GJ 9827

GJ 9827 is a star in the constellation of Pisces. It is a K-type main-sequence star with an apparent magnitude of 10.250.[2] It is 97 light-years (30 parsecs) away, based on parallax.[1]

GJ 9827
Observation data
Epoch J2000      Equinox J2000
Constellation Pisces
Right ascension 23h 27m 04.8377s[1]
Declination −01° 17′ 10.5856″[1]
Apparent magnitude (V) 10.250[2]
Characteristics
Spectral type K6 V[2]
B−V color index +1.30[3]
Astrometry
Radial velocity (Rv)27.00±3.7[4] km/s
Proper motion (μ) RA: 376.019±0.094[1] mas/yr
Dec.: 216.071±0.070[1] mas/yr
Parallax (π)33.6855 ± 0.0611[1] mas
Distance96.8 ± 0.2 ly
(29.69 ± 0.05 pc)
Details
Mass0.593±0.018[5] M☉
Radius0.579±0.018[5] R☉
Luminosity0.119+0.035
−0.029[2] L☉
Surface gravity (log g)4.682±0.021[5] cgs
Temperature4,294±52[5] K
Metallicity [Fe/H]−0.26±0.08[5] dex
Rotation16.9+2.14
−1.5 d[6]
Rotational velocity (v sin i)1.3±1.5[2] km/s
Other designations
BD−02° 5958, GJ 9827, HIP 115752, LTT 9542, 2MASS J23270480-0117108, K2-135[7]
Database references
SIMBADdata
ARICNSdata

The rotation period of the star cannot be determined as in 2020, and can be either around 15 or 30 days, depending on interpretation of available data.[5]

Planetary system

It has 3 transiting planets seen by the Kepler space observatory in their K2 survey. As of October 2017, it is the closest star discovered to have transiting exoplanets found by either the Kepler or K2 missions. The planets (b, c, d) have radii of 1.62, 1.27, and 2.09 times that of the Earth, and periods of 1.209, 3.648, and 6.201 days (ratios 1:3:5).[2] Because of its close distance the system is considered an excellent target for studying atmospherics of exoplanets.

In late 2017, the masses of all three planets were determined using the Planet Finder Spectrograph on the Magellan II Telescope. Planet b was found to be very iron-rich, planet c appears to be mainly rocky, and planet d is a typical volatile-rich planet. GJ 9827b is noted as being one of the densest planets yet found, with its mass containing about ≥50% iron.[2]

More precise radial velocity measurements released in late February 2018 revealed that all three planets have a lower density than Earth and have some amount of volatiles in their compositions. GJ 9827b and c are mainly rocky (containing less than 1% mass fraction of water, and negligible helium and hydrogen) with very thin volatile envelopes,[5] while GJ 9827d is more akin to a Mini-Neptune. The loss of primordial atmosphere was indirectly confirmed in 2020 as no helium was detected at Gliese 9827 d though.[8] With a mass of about 1.5 M⊕, GJ 9827c is one of the least massive planets detected with radial velocity.[9] The atmospheres for GJ 9827b and GJ 9827d were not detected at all in 2021 observations.[10]

The GJ 9827 planetary system[2][9][5]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b 4.87±0.37 M⊕ 0.01866±0.00019 1.2089765(23) 0 88.0+1.1
−1.9°		 1.529±0.058 R⊕
c 1.92±0.49 M⊕ 0.03896+0.00039
−0.0004		 3.648096(24) 0 89.27+0.51
−1.0°		 1.201±0.046 R⊕
d 3.42±0.62 M⊕ 0.0555+0.00055
−0.00057		 6.20183(1) 0 87.722+0.077
−0.25°		 1.955±0.075 R⊕

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. Rodriguez, Joseph E; Vanderburg, Andrew; Eastman, Jason D; Mann, Andrew W; Crossfield, Ian J. M; Ciardi, David R; Latham, David W; Quinn, Samuel N (2018). "A System of Three Super Earths Transiting the Late K-Dwarf GJ 9827 at 30 pc". The Astronomical Journal. 155 (2): 72. arXiv:1709.01957. Bibcode:2018AJ....155...72R. doi:10.3847/1538-3881/aaa292. S2CID 55459523.
  3. Mumford, G. S. (1956). "Photoelectric observations of red dwarf stars". Astronomical Journal. 61: 213–218. Bibcode:1956AJ.....61..213M. doi:10.1086/107329.
  4. Kharchenko, N. V.; et al. (2007). "Astrophysical supplements to the ASCC-2.5: Ia. Radial velocities of ~55000 stars and mean radial velocities of 516 Galactic open clusters and associations". Astronomische Nachrichten. 328 (9): 889. arXiv:0705.0878. Bibcode:2007AN....328..889K. doi:10.1002/asna.200710776. S2CID 119323941.
  5. Kosiarek, Molly R.; et al. (2020). "Physical Parameters of the Multi-Planet Systems HD 106315 and GJ 9827". The Astronomical Journal. 161: 47. arXiv:2009.03398. doi:10.3847/1538-3881/abca39. S2CID 221534625.
  6. Niraula, Prajwal; et al. (2017). "Three Super-Earths Transiting the Nearby Star GJ 9827". The Astronomical Journal. 154 (6): 266. arXiv:1709.01527. Bibcode:2017AJ....154..266N. doi:10.3847/1538-3881/aa957c. S2CID 59939000.
  7. "BD-02 5958". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 8 October 2017.
  8. Kasper, David; Bean, Jacob L.; Oklopčić, Antonija; Malsky, Isaac; Kempton, Eliza M.-R.; Désert, Jean-Michel; Rogers, Leslie A.; Mansfield, Megan (2020). "Non-detection of Helium in the Upper Atmospheres of Three Sub-Neptune Exoplanets". The Astronomical Journal. 160 (6): 258. arXiv:2007.12968. Bibcode:2020AJ....160..258K. doi:10.3847/1538-3881/abbee6. S2CID 220793801.
  9. Prieto-Arranz, J.; et al. (2018). "Mass determination of the 1:3:5 near-resonant planets transiting GJ 9827 (K2-135)". Astronomy and Astrophysics. 618. A116. arXiv:1802.09557. Bibcode:2018A&A...618A.116P. doi:10.1051/0004-6361/201832872. S2CID 119379575.
  10. A multi-wavelength look at the GJ 9827 system No evidence of extended atmospheres in GJ 9827 b and d from HST and CARMENES data, 2021, arXiv:2101.06277
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