GJ 876 c is the planet that revealed the first known pair of exoplanets locked in step. It is a gas giant orbiting GJ 876, a red dwarf about 15 light-years away, every 30 days, about half the period of its outer sibling, GJ 876 b. For every two orbits of c, b completes one.
It was announced in January 2001 by Geoffrey Marcy, Debra Fischer, Paul Butler, Steve Vogt and Jack Lissauer, using the radial velocity method, which detects the slight back-and-forth motion planets' gravity gives their star. Keck Observatory described it as the first known multi-planet system in a 2 to 1 resonance, and noted that the planets' synchronised orbits had at first made the second planet hard to detect. GJ 876 c has a minimum mass of about half Jupiter's.
How the two came to be locked together became a puzzle in its own right. In 2002 a study led by Man Hoi Lee showed that the pair sit deep in three linked resonances at once, in a different arrangement from Jupiter's moons Io and Europa because the GJ 876 orbits are much more stretched. They found that the outer planet drifting inward through the disk the planets formed in would have captured them into this resonance. In 2010 a fourth planet, GJ 876 e, was found in step with both, making a chain of three. According to the NASA Exoplanet Archive, GJ 876 c receives about three-quarters of the light Earth gets from the Sun, but as a gas giant it has no surface.