🪐 Astronomers Found Two Giant Planets Less Dense Than Cotton Candy
Astronomers have confirmed the existence of two of the puffiest planets ever discovered — gas giants roughly the size of Jupiter, but with densities so low they are less dense than cotton candy.
The pair, named TOI-791 b and TOI-791 c, orbit an F7-type star about 1,110 light-years from Earth in the southern constellation Volans. Their numbers are almost hard to believe: TOI-791 b has an average density of just 0.038 g/cm³, while TOI-791 c comes in at 0.047 g/cm³.
For comparison, Jupiter’s average density is about 1.33 g/cm³. Cotton candy is roughly 0.05 g/cm³. Earth is around 5.5 g/cm³.
That makes these planets not just “fluffy” by astronomical standards — they are among the lowest-density giant planets ever detected.
The discovery, published in Monthly Notices of the Royal Astronomical Society, is especially valuable because the two planets appear to be locked in a rare 5:3 orbital resonance: for every five orbits of the inner planet, the outer one completes almost exactly three. This gravitational interaction slightly shifts the timing of their transits across the star, allowing astronomers to estimate their masses.
🔹 The planets were first spotted by volunteers in the Planet Hunters TESS citizen-science project
🔹 Confirmation required eight years of observations
🔹 Data from the ASTEP telescope at Antarctica’s Concordia Station were crucial
🔹 Each transit lasts more than 11 hours — unusually long for ground-based observations
🔹 Only a handful of systems are known to contain multiple super-puff planets
The leading idea is that these worlds may have relatively small cores surrounded by enormous hydrogen- and helium-rich atmospheres. But exactly how such diffuse planets form — and how they keep their atmospheres for so long — remains an open question.
Important caveat: these measurements come from transits and orbital timing effects, not from direct imaging. The densities are robust within the current model, but the planets’ true atmospheric composition will require follow-up observations — potentially with the James Webb Space Telescope.
Super-puff planets are strange because they sit at the edge of what our planet-formation models can comfortably explain.
If a giant planet can be less dense than cotton candy and still hold itself together, what else is out there that our theories have not yet learned to expect?
📄 Source: https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stag864