Super-puff
{{Short description|Planet with a mass slightly above Earth but with a radius larger than Neptune}}
File:Super-puff_planet-bpk.png
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.[https://arxiv.org/abs/1910.12988 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 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.{{Cite web|url=https://www.sciencealert.com/adorably-named-super-puff-planets-are-like-nothing-in-the-solar-system|title=Astronomers Confirm the Existence of Planets That Have the Lightness of Cotton Candy|date=20 December 2019 }}
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.{{cite web |title=These So-Called 'Super-Puff' Worlds Could Be Exoplanets with Rings |url=https://www.discovermagazine.com/the-sciences/these-so-called-super-puff-worlds-could-be-exoplanets-with-rings |website=Discover Magazine |access-date=11 January 2020}}