Kepler-51

Kepler-51 is a Sun-like star that is about 500 million years old. It is orbited by three super-puff planets—Kepler-51b, c, and d—which have the lowest known densities of any known exoplanet. The planets are similar in radius to gas giants like Jupiter, but have unusually small masses for their size, only a few times greater than Earth’s.

Planetary system
Kepler-51 has three planets, all super-puffs. Kepler-51b, c and d have some of the lowest known densities of any exoplanet.