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Concept
Gliese 876
Résumé
Gliese 876 is a red dwarf star away from Earth in the constellation of Aquarius. It is one of the closest known stars to the Sun confirmed to possess a planetary system with more than two planets, after GJ 1061, YZ Ceti, Tau Ceti, and Wolf 1061; as of 2018, four extrasolar planets have been found to orbit the star. The planetary system is also notable for the orbital properties of its planets. It is the only known system of orbital companions to exhibit a near-triple conjunction in the rare phenomenon of Laplace resonance (a type of resonance first noted in Jupiter's inner three Galilean moons). It is also the first extrasolar system around a normal star with measured coplanarity. While planets b and c are located in the system's habitable zone, they are giant planets believed to be analogous to Jupiter.
Gliese 876 is located fairly close to the Solar System. According to astrometric measurements made by the Gaia space observatory, 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.
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.
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