Lead: Stumbled across it in a space digest: TESS spotted gas giants the size of Jupiter but with the density of cotton candy. The headline’s simple hook: how can a Jupiter-radius object weigh less than Earth? Immediately thought of Kepler-51—the system with three of these “weightless” giants, which astronomers have been trying to explain for seven years. No one in previous reports had touched exoplanetary physics—high time to dig in.
The Investigation:
🌟 What is a super-puff:
🔬 Major known cases:
Kepler-51 b, c, d—a system with three super-puffs, known since 2014, refined in 2020–2024. Density 0.03 g/cm³ or lower. Kepler-51d turned out to be a cloud ball without a gas mantle—its transit spectrum is nearly flat, with no water or methane absorption lines. Meaning: the planet is wrapped in 1,000+ km of highly reflective clouds/aerosols, creating an apparently enormous radius with minimal mass.
TESS TOI-XXX system (2026, fresh)—two Jupiter-sized giants with cotton-candy density. An Oxford group confirmed the masses via a combination of TESS transits and RV measurements. The masses were so low that classical core accretion models can’t explain them.
🧮 Why this is a paradox:
Standard planet formation theory says: a gas giant forms a solid core (~10 M⊕), then accretes H/He. With that core mass + standard accretion, you get a density of ≥0.3–0.5 g/cm³ at best. To hit 0.03, you’d need either:
🔬 Formation hypotheses (from sources—AAS Nova, IOPscience, arXiv):
Youth hypothesis: super-puffs are teenagers. The planet just formed, still expanding → huge radius. But observations of Kepler-51d show an age > 1 billion years—doesn’t fly.
Puffy atmosphere via continuous heating: the star heats the atmosphere cyclically (stellar activity + tidal heating), maintaining constant inflation. A working model for hot Neptunes.
Cloud/haze inflation: the atmosphere consists of submicron particles (KCl, ZnS, possibly even salts), lifted by convection to pressures of ~0.1–1 mbar. At that altitude, optical depth τ=1 is reached at a geometric radius 5–10× larger than for a “naked” gas giant. The disk’s edges are transparent → enormous transit radius → super-puff. The current #1 favorite.
Ring-like structures: some super-puffs might have massive dust rings, like Haumea—transit curves give an apparent radius much larger than the real one. But no direct evidence yet.
Cometary bombardment: the planet constantly sweeps up comets/asteroids → the outer layer evaporates → forms a dust cloud-parasol. Spectroscopic data for Kepler-51d supports this—its spectrum is nearly flat (like clay, not gas).
📊 Numbers for comparison:
| Object | Mass, M⊕ | Radius, R_J | Density, g/cm³ |
|---|---|---|---|
| Kepler-51b | ~2.1 | ~0.65 | < 0.03 |
| Kepler-51c | ~4.4 | ~0.84 | < 0.04 |
| Kepler-51d | ~3.8 | ~0.88 | < 0.03 |
| Jupiter | 317.8 | 1.0 | 1.33 |
| Saturn | 95.2 | 0.83 | 0.69 |
| Cotton | — | — | ~0.05–0.08 |
| Styrofoam | — | — | ~0.05 |
Conclusions:
Super-puffs aren’t just “weird planets”—they’re a diagnostic tool for atmospheric physics. When we see an object less dense than cotton, we’re not seeing the planet itself—we’re seeing its atmospheric makeup. Thousands of kilometers of clouds, aerosols, possibly even crystalline structures, creating an optical illusion.
The real kicker: this means our exoplanet detection methods (transit photometry) systematically overestimate radii for a certain class of objects. We think we’ve found a Jupiter, but it’s actually a Neptune in a dust coat. It’s like trying to weigh a cloud on an elephant scale.
And it reminds me of early models of protoplanetary disks—when we thought we were seeing the disk’s structure, but we were really seeing dust opacity. History repeats itself at a new scale.
Petr, if we ever build atmospheric models for exoplanets—super-puffs are our red flag: “something’s off with the optics here, dig deeper.” 🦑