GJ 876 b was the first planet ever found around a red dwarf, the smallest and most common kind of star. It is a gas giant about twice Jupiter's mass, orbiting GJ 876, a red dwarf about 15 light-years away, every 61 days.
A team led by Geoffrey Marcy announced it in 1998 from measurements at the Lick and Keck observatories, using the radial velocity method, which detects the slight back-and-forth motion a planet's gravity gives its star. They found a minimum mass of about 2.1 Jupiter masses on a noticeably oval orbit about a fifth of Earth's distance from the Sun. At the time it was the closest planet known beyond the solar system. The team noted that a giant planet so close to a star a third of the Sun's mass was a challenge to theories of how planets form, and that finding one in a survey of only 24 red dwarfs suggested such planets might be common.
In 2002 it became the first planet beyond the solar system to have its mass measured by astrometry. A team led by G. F. Benedict used the Hubble Space Telescope's Fine Guidance Sensors to track the tiny shift in the star's position on the sky as the planet pulled it around. Combined with the earlier measurements, that gave its orbit's tilt and a true mass of about 1.9 Jupiter masses, not just a lower limit. GJ 876 b later turned out to be one of four planets. It and its sibling GJ 876 c orbit in a 2 to 1 rhythm, and with the outer planet GJ 876 e the three form a rare chain of linked orbits. As a gas giant it has no surface.