“Super-Puffs” may sound like a new breakfast cereal. But it’s actually the nickname for a unique and rare class of young exoplanets that have the density of cotton candy. Nothing like them exists in our solar system.

New data from NASA’s Hubble Space Telescope have provided the first clues to the chemistry of two of these super-puffy planets, which are located in the Kepler 51 system. This exoplanet system, which actually boasts three super-puffs orbiting a young Sun-like star, was discovered by NASA’s Kepler space telescope in 2012. However, it wasn’t until 2014 when the low densities of these planets were determined, to the surprise of many.

The recent Hubble observations allowed a team of astronomers to refine the mass and size estimates for these worlds — independently confirming their “puffy” nature. Though no more than several times the mass of Earth, their hydrogen/helium atmospheres are so bloated they are nearly the size of Jupiter. In other words, these planets might look as big and bulky as Jupiter, but are roughly a hundred times lighter in terms of mass.

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