Interior structures of cold super-puff planets studied for consistency with accretion theory

Star system: Kepler-51 | Distance from the Sun: 2615 light years

Super-puffs are a class of exoplanets characterized by sub-Saturn masses and extremely low bulk densities. Because these planets have such a low density, they are often compared to cotton candy due to their puffy appearance. While some of these planets can be explained by standard core accretion theory—the process where planets form through the gradual accumulation of dust and gas in a protoplanetary disk—others present a significant challenge to these traditional formation models.

Researchers computed the hydrostatic interior structures of 34 cold super-puffs to see if their physical properties align with current planetary formation models. By adjusting for planet age, they determined whether the observed masses and radii could be explained by standard processes. This modeling approach helps clarify which planets fit established theories and which require alternative explanations.

The analysis reveals that 28 of the 34 studied planets are consistent with standard core accretion models. However, six specific planets—HIP 41378 f, Kepler-30 d, Kepler-51 d, Kepler-177 c, TOI-1420 b, and WASP-107 b—defy these expectations. These planets may require non-standard explanations, such as the presence of exo-rings, or additional heat sources. The study also tested whether radiogenic heating from radioactive decay or late-stage impacts could explain the inflation of these planets. While radiogenic heating proved insufficient, late impacts with smaller objects can plausibly explain the inflation for over one billion years.

Key findings
- 28 of the 34 examined cold super-puffs are consistent with core accretion theory.
- Six planets including Kepler-51 d are inconsistent with standard accretion models.
- Non-standard factors like exo-rings may explain the physical properties of the outliers.
- Late-stage impacts with smaller objects can cause planetary inflation for over 1 billion years.
- Radiogenic heating is insufficient to account for the low densities observed in super-puffs.

Scientific study

Do Super-Puffs Defy Core Accretion? Population-Wide Interior Structure Constraints (external link)