A super-puff is a type of

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

with a

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

only a few times larger than

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

's but with a radius larger than that of

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

, giving it a very low mean

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

.The Featureless Transmission Spectra of Two Super-Puff Planets
Jessica E. Libby-Roberts, Zachory K. Berta-Thompson, Jean-Michel Desert, Kento Masuda, Caroline V. Morley, Eric D. Lopez, Katherine M. Deck, Daniel Fabrycky, Jonathan J. Fortney, Michael R. Line, Roberto Sanchis-Ojeda, Joshua N. Winn, 28 Oct 2019 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 Kepler-51 is a Sun-like star that is only about 500 million years old. It is orbited by three 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 Nept ...

which are all

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

-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 Kepler-87c is a planet orbiting Kepler-87, a star slightly more massive than the Sun and nearing the end of its main-sequence period. Characteristics Despite being larger than Neptune Neptune is the eighth planet from the Sun and the ...

. One hypothesis is that a super-puff has continuous outflows of dust to the top of its atmosphere (for example,

Gliese 3470 b Gliese 3470 b, abbreviated as GJ 3470b, is an exoplanet orbiting the star Gliese 3470, located in the constellation Cancer. With a mass of just under 14 Earth-masses and a radius approximately 4.3 times that of Earth's, it is likely something aki ...

)- 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 HIP 41378 f (also known as EPIC 211311380 f) is an exoplanet orbiting around the F-type star HIP 41378. It has an anomalously large radius (9.2 ) for a planet of its size and temperature, therefore the radius may actually be that of an opticall ...

References

Types of planet * {{exoplanet