HD 180902

HD 180902

Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Sagittarius
Right ascension	19^h 19^m 17.708^s[1]
Declination	−23° 33′ 29.36″[1]
Apparent magnitude (V)	7.785[2]
Characteristics
Spectral type	K0 III/IV[3]
B−V color index	+0.94[4]
Astrometry
Radial velocity (Rv)	−2.781±0.140[2] km/s
Proper motion (μ)	RA: 37.832 mas/yr[1] Dec.: −20.334 mas/yr[1]
Parallax (π)	9.5339 ± 0.0521 mas[1]
Distance	342 ± 2 ly (104.9 ± 0.6 pc)
Absolute magnitude (MV)	2.5±0.3[4]
Orbit[5]
Primary	HD 180902 A
Companion	HD 180902 B
Period (P)	5880±440 d
Semi-major axis (a)	7.15±0.69 AU
Eccentricity (e)	0.335±0.025
Periastron epoch (T)	2441100±1200 JD
Argument of periastron (ω) (secondary)	73.3±1.6°
Semi-amplitude (K1) (primary)	898±28 km/s
Details
HD 180902 A
Mass	1.698±0.085[2] M☉
Radius	4.247±0.212[2] R☉
Luminosity	9.4±0.5[4] L☉
Temperature	5,030±44[4] K
Metallicity [Fe/H]	0.04±0.03[4] dex
Age	2.8±0.7[4] Gyr
HD 180902 B
Mass	44.53^+12.91 −5.88[2] MJup
Other designations
CD−23° 15276, HD 180902, HIP 94951, SAO 187961, PPM 269620[6]
Database references
SIMBAD	data

HD 180902 is a star with two or more orbiting companions in the southern constellation of Sagittarius. This system is located at a distance of approximately 342 light years from the Sun based on parallax measurements, but is drifting closer with a radial velocity of −2.8 km/s.[2] It has an absolute magnitude of 2.5,[4] but at that distance the apparent visual magnitude of the system is 7.8,[2] which is too faint to be seen with the naked eye.

The spectrum of the primary, component A, presents as an evolving subgiant star[5] with a stellar classification of K0 III/IV.[3] It is an estimated 2.8[4] billion years old with 1.7 times the mass of the Sun. The star has expanded to 4.2 times the radius of the Sun[2] and is radiating 9.4 times the Sun's luminosity from an enlarged photosphere at an effective temperature of 5,030 K.[4]

Companions

HD 180902 b was discovered using the Doppler spectroscopy method with observations taken at the W. M. Keck Observatory.[4] The radial velocities showed a long term linear trend in the data indicating an additional companion of unknown nature with a longer orbital period.[4] This was subsequently shown to be due to an orbiting brown dwarf or low mass stellar companion, designated component B.[5]

There is a second unconfirmed planet, HD 180902 c, with a mass at least twice that of Neptune and an orbital period of 15 days.[5]

The HD 180902 planetary system[5]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	≥1.685±0.041 MJ	1.40±0.11	510.9±1.5	0.107±0.022	—	—
c (unconfirmed)	≥0.099±0.014 MJ	0.139±0.011	15.9058±0.0055	0.28±0.13	—	—

References

1. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
2. Kervella, Pierre; et al. (March 2019), "Stellar and substellar companions of nearby stars from Gaia DR2. Binarity from proper motion anomaly", Astronomy & Astrophysics, 623: 23, arXiv:1811.08902, Bibcode:2019A&A...623A..72K, doi:10.1051/0004-6361/201834371, S2CID 119491061, A72.
3. Houk, N.; Smith-Moore, M. (1988). Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Declinations -26°.0 to -12°.0. Vol. 4. Bibcode:1988mcts.book.....H.
4. Johnson, John Asher; et al. (2010). "Retired A Stars and Their Companions. IV. Seven Jovian Exoplanets from Keck Observatory". Publications of the Astronomical Society of the Pacific. 122 (892): 701–711. arXiv:1003.3445. Bibcode:2010PASP..122..701J. doi:10.1086/653809.
5. Luhn, Jacob K.; et al. (2019). "Retired A Stars and Their Companions. VIII. 15 New Planetary Signals around Subgiants and Transit Parameters for California Planet Search Planets with Subgiant Hosts". The Astronomical Journal. 157 (4). 149. arXiv:1811.03043. Bibcode:2019AJ....157..149L. doi:10.3847/1538-3881/aaf5d0. S2CID 102486961.
6. "HD 180902". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-09-21.