,906 W/m²), TOI-178 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star TOI-178 is classified as an orange K-type star of spectral type K with an effective photospheric temperature of 4,316 K, a radius of 0.662 R☉, a mass of 0.647 M☉, an estimated bolometric luminosity of 0.136 L☉. Stellar evolution modeling indicates an estimated stellar age of 6 Gyr and a metallicity of [Fe/H] = -0.23 dex (metal-poor relative to the Sun). From Earth, TOI-178 shines at apparent V-band magnitude 11.955 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The TOI-178 system is a multi-planet architecture hosting 6 confirmed worlds, with TOI-178 d sharing its stellar neighborhood alongside TOI-178 b, TOI-178 c, TOI-178 e, TOI-178 f, TOI-178 g.
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-178 |
| Spectral Type | K |
| Effective Temp (Teff) | 4,316 K |
| Stellar Radius | 0.6620 R☉ |
| Stellar Mass | 0.6470 M☉ |
| Bolometric Luminosity | 0.136 L☉ |
| Metallicity [Fe/H] | -0.230 dex |
| Stellar Age | 6.000 Gyr |
| Apparent Magnitude | V: 11.955 · G: 11.152 |
| System Architecture | 1 Star, 6 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.059200 AU |
| Orbital Period | 6.558 days |
| Mean Orbital Velocity | 98.2 km/s |
| Orbital Eccentricity (e) | 0.00680 |
| Orbital Inclination (i) | 88.5800° |
| Mean Bulk Density | 1.46 g/cm³ (15.5 km/s vesc) |
| Insolation Flux | 24.9125 S⊕ (~33,906 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 204.500 LY (62.699 pc) |