Cotton Candy Planets: A Puffy Mystery Even the James Webb Telescope Can't Solve

Some planets are so light and fluffy they are called "cotton candy planets." They float in space with their secrets hidden by a thick fog. Even the powerful James Webb Space Telescope cannot see through this haze. Astronomers have a big question: how did these strange, low-density worlds form?

"These super light planets are very rare," said astronomer Jessica Libby-Roberts. "They do not fit our normal ideas about how gas giants form. Explaining one is hard, but this system has three!"

The main planet being studied is named Kepler-51d. It is part of a system with four planets orbiting a young, Sun-like star that is 2,615 light-years away. NASA's Kepler Space Telescope first found these planets by watching them pass in front of their star, an event called a transit. By observing how much starlight they blocked, scientists measured their sizes. By studying how their gravity affects each other, they calculated their masses.

Planets 51b, c, and d are each about the same size as Saturn, a gas giant in our solar system. But their masses are shockingly small—only a few times heavier than Earth. In contrast, Saturn is 95 times heavier than Earth. This means these planets are the size of Saturn but far less dense.

This gives them an incredibly low density. Their densities are more like cotton candy than any planet near us. A fourth planet, 51e, was found in 2024, but its exact mass and size are still unknown.

These cotton candy planets are unlike the gas giants in our solar system. For Jupiter and Saturn, large, solid cores formed first. Then, their strong gravity pulled in huge amounts of gas from the disk of material around the young Sun.

The planets around Kepler-51 seem to be built in a very different way. "Kepler-51d seems to have a tiny core and a huge atmosphere," explained Libby-Roberts. "This gives it a density like cotton candy." Current science cannot explain how such a small core could gather such a large, puffy atmosphere.

In 2020, Jessica Libby-Roberts led a team to study the Kepler-51 system with the Hubble Space Telescope. They used a method called spectroscopy, hoping to find the chemical fingerprint of the planets' atmospheres. The gases could reveal where the planets formed and why they are so puffy. Scientists initially thought the atmospheres would be full of light gases like hydrogen and helium.

However, Hubble found nothing. It did not see signs of any specific chemicals. The team concluded that a thick, blank haze was covering the planets, blocking Hubble's view.

Recently, Libby-Roberts used a more powerful tool: the James Webb Space Telescope (JWST). Her team used JWST's Near-Infrared Spectrometer to try and look deeper into the atmosphere of Kepler-51d. The goal was to use transit spectroscopy. As the planet passes in front of its star, some starlight filters through its atmosphere. Molecules in the atmosphere absorb specific colors of that light, leaving dark lines in the star's spectrum that act like a chemical fingerprint.

"A star's light is filtered through the planet's atmosphere before it reaches our telescopes," said Libby-Roberts. "If we look across a range of wavelengths, we get a sort of fingerprint of the planet's atmosphere."

Yet, once again, the results showed no clear chemical signs. The spectrum was flat. This means the haze around Kepler-51d is the thickest ever seen on an exoplanet. It is so dense that even JWST cannot see through it. JWST looks at longer, more penetrating infrared light than Hubble.

"It seems very similar to the haze we see on Saturn's moon Titan," said co-researcher Suvrath Mahadevan. Titan has hydrocarbons like methane. "But Kepler-51d's haze is at a much larger scale. It seems to have a huge amount of haze, almost the size of Earth."

This discovery makes the mystery even deeper, especially because these planets orbit very close to their star. If planets 51b, c, and d were placed in our solar system, they would all fit inside the orbit of Venus. No current theory easily explains how such light, puffy planets could form so close to a star.

"It's possible that 51d formed further away and moved inward," said Libby-Roberts. "But we are still left with many questions. What is it about this system that created three really odd planets? It's a combination of extremes we haven't seen anywhere else."

One idea is that we are seeing these planets in a brief, young phase. The Kepler-51 system is about 500 million years old, which is young compared to our 4.5-billion-year-old solar system. Because the star is young, it is more active. Its strong stellar wind might be blowing away the outer layers of the planets' puffy atmospheres. If astronomers could watch the system a billion years from now, much of the gas might be gone, leaving only the small cores.

Answers may still come. Another team of scientists is using JWST to observe Kepler-51b. They hope to find out what its atmosphere is made of and whether it is also covered in haze. If they succeed, the clues they find could help explain its sibling planets, 51c and d, potentially solving the puzzle of this strange family of planets.

The new measurements of Kepler-51d are described in a study published in the March 16 issue of The Astronomical Journal.