HD 106315

HD 106315, or K2-109, is a single star with a pair of close-orbiting exoplanets, located in the constellation of Virgo. Based on parallax measurements, this system lies at a distance of 356 light years from the Sun.^[1] At that range, the star is too faint to be seen with the naked eye, as it has an apparent visual magnitude of 8.95.^[2] But it is slowly drifting closer with a radial velocity of −3 km/s.^[1] As of 2020^[update], multiplicity surveys have not detected any stellar companions to HD 106315.^[5]

HD 106315

Observation data Epoch J2000      Equinox J2000
Constellation	Virgo
Right ascension	12h 13m 53.3962s[1]
Declination	−00° 23′ 36.5534″[1]
Apparent magnitude (V)	8.951[2]
Characteristics
Evolutionary stage	Main sequence
Spectral type	F5V[3]
B−V color index	0.45[2]
Astrometry
Radial velocity (Rv)	−3.2±0.3[1] km/s
Proper motion (μ)	RA: −2.36±0.03 mas/yr[1] Dec.: 11.943±0.019 mas/yr[1]
Parallax (π)	9.1725±0.0221 mas[1]
Distance	355.6 ± 0.9 ly (109.0 ± 0.3 pc)
Details
Mass	1.105+0.028 −0.036[4] M☉
Radius	1.286+0.049 −0.040[4] R☉
Luminosity	2.432+0.057 −0.234[4] L☉
Surface gravity (log g)	4.261+0.027 −0.024[4] cgs
Temperature	6,300±37[4] K
Metallicity [Fe/H]	−0.268+0.060 −0.071[4] dex
Rotation	4.78±0.15[5]
Rotational velocity (v sin i)	12.9±0.4[4] km/s
Age	3.987+0.802 −0.516[4] Gyr
Other designations
BD+00 2910, HD 106316, TYC 4940-868-1, GSC 04940-00868, 2MASS J12135339-0023365, K2-109, Gaia DR3 3698307419878650240[6]
Database references
SIMBAD	data

The spectrum of HD 106315 presents as an ordinary F-type main-sequence star with a stellar classification of F5V,^[3] indicating it is generating energy through hydrogen fusion at its core. It is estimated to be roughly four^[4] billion years old but is spinning quickly with a rotation period of 5 days. The star is relatively metal-poor, having 60% of solar concentration of iron. It has only a low level of magnetic activity in its chromosphere, showing a minimal level of star spot coverage.^[5] The star has 11% more mass and a 29% larger radius than the Sun. It is radiating 2.4 times the luminosity of the Sun from its photosphere at an effective temperature of 6,300 K.^[4]

Planetary system

Two planets were detected by the transit method in 2017,^[4] using data from the extended Kepler mission (K2). Their large planetary radii imply both planets have a massive steam atmosphere for planet b and hydrogen-helium atmosphere for planet c.^[5] The planetary system of HD 106315 is rather unstable and current planetary orbits are the outcome of violent dynamical history,^[7] strongly affected by relativistic effects.^[8] The orbits of planets are nearly coplanar, and orbit of c is well aligned with the equatorial plane of the star, misalignment been equal to -10^+3.6
_−3.8°.^[9]

Since 2017, a third outer planet with mass above 45M_E is suspected to exist in the system.^[2]

Artist's impression of the two known planets in the HD 106315 system and their size comparison with Earth and Neptune

The HD 106315 planetary system

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	10.5±3.1[5] M🜨	0.0924+0.0011 −0.0012	9.55288±0.00021	0	87.6+3.0 −1.7°	2.4±0.2 R🜨
c	12.0±3.8[5] M🜨	0.1565+0.0019 −0.0020	21.05652±0.00012	0	88.89+0.69 −0.51°	4.379±0.086 R🜨