Astronomers Find 2 Jupiter-Size Super-Puffs 1,110 Light-Years Away as Densities Fall Below Cotton Candy

Q: How can planets the size of Jupiter be lighter than cotton candy?

They can be Jupiter-sized yet “lighter than cotton candy” because size and density are not the same thing. These newly found worlds are enormous in volume but contain relatively little mass, so their average density is extraordinarily low. Astronomers classify them as rare “super-puff” exoplanets. Although they are about the size of Jupiter, they are made mostly of hydrogen and helium, the lightest elements, in very swollen, diffuse atmospheres. That means a huge amount of space is filled with very little material. Their densities are about 0.038 and 0.047 grams per cubic centimeter, compared with roughly 0.05 g/cm³ for cotton candy and 1.33 g/cm³ for Jupiter. In other words, Jupiter is roughly 28 to 35 times denser. This does not mean the planets weigh less than a bag of cotton candy overall. It means that, cubic centimeter for cubic centimeter, their material is less dense on average. A planet can still be large and massive, but if its gas envelope is extremely expanded, its density can remain exceptionally low. Researchers think such planets likely formed in gas-rich disks around young stars, probably farther from their star where gas could accumulate into a thick envelope. Over time, some may lose material, but these two still appear unusually inflated. Because fewer than 40 super-puffs are confirmed among more than 6,000 known exoplanets, they are valuable for testing theories of how planets form and evolve.

3 articles · Updated · The Associated Press · Jun 25

Two giant exoplanets orbiting the Sun-like star TOI-791 were identified as the lightest known planets of their size, with densities lower than cotton candy despite being roughly Jupiter-scale.
Earth-based telescope measurements of their orbits, combined with detections by NASA’s TESS mission, showed the pair are up to 35 times less dense than Jupiter.
George Dransfield’s team reported the discovery in Monthly Notices of the Royal Astronomical Society and said follow-up observations by NASA’s Webb telescope are needed to confirm the planets’ hydrogen-helium makeup.
Fewer than 40 of the nearly 6,300 confirmed exoplanets are classified as super-puffs, making the TOI-791 pair a rare clue to how planets form and lose material over time.