Key parameters at a glance

A radius of 2.20 Earth radii
A mass of 5.46 Earth masses
Surface gravity around 1.13 g
An orbital period of 36.116 days
Semi-major axis 0.1417 AU
Equilibrium temperature 234 K (-39 °C)
Distance from Earth 20.37 light-years
Earth Similarity Index 0.693
Travel time at Voyager 1 speed 359,164 years
Almost certainly tidally locked to its host star given the close orbit

Context from the literature

HN Librae, also known as Gliese 555, is a red dwarf with one or more orbiting exoplanets in the constellation Libra. With an apparent visual magnitude of 11.32, it can only be viewed through a telescope. The system is located at a distance of 20.4 light years based on parallax measurements, but is drifting closer to the Sun with a radial velocity of −1.4 km/s. It does not appear to belong to any known stellar moving group or association.

Excerpted from Wikipedia · full article

HN Lib b Planet Profile

Physical Specs

Radius

2.20 R⊕

Radius (Jupiter)

0.196 R♃

Mass

5.46 M⊕

Mass (Jupiter)

0.017 M♃

Density

2.82 g/cm³

Surface Gravity

1.13 g (Earth)

Classification

Planet Type

Sub-Neptune

ESI Score 0.693

HZ Position Habitable

Controversial No

Discovery

Year 2023

Method Radial Velocity

Facility Calar Alto Observatory

Telescope 3.5 m telescope

Size rank in cohort

Rank by radius

#1673of 1978

top 84.5%

This planet

2.20R⊕

Sub-Neptune median

2.71R⊕

Nearest-size peers

HD 110067 b 2.20 R⊕ Kepler-169 e 2.20 R⊕ Kepler-122 d 2.20 R⊕

Metric	Earth	HN Lib b	Jupiter
Radius (R⊕)	1.00	2.20	11.21
Mass (M⊕)	1.00	5.46	317.83
Density (g/cm³)	5.51	2.82	1.33
Surface Gravity (g)	1.00	1.13	2.53
Insolation (S⊕)	1.00	0.50	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Minimum mass (M sin i)	5.460 M⊕	Lower bound from radial-velocity fitting.

Star Catalogue Identifiers

HIP

HIP 71253

TIC

TIC 294401924

Gaia DR2

Gaia DR2 6324325225803432320

Gaia DR3

Gaia DR3 6324325225803432320

System

HN Lib

Percentile among Sub-Neptune cohort

Radius 2.200 R⊕ · percentile 15 / cohort 1978

Mass 5.460 M⊕ · percentile 15 / cohort 1978

Orbital period 36.12 d · percentile 78 / cohort 1946

Distance 6.24 pc · percentile 1 / cohort 1975

ESI 0.693 · percentile 92 / cohort 1978

Orbit & Habitability

Orbital Dynamics

Period

36.116 days

Semi-major axis

0.1417 AU

Eccentricity

0.079

Inclination

— °

Year Length

A year here lasts approximately 36.12 Earth days (9.9% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.1417 AU.

Eq. Temperature

234K

(-39 °C)

Insolation (S⊕)

0.50

Earth = 1.00

Earth Similarity (ESI)

0.693

0 = alien, 1 = Earth-identical

Within the conservative habitable zone — surface water would be frozen. Close-orbiting tidally-locked around a cool host.

Discovery Paper

Primary reference

Gonz Aacute Lez Aacute Lvarez et al. 2023

Instrument

CARMENES

Publication

2023-07

Observation locale

Ground

Discovery cohort

Planets confirmed in 2023 at Calar Alto Observatory (3 shown).

Wolf 1069 b · Rocky Terrestrial GJ 3988 b · Super-Earth GJ 724 b · Sub-Neptune

Host System: HN Lib

Spectral Class

M-type red dwarf

Effective Temperature

3,347 K — cooler than the Sun (5,778 K)

Estimated Age

4.40 Gyr — younger than the Sun (4.6 Gyr)

Stellar Radius

0.299 R☉

Stellar Mass

0.291 M☉

Metallicity [Fe/H]

-0.18

Extended Stellar Properties

Surface gravity (log g)

4.760 dex

Stellar density

13.236 g/cm³

Systemic radial velocity

-1.43 km/s

Rotational v·sin i

2.00 km/s

Rotation period

96.00 days

Activity index (log R'HK)

-5.226

Metallicity ratio

[Fe/H]

Distance

6.24 parsec

Light-years 20.37 ly

V-band magnitude

11.31 mag

Voyager-speed travel 359,164 yr

Multi-band Host Photometry — 8 bands

Astrometric Data

Parallax

160.114 mas

Total Proper Motion

691.347 mas/yr

PM Right Ascension

-355.04 mas/yr

PM Declination

593.22 mas/yr

Galactic Cartesian (pc)

x = -0.763 · y = -0.609 · z = -0.217

Equatorial (J2000)

RA 218.56848° · Dec -12.51701°

Galactic ℓ, b

338.420° · 43.157°

Ecliptic λ, β

220.178° · 2.479°

HTM-20 index

611189742

Wikipedia → Image: Wikimedia Commons · PopePompus · CC BY-SA 4.0