Key parameters at a glance

A radius of 3.20 Earth radii
A mass of 8.00 Earth masses
Surface gravity around 0.78 g
An orbital period of 28.580 days
Semi-major axis 0.1752 AU
Equilibrium temperature 600 K (327 °C)
Distance from Earth 174.85 light-years
Earth Similarity Index 0.367
Travel time at Voyager 1 speed 3,083,456 years

5 siblings around HD 191939

HD 191939 c shares its host star with 5 other confirmed planets. Side-by-side measurements below.

Planet	Type	Radius (R⊕)	Mass (M⊕)	Period (d)	Eq. T (K)	Year
HD 191939 b	Sub-Neptune	3.41	10.00	8.880	880	2020
HD 191939 c this	Sub-Neptune	3.20	8.00	28.580	600	2020
HD 191939 d	Sub-Neptune	3.00	2.80	38.353	540	2020
HD 191939 e	Gas Giant	13.00	112.20	101.120	390	2022
HD 191939 g	Sub-Neptune	3.74	13.50	284.000	278	2022
HD 191939 f	Gas Giant	13.20	915.35	2,898.000	125	2022

HD 191939 c Planet Profile

Physical Specs

Radius

3.20 R⊕

Radius (Jupiter)

0.285 R♃

Mass

8.00 M⊕

Mass (Jupiter)

0.025 M♃

Density

1.35 g/cm³

Surface Gravity

0.78 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.367

HZ Position Inner

Controversial No

Discovery

Year 2020

Method Transit

Facility Transiting Exoplanet Survey Satellite (TESS)

Telescope 0.1 m TESS Telescope

Size rank in cohort

Rank by radius

#357of 1978

top 18.0%

This planet

3.20R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

Kepler-1710 b 3.20 R⊕ Kepler-1294 b 3.19 R⊕ Kepler-351 c 3.19 R⊕

Metric	Earth	HD 191939 c	Jupiter
Radius (R⊕)	1.00	3.20	11.21
Mass (M⊕)	1.00	8.00	317.83
Density (g/cm³)	5.51	1.35	1.33
Surface Gravity (g)	1.00	0.78	2.53
Insolation (S⊕)	1.00	21.00	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	8.000 M⊕	Direct mass measurement (not derived).
Minimum mass (M sin i)	7.550 M⊕	Lower bound from radial-velocity fitting.

Star Catalogue Identifiers

HD 191939

HIP

HIP 99175

TIC

TIC 269701147

Gaia DR2

Gaia DR2 2248126315275354496

Gaia DR3

Gaia DR3 2248126315275354496

System

HD 191939

Percentile among Sub-Neptune cohort

Radius 3.195 R⊕ · percentile 82 / cohort 1978

Mass 8.000 M⊕ · percentile 54 / cohort 1978

Orbital period 28.58 d · percentile 72 / cohort 1946

Distance 53.61 pc · percentile 12 / cohort 1975

ESI 0.367 · percentile 45 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

28.580 days

Semi-major axis

0.1752 AU

Eccentricity

0.034

Inclination

89.10 °

Year Length

A year here lasts approximately 28.58 Earth days (7.8% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.1752 AU.

Transit Fingerprint

Depth

0.108 %

Duration

4.162 h

Impact parameter b

0.630

Rp / R★

0.031180

Transit midpoint (BJD)

2,459,440.5491

Photometric dip

Transit produces a flux drop of 1,081 ppm lasting ≈ 4.16 h.

Extended Orbital Architecture

Planet / star radius ratio

0.031180

Semi-major axis / stellar radius

40.000

Impact parameter (b)

0.630

RV semi-amplitude (K)

1.930 m/s

Transit mid-time (BJD)

2,459,440.5491

Long. of periastron (ω)

-90.00°

Angular separation (arcsec)

3.27000

Eq. Temperature

600K

(327 °C)

Insolation (S⊕)

21.00

Earth = 1.00

Earth Similarity (ESI)

0.367

0 = alien, 1 = Earth-identical

Inside the inner HZ boundary (too hot for liquid water) — the surface is too hot to retain surface water.

Detection Signatures

Transit photometry

A transit dip has been observed from at least one telescope.

Radial velocity

Stellar wobble has been measured spectroscopically.

Discovery Paper

Primary reference

Badenas Agusti et al. 2020

Instrument

TESS CCD Array

Publication

2020-09

Observation locale

Space

Discovery cohort

Planets confirmed in 2020 at Transiting Exoplanet Survey Satellite (TESS) (12 shown).

TOI-700 d · Rocky Terrestrial TOI-237 b · Super-Earth TOI-700 b · Rocky Terrestrial TOI-1266 c · Super-Earth LTT 3780 c · Sub-Neptune TOI-700 c · Sub-Neptune TOI-776 c · Sub-Neptune TOI-942 c · Neptune-like TOI-776 b · Super-Earth TOI-1266 b · Sub-Neptune TOI-540 b · Rocky Terrestrial LHS 1815 b · Rocky Terrestrial

Host System: HD 191939

Spectral Class

G-type yellow (Sun-like)

Effective Temperature

5,348 K — cooler than the Sun (5,778 K)

Estimated Age

8.70 Gyr — older than the Sun (4.6 Gyr)

Stellar Radius

0.940 R☉

Stellar Mass

0.810 M☉

Metallicity [Fe/H]

-0.15

Extended Stellar Properties

Surface gravity (log g)

4.300 dex

Stellar density

1.370 g/cm³

Systemic radial velocity

-9.50 km/s

Rotational v·sin i

2.00 km/s

Activity index (log R'HK)

-5.110

Metallicity ratio

[Fe/H]

Distance

53.61 parsec

Light-years 174.85 ly

V-band magnitude

8.97 mag

Voyager-speed travel 3,083,456 yr

Multi-band Host Photometry — 11 bands

Astrometric Data

Parallax

18.625 mas

Total Proper Motion

163.283 mas/yr

PM Right Ascension

150.26 mas/yr

PM Declination

-63.91 mas/yr

Galactic Cartesian (pc)

x = 0.208 · y = -0.333 · z = 0.919

Equatorial (J2000)

RA 302.02562° · Dec 66.85030°

Galactic ℓ, b

99.987° · 17.722°

Ecliptic λ, β

16.039° · 77.471°

HTM-20 index

-1738980752