The star is known to be a low activity flare star, which means it undergoes random increases in luminosity because of magnetic activity at the surface. The spectrum shows emission lines of calcium in the H and K bands. The emission of X-rays has been detected from the corona of this star.[10] These may be caused by magnetic loops interacting with the gas of the star's outer atmosphere. No large-scale star spot activity has been detected.[2]
Asubstellar companion was discovered in 1994 by Caltech astronomers Kulkarni, Tadashi Nakajima, Keith Matthews, and Rebecca Oppenheimer, and Johns Hopkins scientists Sam Durrance and David Golimowski. It was confirmed in 1995 as Gliese 229B,[12][13] It was the first brown dwarf to be confirmed. Although too small to sustain hydrogen-burning nuclear fusion as in a main sequence star, with a mass of around 40 to 60 times that of Jupiter (0.06 solar masses),[5][9] it is still too massive to be a planet. As a brown dwarf, its core temperature is high enough to initiate the fusion of deuterium with a proton to form helium-3, but it is thought that it used up all its deuterium fuel long ago.[14] This object has a surface temperature of 950 K.[15]
The most recent parameters for Gliese 229 B as of 2022 come from a combination of data from radial velocity, astrometry, and imaging, showing that it is about 60.4 times the mass of Jupiter, and on an eccentric orbit with a semi-major axis of about 28.9 AU and an orbital period of about 217 years.[5]
Inconsistencies between the measured mass and luminosity of Gliese 229 B suggest that it may in fact be an unresolved binary brown dwarf.[3][16]
In March 2014, a super-Neptune mass planet candidate was announced in a much closer-in orbit around GJ 229.[17] Given the proximity to the Sun, the orbit of GJ 229 Ab might be fully characterized by the Gaia space-astrometry mission or via direct imaging. In 2020, a super-Earth mass planet was discovered around GJ 229. GJ 229 Ac orbits the star closer in than GJ 229 Ab, located towards the outer edge but still well inside the star's habitable zone and in that sense similar to Mars in our own Solar System. While considering GJ 229 Ab an unconfirmed candidate, the study estimated a significantly lower minimum mass for it.[18] As of 2022[update], most sources consider both planets to be confirmed.[5][19][20][21]
If the planets Gliese 229 Ab & c orbit in the same plane as the brown dwarf Gliese 229 B, their true masses would be significantly greater than their minimum masses, making them both nearly as massive as Saturn.[nb 2]
^ abWhite, Stephen M.; Jackson, Peter D.; Kundu, Mukul R. (December 1989). "A VLA survey of nearby flare stars". Astrophysical Journal Supplement Series. 71: 895–904. Bibcode:1989ApJS...71..895W. doi:10.1086/191401.
^Díez Alonso, E.; Caballero, J. A.; Montes, D.; De Cos Juez, F. J.; Dreizler, S.; Dubois, F.; Jeffers, S. V.; Lalitha, S.; Naves, R.; Reiners, A.; Ribas, I.; Vanaverbeke, S.; Amado, P. J.; Béjar, V. J. S.; Cortés-Contreras, M.; Herrero, E.; Hidalgo, D.; Kürster, M.; Logie, L.; Quirrenbach, A.; Rau, S.; Seifert, W.; Schöfer, P.; Tal-Or, L. (2019). "CARMENES input catalogue of M dwarfs. IV. New rotation periods from photometric time series". Astronomy and Astrophysics. 621: A126. arXiv:1810.03338. Bibcode:2019A&A...621A.126D. doi:10.1051/0004-6361/201833316. S2CID111386691.
^Gliese, W. (1969). "Catalogue of Nearby Stars". Veröffentlichungen des Astronomischen Rechen-Instituts Heidelberg. 22: 1. Bibcode:1969VeARI..22....1G.
^Tuomi, Mikko; et al. (2014). "Bayesian search for low-mass planets around nearby M dwarfs – Estimates for occurrence rate based on global detectability statistics". Monthly Notices of the Royal Astronomical Society. 441 (2): 1545. arXiv:1403.0430. Bibcode:2014MNRAS.441.1545T. doi:10.1093/mnras/stu358. S2CID32965505.
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and the absolute visual magnitude of the Sun 
, the visual luminosity can be calculated by 
