Summary:
- Kepler-51d is an exoplanet (a planet orbiting a star outside our solar system) that is classified as a "superpuff" due to its extremely low density.
- Superpuff planets are rare and have a density lower than the density of Saturn, which is the least dense planet in our solar system.
- Kepler-51d is about 2.5 times the size of Earth but has only about 1% of Earth's mass, making it one of the least dense exoplanets known.