Cotton candy planets: Scientists find two giant worlds with extremely low density

Fluffy giants in the cosmos

Astronomers have identified two exoplanets so light that scientists are comparing them to cotton candy. The worlds, located in the Kepler-51 system roughly 2,600 light-years from Earth, possess densities far lower than any planet in our solar system.

These are not your average gas giants. Their density readings are so extreme that researchers initially doubted their own calculations. The planets, designated Kepler-51d and Kepler-51e, orbit a Sun-like star and belong to a rare class known as 'super-puffs'.

How light are they?

To put it in perspective, Saturn is the least dense planet in our solar system — it would float in water. These newly discovered exoplanets are significantly lighter. Their density is comparable to cotton candy, which is mostly air.

Data from the Kepler space telescope and follow-up observations from ground-based observatories revealed their size and mass. While they are as wide as Jupiter, they weigh a fraction of what standard gas giants should. One of the planets has a density of roughly 0.1 grams per cubic centimetre. By comparison, water has a density of 1 gram per cubic centimetre, and Earth's is about 5.5 grams per cubic centimetre.

What makes them so puffy?

The leading explanation is that these planets have extremely inflated atmospheres. They likely formed with a small, rocky core and then accumulated vast envelopes of hydrogen and helium gas. The intense radiation from their star heats these atmospheres, causing them to expand like a balloon.

Another possibility is that the planets are relatively young. Over billions of years, their atmospheres could cool and contract, making them denser. But for now, they remain a puzzle. Researchers say these super-puffs challenge existing models of planetary formation and atmospheric retention.

Implications for planet hunting

Discoveries like this reshape how astronomers look for habitable worlds. If such low-density planets exist, it suggests that planetary systems can host a wider variety of worlds than previously thought. This also affects how scientists estimate the size and composition of exoplanets based on transit data.

The findings have been published in a peer-reviewed journal, and the team plans further observations using the James Webb Space Telescope. Those studies will aim to probe the chemical makeup of the planets' atmospheres.

For now, these cotton candy planets remain a reminder that the universe is stranger — and lighter — than we imagined. What comes next is a closer look at their skies.