Astronomers have taken a closer look at one of the strangest planets ever discovered, and it is raising more questions than answers.
The planet, called Kepler-51d, is so light and fluffy that scientists often compare it to cotton candy.
Now, new observations from NASA’s James Webb Space Telescope suggest that a thick layer of haze surrounds the planet, hiding key details about what it is made of and how it formed.
Kepler-51d is located about 2,600 light-years away from Earth in the constellation Cygnus. It is part of a system with several unusual planets known as “super-puffs.”
These planets are roughly the size of Saturn but have only a few times the mass of Earth, making them incredibly low in density. Among them, Kepler-51d is the least dense and coolest.
Normally, gas giant planets form with a solid, heavy core that pulls in large amounts of gas.
They are also usually found far from their host stars, where conditions allow them to grow. But Kepler-51d does not follow these rules.
It appears to have a very small core and orbits its star at a distance similar to Venus’s orbit around the Sun. This makes it difficult to explain how such a planet could form and survive.
To investigate, researchers led by Penn State used the James Webb Space Telescope to study the planet’s atmosphere.
They analyzed how the planet blocks and filters light from its star as it passes in front of it. This method usually reveals a “fingerprint” of the chemicals in a planet’s atmosphere.
However, in this case, the results were surprising. Instead of clear signals from specific molecules, the team found that the light was being absorbed in a smooth, continuous way. This suggests that a very thick layer of haze is covering the planet, preventing scientists from seeing what lies beneath.
The haze may be similar to the smog-like atmosphere of Titan, Saturn’s largest moon, which is rich in hydrocarbons such as methane. But on Kepler-51d, the haze appears to be much more extreme—possibly extending as far as the size of Earth itself. This could make it one of the thickest atmospheric haze layers ever observed on a planet.
The researchers also considered other explanations, such as the presence of rings around the planet. Rings could block starlight and make the planet appear larger and less dense than it really is. However, the data does not match what scientists would expect from rings, making haze the most likely explanation.
Even with these new findings, many mysteries remain. Scientists still do not know how Kepler-51d formed, why it has such a low density, or how it managed to hold onto its atmosphere despite strong stellar winds from its active star.
Studying unusual planets like Kepler-51d helps astronomers understand the full range of worlds in the universe. Each new discovery challenges existing theories and brings scientists closer to understanding how planets—including our own—came to be.
