Super-puff

A super-puff is a type of exoplanet with a mass only a few times larger than Earth's but with a radius larger than that of Neptune, giving it a very low mean density. They are cooler and less massive than the inflated low-density hot-Jupiters.

The most extreme examples known are the three planets around Kepler-51 which are all Jupiter-sized but with densities below 0.1 g/cm3. These planets were discovered in 2012 but their low densities were not discovered until 2014. Another example is Kepler-87c.

One hypothesis is that a super-puff has continuous outflows of dust to the top of its atmosphere (for example, Gliese 3470 b), so the apparent surface is really dust at the top of the atmosphere. Another possibility is that some of the super-puff planets are smaller planets with large ring systems, like HIP 41378 f.