Gliese 900

Gliese 900 (GJ 900, BD+00 5017) is a triple star system, located 68 light-years from Earth in the constellation Pisces. It is made up of three main sequence stars: one is a K-type star, the two others are M-dwarf stars. The two M-dwarfs form a binary system with a period of 36 years, and this system has a period of 80 years around the primary component. With an apparent magnitude of 9.546, Gliese 900 is not visible to the naked eye. A widely separated planet has been detected around the system.

Stellar system
Gliese 900 is a hierarchical star system, made up of three main sequence stars: The primary component (Gliese 900 A) is a K5-K7 type star, that has 0.64–0.67 times the mass of the Sun, 0.72 times its radius, and 12% its luminosity. A light curve from the Transiting Exoplanet Survey Satellite (TESS) shows that its rotational period is 12 days. Gliese 900 A has a high level of chromospheric and coronal activity, although its apparent brightness presents little variation. The other components are red dwarf stars. Gliese 900 B has a spectral type of M3–M4 and a mass between 0.24 and 0.34 solar masses. Gliese 900 C has a spectral type of M5–M6 and a mass between 0.16 and 0.24 solar masses.

The system is young, about 200 million years old, and is a likely member (99.7% probability) of the nearby moving group Carina-Near. It is a source of X-ray emission, with an observed flux of $6.66$, and is also a source of ultraviolet emission. The emission of X-rays is typical of young stars, and classifies it as one. The TESS light curve identifies stellar flares on this star.

Orbit
Gliese 900 B and C form an inner pair (named Gliese 900 BC) with an orbital period of around 36 years. Gliese 900 BC and Gliese 900 A orbit the system's center of mass with a period of 80 years. B was separated from A by 751milliarcseconds and C was separated from A by 708mas. This separation changes over time. It was identified as a multiple star system in 2002 by Eduardo L. Martín, using adaptive optics-corrected images at the 8.2 m Subaru Telescope. When first observed, the A–B and A–C separations were of 0.51 and 0.76 arcsecs respectively. A further study by Malogolovets et al. (2007) identified this system as a hierarchical triple.

Other stars in the system
Malogolovets et al. (2007) reported two other objects in 2MASS images (potentially late red dwarfs) that would be the components D and E and make the system quintuple, being "very likely" to be bound to the system. However, these faint stars haven't been confirmed as members of the GJ 900 system, and are likely not associated.

Motion
Gliese 900 is located 68 light-years from Earth, based on parallax measurements by the Gaia spacecraft. The BP-RP spectra suggest a distance of 67.7 ly. The space velocity components of this system are U = $4.84$, V = $9.15$ and W = $6.76$. Gliese 900 is part of the thin disk population of the Milky Way. It was once classified as part of the IC 2602 supercluster. A newer analysis using kinematics from the Gaia spacecraft suggest that Gliese 900 has a 99.7% chance of being a member of the Carina-Near moving group and a 0.3% chance of being a field star, i.e. not associated to any star cluster or stellar association.

Planetary system
A 2024 study led by Austin Rothermich identified CWISE J233531.55+014219.6 (abbreviated to CW2335+0142) as a proper motion companion to Gliese 900, with 99.5% probability. This object, also called Gliese 900 b or Gliese 900 (ABC)b, is a planetary-mass object that has 10.5 times the mass of Jupiter (or 0.01 times the mass of the Sun ) and a spectral type T9. It was found to be at an angular separation of 587" from Gliese 900. At the estimated distance to this system, it translates to a projected separation of 12,000 astronomical units.

, Gliese 900 b has the largest observed separation of any known planet, and assuming a circular orbit, the longest orbital period. The orbital period is estimated at 1.27 or 1.4 million years based on the projected separation. Due to the similar spectral type, orbital separation and age, CW2335+0142 has been compared to COCONUTS-2b by the discovery team.