HD 21693 c
Astrophysical Synthesis & Habitability Characterization
HD 21693 c is a confirmed sub-neptune exoplanet orbiting the star HD 21693, located approximately 108.5 light-years (33.26 parsecs) from the Solar System. First announced in 2019 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 4.34 Earth radii (0.39 Jupiter radii) and has a measured mass of 17.37 Earth masses. Combining its mass and radius yields a mean bulk density of 1.17 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.92 g⊕ (9.02 m/s²), and an escape velocity of 22.4 km/s.
In its orbital architecture around HD 21693, HD 21693 c completes one full revolution every 53.736 Earth days at a semi-major axis of 0.2586 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 52.4 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.07 carries the world between 0.2405 AU at periapsis and 0.2767 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 492 K (+219°C / +426°F) and receiving 9.27 S⊕ (~12,618 W/m²), HD 21693 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 21693 is classified as a solar-type yellow star of spectral type G8 V with an effective photospheric temperature of 5,430 K, a radius of 0.9144 R☉, a mass of 0.8 M☉, an estimated bolometric luminosity of 0.652 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0 dex (near-solar heavy-element abundance). From Earth, HD 21693 shines at apparent V-band magnitude 7.95 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HD 21693 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 21693 c sharing its stellar neighborhood alongside HD 21693 b.
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: La Silla Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 21693 |
| Spectral Type | G8 V |
| Effective Temp (Teff) | 5,430 K |
| Stellar Radius | 0.9144 R☉ |
| Stellar Mass | 0.8000 M☉ |
| Bolometric Luminosity | 0.652 L☉ |
| Metallicity [Fe/H] | 0.000 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 7.950 · G: 7.757 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.258600 AU |
| Orbital Period | 53.736 days |
| Mean Orbital Velocity | 52.4 km/s |
| Orbital Eccentricity (e) | 0.07000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.17 g/cm³ (22.4 km/s vesc) |
| Insolation Flux | 9.2712 S⊕ (~12,618 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 108.500 LY (33.255 pc) |