Like cotton candy: astronomers have found one of the lightest planets in an unusual system.

Astronomers using the James Webb Space Telescope have discovered a new planet orbiting the star Kepler-51, located 2,615 light-years away from us. This star is also home to three known planets, which are among the lightest and least dense ever found. Their density is comparable to that of cotton candy. It is still unclear whether this new world shares similar characteristics, and scientists do not understand how such strange worlds could form within a single planetary system. The study has been published in the Astronomical Journal, according to Space.

According to scientists, these so-called "super-puffy" planets have very low density, small mass, but large size, and they are extremely rare. Previously, scientists were aware of three planets orbiting the star Kepler-51, each the size of Saturn, but with masses only 3 to 6 times that of Earth. Their density is akin to that of cotton candy, making them some of the lightest known planets outside our solar system.

The star Kepler-51 is a younger version of the Sun, with a mass just 4% less and a brightness 30% lower. Kepler-51 is about 7 times younger than the Sun, which is 4.6 billion years old, and when our star was at the same age, its brightness was comparable.

Previously, scientists discovered that the planets Kepler-51b, Kepler-51c, and Kepler-51d orbit this star. Now, astronomers have identified a fourth planet in this system — Kepler-51e.

Kepler-51d is the puffiest planet in the system. Many planets with extremely low density orbit very close to their stars, and the warmth from the stars causes them to expand. However, Kepler-51d completes a full orbit around its star in 130 days, yet it emits less heat than the Sun, meaning the planet should not be expanding. Nevertheless, the density of Kepler-51d is only 0.0381 g/cm³. For comparison, the density of water is 1 g/cm³, Earth’s density is 5.5 g/cm³, and Saturn, the least dense planet in our solar system, has a density of 0.69 g/cm³.

As for the new planet Kepler-51e, its mass is similar to that of the other three planets. This planet completes its orbit around its star in 264 days, and it influences the orbits of the other planets in the system.

Scientists are still unsure if Kepler-51e is as super-puffy as its neighbors. It might be a regular gas giant like Neptune, or it could even be a super-Earth.

The discovery of the new planet prompted scientists to remeasure the masses of the three known planets, revealing they are slightly heavier than previously thought, especially Kepler-51b. Although this makes them a bit denser, they still rank among the lightest planets with the lowest density discovered so far.

According to scientists, super-puffy planets are very rare, and typically only one such world exists in a planetary system. However, in this case, there are several such worlds orbiting the star Kepler-51, which is quite unusual.

Scientists believe these planets have tiny cores and vast, puffy atmospheres composed of hydrogen or helium. However, how they formed and why their atmospheres have not been obliterated by the intense radiation from the star (all four planets are closer to their star than Earth is to the Sun) remains a mystery.