Gliese 876

Gliese 876 is located fairly close to the Solar System. According to astrometric measurements made by the Gaia spacecraft, the star shows a parallax of 214.038 milliarcseconds, which corresponds to a distance of . Despite being located so close to Earth, the star is so faint that it is invisible to the naked eye and can only be seen using a telescope. ==Stellar characteristics==

light curve for IL Aquarii, adapted from Hosey et al. (2015) As a red dwarf, Gliese 876 is much less massive than the Sun: estimates suggest it has only 35% of the mass of the Sun. The surface temperature of Gliese 876 is cooler than the Sun and the star has a smaller radius. These factors combine to make the star only 1.3% as luminous as the Sun, and most of this is at infrared wavelengths. Estimating the age and metallicity of cool stars is difficult due to the formation of diatomic molecules in their atmospheres, which makes the spectrum extremely complex. By fitting the observed spectrum to model spectra, it is estimated that Gliese 876 has a slightly lower abundance of heavy elements compared to the Sun (around 75% the solar abundance of iron). Based on chromospheric activity the star is likely to be around 6.5 to 9.9 billion years old, depending on the theoretical model used. However, its membership among the young disk population suggest that the star is less than 5 billion years old but the long rotational period of the star implies that it is at least older than 100 million years. Like many low-mass stars, Gliese 876 is a variable star, classified as a BY Draconis variable. Its brightness fluctuates by around 0.04 magnitudes. This type of variability is thought to be caused by large starspots moving in and out of view as the star rotates. Gliese 876 emits X-rays, as most Red Dwarfs do. The variability of the star's brightness was first detected by Edward W. Weis. It was given its variable star designation, IL Aquarii, in 1997. ==Planetary system==

Observation history On June 23, 1998, an extrasolar planet was announced in orbit around Gliese 876 by two independent teams led by Geoffrey Marcy and Xavier Delfosse. In 2014, reanalysis of the existing radial velocities suggested the possible presence of two additional planets, which would have almost the same mass as Gliese 876 d, In 2018 a study using hundreds of new radial velocity measurements found no evidence for any additional planets. None of these planets transit the star from the perspective of Earth, making it difficult to study their properties. GJ 876 is a candidate parent system for the ʻOumuamua object. The trajectory of this interstellar object took it near the star about 820,000 years ago with a velocity of 5 km/s, after which it has been perturbed by six other stars. Orbital arrangement Gliese 876 has a notable orbital arrangement. It is the first planetary system around a normal star to have mutual inclination between planets measured without transits (previously the mutual inclination of the planets orbiting the pulsar PSR B1257+12 had been determined by measuring their gravitational interactions The presence of surface liquid water and life is possible on sufficiently massive satellites should they exist. ; Gliese 876 b Gliese 876 b, discovered in 1998, is around twice the mass of Jupiter and revolves around its star in an orbit taking 61.104 days to complete, at a distance of only 0.21 AU, less than the distance from the Sun to Mercury. Its temperature makes it more likely to be a Class II or Class III planet in the Sudarsky model. The presence of surface liquid water and life is possible on sufficiently massive satellites should they exist. ; Gliese 876 e Gliese 876 e, discovered in 2010, has a mass similar to that of the planet Uranus and its orbit takes 124 days to complete. ==See also==