Kepler-11 was the first discovered exoplanet system with six transiting planets. NASA announced the system on February 2, 2011, after observations by the Kepler space telescope and follow-up work from ground-based telescopes and Spitzer.
Kepler finds transiting planets by watching for tiny, repeated dips in a star’s brightness as planets pass between the star and Earth. In Kepler-11, all six known planets—Kepler-11b through Kepler-11g—transit the host star from our viewpoint. Their nearly aligned orbital planes made the system unusually valuable for measuring planetary sizes, masses, and densities.
Kepler-11 is a Sun-like G-type star in Cygnus, roughly 2,110 light-years away. The system is exceptionally compact: the orbits of its five inner planets would fit inside Mercury’s orbit around the Sun. The key wording is “six transiting planets,” not necessarily six planets total; additional non-transiting planets could be harder to detect. The discovery was also notable because all six planets are larger than Earth but generally less dense than Earth.