YZ Cet d
Astrophysical Synthesis & Habitability Characterization
YZ Cet d is a confirmed terrestrial exoplanet orbiting the star YZ Cet, located approximately 12.1 light-years (3.71 parsecs) from the Solar System. First announced in 2017 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 1.03 Earth radii and has a measured mass of 1.09 Earth masses. Combining its mass and radius yields a mean bulk density of 5.5 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 1.03 g⊕ (10.1 m/s²), and an escape velocity of 11.5 km/s.
In its orbital architecture around YZ Cet, YZ Cet d completes one full revolution every 4.656 Earth days at a semi-major axis of 0.02851 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 66.6 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.07 carries the world between 0.0265 AU at periapsis and 0.0305 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 357 K (+84°C / +183°F) and receiving 2.70 S⊕ (~3,675 W/m²), YZ Cet d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star YZ Cet is classified as a low-mass red dwarf of spectral type M4.5 V with an effective photospheric temperature of 3,151 K, a radius of 0.157 R☉, a mass of 0.142 M☉, an estimated bolometric luminosity of 0.00218 L☉. Stellar evolution modeling indicates an estimated stellar age of 3.8 Gyr and a metallicity of [Fe/H] = -0.18 dex (metal-poor relative to the Sun). From Earth, YZ Cet shines at apparent V-band magnitude 12.074 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The YZ Cet system is a multi-planet architecture hosting 3 confirmed worlds, with YZ Cet d sharing its stellar neighborhood alongside YZ Cet b, YZ Cet 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: La Silla Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | YZ Cet |
| Spectral Type | M4.5 V |
| Effective Temp (Teff) | 3,151 K |
| Stellar Radius | 0.1570 R☉ |
| Stellar Mass | 0.1420 M☉ |
| Bolometric Luminosity | 0.00218 L☉ |
| Metallicity [Fe/H] | -0.180 dex |
| Stellar Age | 3.800 Gyr |
| Apparent Magnitude | V: 12.074 · G: 10.429 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.028510 AU |
| Orbital Period | 4.656 days |
| Mean Orbital Velocity | 66.6 km/s |
| Orbital Eccentricity (e) | 0.07000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 5.5 g/cm³ (11.5 km/s vesc) |
| Insolation Flux | 2.7000 S⊕ (~3,675 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 12.100 LY (3.712 pc) |