Key parameters at a glance

A radius of 1.79 Earth radii
A mass of 3.85 Earth masses
Surface gravity around 1.20 g
An orbital period of 14.237 days
Semi-major axis 0.0808 AU
Equilibrium temperature 336 K (63 °C)
Distance from Earth 18.20 light-years
Earth Similarity Index 0.775
Travel time at Voyager 1 speed 320,981 years
Almost certainly tidally locked to its host star given the close orbit

1 sibling around GJ 251

GJ 251 b shares its host star with 1 other confirmed planet. Side-by-side measurements below.

Planet	Type	Radius (R⊕)	Mass (M⊕)	Period (d)	Eq. T (K)	Year
GJ 251 b this	Super-Earth	1.79	3.85	14.237	336	2020
GJ 251 c	Super-Earth	1.80	3.88	53.647	216	2025

GJ 251 b Planet Profile

Physical Specs

Radius

1.79 R⊕

Radius (Jupiter)

0.160 R♃

Mass

3.85 M⊕

Mass (Jupiter)

0.012 M♃

Density

3.69 g/cm³

Surface Gravity

1.20 g (Earth)

Classification

Planet Type

Super-Earth

ESI Score 0.775

HZ Position Inner

Controversial No

Discovery

Year 2020

Method Radial Velocity

Facility W. M. Keck Observatory

Telescope 10 m Keck I Telescope

Size rank in cohort

Rank by radius

#271of 1176

top 23.0%

This planet

1.79R⊕

Super-Earth median

1.60R⊕

Nearest-size peers

Kepler-376 c 1.79 R⊕ Kepler-272 c 1.79 R⊕ Kepler-204 c 1.79 R⊕

Metric	Earth	GJ 251 b	Jupiter
Radius (R⊕)	1.00	1.79	11.21
Mass (M⊕)	1.00	3.85	317.83
Density (g/cm³)	5.51	3.69	1.33
Surface Gravity (g)	1.00	1.20	2.53
Insolation (S⊕)	1.00	2.37	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Minimum mass (M sin i)	3.850 M⊕	Lower bound from radial-velocity fitting.

Star Catalogue Identifiers

HD 265866

HIP

HIP 33226

TIC

TIC 68581262

Gaia DR2

Gaia DR2 939072613334579328

Gaia DR3

Gaia DR3 939072613334579328

System

GJ 251

Percentile among Super-Earth cohort

Radius 1.790 R⊕ · percentile 76 / cohort 1176

Mass 3.850 M⊕ · percentile 69 / cohort 1176

Orbital period 14.24 d · percentile 76 / cohort 1164

Distance 5.58 pc · percentile 0 / cohort 1171

ESI 0.775 · percentile 88 / cohort 1176

Orbit & Habitability

Orbital Dynamics

Period

14.237 days

Semi-major axis

0.0808 AU

Eccentricity

0.000

Inclination

— °

Year Length

A year here lasts approximately 14.24 Earth days (3.9% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0808 AU.

Extended Orbital Architecture

RV semi-amplitude (K)

2.010 m/s

Transit mid-time (BJD)

2,458,626.8100

Long. of periastron (ω)

0.00°

Angular separation (arcsec)

14.50000

Eq. Temperature

336K

(63 °C)

Insolation (S⊕)

2.37

Earth = 1.00

Earth Similarity (ESI)

0.775

0 = alien, 1 = Earth-identical

Inside the inner HZ boundary (too hot for liquid water) — liquid water is physically possible with adequate atmospheric pressure. Close-orbiting tidally-locked around a cool host.

Discovery Paper

Primary reference

Stock et al. 2020

Instrument

HIRES Spectrometer

Publication

2020-11

Observation locale

Ground

Host System: GJ 251

Spectral Class

M-type red dwarf

Effective Temperature

3,342 K — cooler than the Sun (5,778 K)

Estimated Age

6.80 Gyr — older than the Sun (4.6 Gyr)

Stellar Radius

0.360 R☉

Stellar Mass

0.350 M☉

Metallicity [Fe/H]

0.07

Extended Stellar Properties

Surface gravity (log g)

4.870 dex

Stellar density

10.053 g/cm³

Systemic radial velocity

22.30 km/s

Rotational v·sin i

2.00 km/s

Rotation period

122.10 days

Metallicity ratio

[Fe/H]

Distance

5.58 parsec

Light-years 18.20 ly

V-band magnitude

9.89 mag

Voyager-speed travel 320,981 yr

Multi-band Host Photometry — 12 bands

Astrometric Data

Parallax

179.164 mas

Total Proper Motion

828.347 mas/yr

PM Right Ascension

-726.39 mas/yr

PM Declination

-398.13 mas/yr

Galactic Cartesian (pc)

x = -0.198 · y = 0.812 · z = 0.549

Equatorial (J2000)

RA 103.70025° · Dec 33.26646°

Galactic ℓ, b

182.937° · 15.132°

Ecliptic λ, β

101.611° · 10.378°

HTM-20 index

702762585

Observation Record

Photometric series

Stellar spectra

Archive notes