Key parameters at a glance

A radius of 2.50 Earth radii
A mass of 6.81 Earth masses
Surface gravity around 1.09 g
An orbital period of 12.921 days
Semi-major axis 0.0718 AU
Distance from Earth 20.54 light-years
Earth Similarity Index 0.704
Travel time at Voyager 1 speed 362,252 years
Almost certainly tidally locked to its host star given the close orbit

Context from the literature

Gliese 581c is an exoplanet orbiting within the Gliese 581 system. It is the second planet discovered in the system and the third in order from the star. With a mass about 6.8 times that of the Earth, it is classified as a super-Earth.

Excerpted from Wikipedia · full article

2 siblings around GJ 581

GJ 581 c shares its host star with 2 other confirmed planets. Side-by-side measurements below.

Planet	Type	Radius (R⊕)	Mass (M⊕)	Period (d)	Eq. T (K)	Year
GJ 581 e	Super-Earth	1.38	2.48	3.148	—	2009
GJ 581 b	Neptune-like	4.79	20.50	5.369	—	2005
GJ 581 c this	Sub-Neptune	2.50	6.81	12.921	—	2007

GJ 581 c Planet Profile

Physical Specs

Radius

2.50 R⊕

Radius (Jupiter)

0.223 R♃

Mass

6.81 M⊕

Mass (Jupiter)

0.021 M♃

Density

2.39 g/cm³

Surface Gravity

1.09 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.704

HZ Position Unknown

Controversial No

Discovery

Year 2007

Method Radial Velocity

Facility La Silla Observatory

Telescope 3.6 m ESO Telescope

Size rank in cohort

Rank by radius

#1168of 1978

top 59.0%

This planet

2.50R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

Kepler-126 d 2.50 R⊕ Kepler-254 d 2.50 R⊕ GJ 163 c 2.50 R⊕

Metric	Earth	GJ 581 c	Jupiter
Radius (R⊕)	1.00	2.50	11.21
Mass (M⊕)	1.00	6.81	317.83
Density (g/cm³)	5.51	2.39	1.33
Surface Gravity (g)	1.00	1.09	2.53
Insolation (S⊕)	1.00	—	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Best measured mass	6.810 M⊕	Direct mass measurement (not derived).
Minimum mass (M sin i)	5.500 M⊕	Lower bound from radial-velocity fitting.

Star Catalogue Identifiers

HIP

HIP 74995

TIC

TIC 36853511

Gaia DR2

Gaia DR2 6322070093095493504

Gaia DR3

Gaia DR3 6322070093095493504

System

GJ 581

Percentile among Sub-Neptune cohort

Radius 2.500 R⊕ · percentile 40 / cohort 1978

Mass 6.810 M⊕ · percentile 38 / cohort 1978

Orbital period 12.92 d · percentile 44 / cohort 1946

Distance 6.30 pc · percentile 1 / cohort 1975

ESI 0.704 · percentile 93 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

12.921 days

Semi-major axis

0.0718 AU

Eccentricity

0.032

Inclination

47.00 °

Year Length

A year here lasts approximately 12.92 Earth days (3.5% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0718 AU.

Extended Orbital Architecture

RV semi-amplitude (K)

3.100 m/s

Transit mid-time (BJD)

2,455,183.6900

Long. of periastron (ω)

16.00°

Angular separation (arcsec)

11.40000

Eq. Temperature

—

Insolation (S⊕)

—

Earth = 1.00

Earth Similarity (ESI)

0.704

0 = alien, 1 = Earth-identical

Insufficient insolation data. Close-orbiting tidally-locked around a cool host.

Discovery Paper

Primary reference

Udry et al. 2007

Instrument

HARPS Spectrograph

Publication

2007-07

Observation locale

Ground

Discovery cohort

Planets confirmed in 2007 at La Silla Observatory (10 shown).

GJ 674 b · Sub-Neptune HD 219828 b · Neptune-like HD 70573 b · Gas Giant HD 221287 b · Gas Giant HD 171028 b · Gas Giant BD-13 2130 b · Gas Giant HD 156846 b · Gas Giant HD 190647 b · Gas Giant HD 4113 b · Gas Giant HD 100777 b · Gas Giant

Host System: GJ 581

Spectral Class

M-type red dwarf

Effective Temperature

3,500 K — cooler than the Sun (5,778 K)

Estimated Age

4.30 Gyr — younger than the Sun (4.6 Gyr)

Stellar Radius

0.302 R☉

Stellar Mass

0.295 M☉

Metallicity [Fe/H]

-0.08

Extended Stellar Properties

Surface gravity (log g)

4.970 dex

Stellar density

12.224 g/cm³

Systemic radial velocity

9.75 km/s

Rotational v·sin i

2.00 km/s

Rotation period

132.50 days

Activity index (log R'HK)

-5.480

Metallicity ratio

[Fe/H]

Distance

6.30 parsec

Light-years 20.54 ly

V-band magnitude

10.57 mag

Voyager-speed travel 362,252 yr

Multi-band Host Photometry — 12 bands

Astrometric Data

Parallax

158.749 mas

Total Proper Motion

1,225.326 mas/yr

PM Right Ascension

-1,221.47 mas/yr

PM Declination

-97.13 mas/yr

Galactic Cartesian (pc)

x = -0.639 · y = -0.757 · z = -0.134

Equatorial (J2000)

RA 229.85647° · Dec -7.72269°

Galactic ℓ, b

354.072° · 40.022°

Ecliptic λ, β

229.513° · 10.255°

HTM-20 index

2005289755

Observation Record

Photometric series

RV measurements

Stellar spectra

Wikipedia → Image: Wikimedia Commons · Aldaron, a.k.a. Aldaron · CC BY-SA 3.0