{{Short description|Planet with a mass slightly above Earth but with a radius larger than Neptune}} thumb|Artistic representation of a super-puff planet 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.<ref name="two">[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</ref> They are cooler and less massive than the inflated low-density hot-Jupiters.<ref name="two"/>
The most extreme examples known are the three planets around Kepler-51 which are all Jupiter-sized but with densities below 0.1 g/cm<sup>3</sup>.<ref name="two"/> These planets were discovered in 2012 but their low densities were not discovered until 2014.<ref name=sciencealert>{{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 }}</ref> Another example is Kepler-87c.<ref name="two"/>
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.<ref name=sciencealert/> Another possibility is that some of the super-puff planets are smaller planets with large ring systems, like HIP 41378 f.<ref>{{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}}</ref>
A 2026 study theorized that super-Earths and sub-Neptunes are puffy when they are young but shrink in size over time, with systems V1298 Tauri and Kepler-51 given as examples.<ref>[https://www.sciencefocus.com/news/missing-link-planet-forming Most planets in our galaxy are born ‘bloated’, new study suggests], BBC Science Focus, 7 Jan 2026</ref><ref>[https://www.nature.com/articles/s41586-025-09840-z A young progenitor for the most common planetary systems in the Galaxy], Nature, 7 Jan 2026</ref>
==Formation hypotheses== 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.<ref>Lee, E. J.; Chiang, E. (2016). "Breeding super-Earths and birthing super-puffs in transitional disks". The Astrophysical Journal. 817: 90.</ref><ref>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.</ref><ref>Hanf, B.; Kincaid, W.; Schlichting, H.; Cappiello, L.; Tamayo, D. (2025). "Orbital migration through atmospheric mass loss". The Astronomical Journal. 169: 19.</ref> 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.<ref>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.</ref><ref>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.</ref><ref>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.</ref><ref>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.</ref><ref>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.</ref> The persistence of known super-puffs has therefore motivated alternative models of envelope formation and retention.
==References== {{reflist}}
Category:Types of planet *
{{Extrasolar-planet-stub}}{{exoplanet}}