These Giant Planets Are So Light, They're Less Dense Than Cotton Candy

Imagine a planet the size of Jupiter, but so incredibly light it would float on water — if you could find a bathtub big enough. Now imagine two of them, locked in a cosmic dance 1,110 light-years away. That's what an international team of astronomers just found: two "super-puff" planets, TOI-791 b and TOI-791 c, with densities so low they make cotton candy look robust.

Let that satisfying number sink in: TOI-791 b clocks in at 0.038 grams per cubic centimeter. Its sibling, TOI-791 c, is a slightly heftier 0.047 g/cm³. For perspective, a fluffy cloud of carnival-spun sugar is around 0.05 g/cm³. Our own Earth, meanwhile, is a rather chunky 5.5 g/cm³. Jupiter, the gas giant we know and love, is 1.33 g/cm³ — meaning these new finds are 28 to 35 times less dense than the biggest planet in our solar system. Which, if you think about it, is both impressive and slightly terrifying.

A Cosmic Ballet in the Antarctic Dark

These planetary siblings didn't just appear. They formed together from the same swirling cosmic dust and gas, and they've got a rather intimate gravitational relationship. For every five times the inner planet orbits its star, the outer one completes almost exactly three. This 5:3 mean-motion resonance means they're constantly tugging on each other, subtly altering their paths.

Volunteers from the wonderfully named Planet Hunters TESS project first spotted these oddballs in 2019 and 2023, using NASA's Transiting Exoplanet Survey Satellite. But figuring out their true, wispy densities took eight years of intense observation. Part of that data came from the ASTEP telescope in Antarctica.

Turns out, the long, dark Antarctic winter is perfect for stargazing. Astronomers got uninterrupted views of these planets as they crossed in front of their star — events that lasted over 11 hours each. These are the longest continuous planetary crossings ever fully observed from the ground. Because apparently, that's where we are now: using Earth's poles to peer at planets lighter than dessert.

The Mystery of the Puffy Atmosphere

Scientists are still scratching their heads about how these super-puff planets get so… puffy. The leading theory is that they’re mostly enormous atmospheres of hydrogen and helium, wrapped around a relatively small solid core. Imagine a peach where the pit is tiny and the fuzzy skin is miles thick. These massive gas layers likely formed when the planets were far from their star, in the super-cold outer reaches of the early planetary disk, allowing gas to accumulate rapidly.

The next step? The James Webb Space Telescope. Researchers want to use its incredible capabilities to sniff out carbon, nitrogen, and oxygen in these enormous atmospheres. That chemical fingerprint could help unlock the secrets of how these truly bizarre, multi-planet systems came to be. And maybe, just maybe, explain why they're lighter than a snack food.