The most extreme examples known are the three planets aroundKepler-51 which are allJupiter-sized but with densities below 0.1 g/cm3.[1] These planets were discovered in 2012 but their low densities were not discovered until 2014.[2]Another example isKepler-87c.[1]
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.[2] Another possibility is that some of the super-puff planets are smaller planets with large ring systems, likeHIP 41378 f.[3]
The anomalous mass-to-radius ratio of super-puff planets was first interpreted as evidence for the presence of substantial hydrogen-helium envelopes formed billions of years ago within the protoplanetary disk.[6][7][8] In this long-term formation scenario, such envelopes would be prone to erosion through atmospheric escape processes, suggesting that maintaining extremely low densities over gigayear timescales would be difficult.[9][10][11][12][13] The persistence of known super-puffs has therefore motivated alternative models of envelope formation and retention.
^abcdThe 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
^Lee, E. J.; Chiang, E. (2016). "Breeding super-Earths and birthing super-puffs in transitional disks". The Astrophysical Journal. 817: 90.
^Chachan, Y.; Lee, E. J.; Knutson, H. A. (2021). "Radial gradients in dust-to-gas ratio lead to preferred region for giant planet formation". The Astrophysical Journal. 919: 63.
^Hanf, B.; Kincaid, W.; Schlichting, H.; Cappiello, L.; Tamayo, D. (2025). "Orbital migration through atmospheric mass loss". The Astronomical Journal. 169: 19.
^Gao, P.; Zhang, X. (2020). "Deflating super-puffs: Impact of photochemical hazes on the observed mass–radius relationship of low-mass planets". The Astrophysical Journal. 890: 93.
^Chachan, Y.; Jontof-Hutter, D.; Knutson, H. A.; Adams, D.; Gao, P.; et al. (2020). "A featureless infrared transmission spectrum for the super-puff planet Kepler-79d". The Astronomical Journal. 160: 201.
^Cubillos, P.; Erkaev, N. V.; Juvan, I.; Fossati, L.; Johnstone, C. P.; et al. (2017). "An overabundance of low-density Neptune-like planets". Monthly Notices of the Royal Astronomical Society. 466: 1868–1879.
^Thao, P. C.; Mann, A. W.; Feinstein, A. D.; Gao, P.; Thorngren, D.; et al. (2024). "The featherweight giant: Unraveling the atmosphere of a 17 Myr planet with JWST". The Astronomical Journal. 168: 297.
^Wang, L.; Dai, F. (2019). "Dusty outflows in planetary atmospheres: Understanding 'super-puffs' and transmission spectra of sub-Neptunes". The Astrophysical Journal Letters. 873: L1.
