CONFIRMED EXOPLANET · Sub-Neptune

HD 191939 b

Orbits Host Star: HD 191939 (Type G9 V) · Distance: 174.800 Light-Years (53.609 pc)

Planetary Radius

3.410 R⊕

Planetary Mass

10.000 M⊕

Surface Gravity

0.86 g / g⊕

Equilibrium Temp

880K (+607°C / +1125°F)

Orbital Period

8.88 days

Discovery Year

2020 (Transit)

Earth Similarity (ESI)

0.25 / 1.00

Insolation Flux

100.00 S⊕

Astrophysical Synthesis & Habitability Characterization

HD 191939 b is a confirmed sub-neptune exoplanet orbiting the star HD 191939, located approximately 174.8 light-years (53.61 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 0.1106% across the stellar disk), this world measures 3.41 Earth radii (0.3 Jupiter radii) and has a measured mass of 10 Earth masses. Combining its mass and radius yields a mean bulk density of 1.39 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.86 g⊕ (8.43 m/s²), and an escape velocity of 19.2 km/s.

In its orbital architecture around HD 191939, HD 191939 b completes one full revolution every 8.88 Earth days at a semi-major axis of 0.0804 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 98.5 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.031 (ranging from 0.0779 AU at periapsis to 0.0829 AU at apoapsis), the planet experiences minimal orbital distance variation.

⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 880 K (+607°C / +1125°F) and receiving 100.00 S⊕ (~136,100 W/m²), HD 191939 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star HD 191939 is classified as a solar-type yellow star of spectral type G9 V with an effective photospheric temperature of 5,348 K, a radius of 0.94 R☉, a mass of 0.81 M☉, an estimated bolometric luminosity of 0.649 L☉. Stellar evolution modeling indicates an estimated stellar age of 8.7 Gyr and a metallicity of [Fe/H] = -0.15 dex (metal-poor relative to the Sun). From Earth, HD 191939 shines at apparent V-band magnitude 8.97 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere. The HD 191939 system is a multi-planet architecture hosting 6 confirmed worlds, with HD 191939 b sharing its stellar neighborhood alongside HD 191939 c, HD 191939 d, HD 191939 e, HD 191939 g, HD 191939 f.

Earth Similarity Index (ESI): 0.25 Earth Baseline = 1.00

0.00 Dissimilar 0.50 Mars Scale 0.70 Analog Regime 1.00 Earth

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	HD 191939
Spectral Type	G9 V
Effective Temp (Teff)	5,348 K
Stellar Radius	0.9400 R☉
Stellar Mass	0.8100 M☉
Bolometric Luminosity	0.649 L☉
Metallicity [Fe/H]	-0.150 dex
Stellar Age	8.700 Gyr
Apparent Magnitude	V: 8.970 · G: 8.775
System Architecture	1 Star, 6 Planets

🪐 Orbital & Physical Dynamics

Semi-Major Axis (a)	0.080400 AU
Orbital Period	8.88 days
Mean Orbital Velocity	98.5 km/s
Orbital Eccentricity (e)	0.03100
Orbital Inclination (i)	88.1000°
Mean Bulk Density	1.39 g/cm³ (19.2 km/s v_esc)
Insolation Flux	100.0000 S⊕ (~136,100 W/m²)
Habitable Zone Status	Exterior / Non-HZ
Celestial Distance	174.800 LY (53.609 pc)