HN Librae

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,^[3] 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.^[2] It does not appear to belong to any known stellar moving group or association.^[4]

HN Librae

A visual band light curve for HN Librae, adapted from Weis (1994)[1]
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Libra
Right ascension	14h 34m 16.81166s[2]
Declination	−12° 31′ 10.4145″[2]
Apparent magnitude (V)	11.32[3]
Characteristics
Spectral type	M4.0V[4]
B−V color index	1.633±0.052[3]
Variable type	BY Dra[5]
Astrometry
Radial velocity (Rv)	−1.36±0.20[2] km/s
Proper motion (μ)	RA: −355.138 mas/yr[2] Dec.: 593.040 mas/yr[2]
Parallax (π)	159.9225±0.0546 mas[2]
Distance	20.395 ± 0.007 ly (6.253 ± 0.002 pc)
Absolute magnitude (MV)	12.41[3]
Details[4]
Mass	0.291±0.013 M☉
Radius	0.299±0.009 R☉
Luminosity (bolometric)	0.010106±0.000069 L☉
Surface gravity (log g)	4.76±0.13 cgs
Temperature	3347±50 K
Metallicity [Fe/H]	−0.18±0.15 dex
Rotation	96±2 d
Rotational velocity (v sin i)	<2.0 km/s
Age	0.8–8.0 Gyr
Other designations
HN Lib, BD−11 3759, GJ 555, HIP 71253, Ci 20 870, LFT 1120, LHS 2945, LPM 532, LTT 5759, NLTT 37751, PLX 3296, PM 14316-1219, Wolf 1481, TYC 5572-804-1, GSC 05572-00804, 2MASS J14341683-1231106[6]
Database references
SIMBAD	data
Exoplanet Archive	data
HN Librae Location of HN Librae in the constellation Libra

This is an M-type main-sequence star, a red dwarf, with a stellar classification of M4.0V. The chromosphere of this star is weakly active, causing starspots that vary the stellar luminosity as it rotates.^[4] It has 29% of the mass of the Sun and 30% of the Sun's girth. On average, the star is radiating just 1% of the luminosity of the Sun from its photosphere at an effective temperature of 3,347 K. The star is spinning slowly with a rotation period of around 96 days.^[4]

Planetary system

In 2019, one planet candidate detected by radial velocity was reported in a preprint (never accepted for publication as of 2024), among 118 planets around M dwarf stars. This would have a minimum mass about 30 times that of Earth and orbit with a period of about 450 days.^[7]

However, later radial velocity observations by the CARMENES survey published in 2023 did not confirm a planet at this period, but instead found a different planet.^[8] This is a super-Earth or mini-Neptune (the discovery paper uses the term "sub-Neptune") with a minimum mass of 5.5 Earths and a period of 36 days, placing it within the habitable zone. A second planet candidate was also found, with a minimum mass of 9.7 Earths and a period of 113 days, but this signal could not be confirmed as having a planetary origin due to its similarity to the rotation period of the star.^[4]

The HN Librae planetary system^[4]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	≥5.46±0.75 M🜨	0.1417±0.0023	36.116+0.027 −0.029	0.079+0.090 −0.055	—	—
c (unconfirmed)	≥9.7±1.9 M🜨	0.3040+0.0048 −0.0051	113.46+0.19 −0.20	—	—	—