What is a Super-puff?

A super-puff is a planet with a super-Earth-like mass (less than 5 M⊕) but a gas-giant radius (larger than 5 R⊕). On a mass–radius diagram, they stand out as extreme outliers, with mean densities as low as as 0.1 or 0.01 gcm^-3 (similar to the density of cotton candy). This is one or even two orders of magnitude lower than most exoplanets. A prime example is the Kepler-51 systems ( Masuda 2014). The host star is a young (~300Myr) sun-like star. It has 3 transiting super-puffs near 1:2:3 mean-motion resonance.

Fig 1. Super-puffs (e.g. Kepler-51 planets) are planets with the anomalously low mean densities (Libby-Roberts et al. 2020).

Why are Super-puffs so puzzling to planet formation theorists?

For starters, the extended atmospheres of super-puffs should undergo rapid mass loss. This issue was noted and discussed extensively by Owen & Wu 2016 . They named this phenomenon “boil-off”. Their models predict that super-puffs would shed their envelopes within only thousands to tens of thousands of years which is much shorter than the age of the planetary systems (hundreds of Myr or older).

Even more puzzling is that the extended atmospheres of super-puffs imply extremely large scale heights. For Kepler-51b, the scale height H is ~3000km compared to 10km on Earth. Such large scale heights should make super-puffs ideal targets for transmission spectroscopy. Yet, recent Hubble observation by Libby-Roberts et al. 2020 revealed that both Kepler-51b and Kepler-51d exhibit completely flat transmission spectra in the near-infrared.

Fig 2. The featureless transmission spectra of Kepler-51 b and d (Libby-Roberts et al. 2020).

If clouds are invoked to explain the flat transmission spectra of Kepler-51b and Kepler-51d, they would need to form at pressures near 0.1 mbar. Under equilibrium chemistry, however, clouds are expected to form where the planets’ pressure–temperature profiles intersect the condensation curves of abundant cloud-forming species. For Kepler-51b and Kepler-51d, this occurs around the 1 bar level which is much deeper than the high-altitude cloud deck required to obscure spectral absorption features.

A Simple Solution: Dusty Outflows

This puzzle persisted for years. Our proposed solution is surprisingly simple (Wang & Dai 2019). We considered a non-hydrostatic atmosphere with a moderate outflow of ~0.1 M⊕ per Gyr. Such an outflow is slow enough to be consistent with the system’s 300 Myr age i.e. preserving a H/He atmosphere. However, it is fast enough to loft small dust particles to high altitudes. The resulting high-altitude opacity both inflates the observed transit radius and suppresses spectral absorption features, simultaneously accounting for the planets’ puffiness and their flat transmission spectra.

The dusty outflow scenario may not be as far-fetched as it first appears, since atmospheric mass loss from sub-Neptune planets is thought to be common, if not ubiquitous.This is evidenced by the pronounced bimodal radius distribution of sub-Neptunes. Within the photoevaporation framework, Sun-like stars remain highly active in X-ray and extreme ultraviolet radiation for the first few hundred Myr of their lifetimes. Kepler-51b and Kepler-51d, orbiting a ~300 Myr-old star, are therefore expected to be actively losing their atmospheres through photoevaporation.

Our work was recognized as the first viable explanation for super-puffs and was featured on the AAS Nova website