Gaseous planets like Jupiter and Saturn have a much lower density than Earth (Saturn could even float in a huge bathtub) but now astronomers have discovered what can be called a new class of even lighter planets, different even from the two largest pyerids in our solar system.
Rather than “gas giants,” these new planets could be classified as “cotton candy” because their density is so low that they look swollen. Thanks to the Kepler telescope, three of them have been found orbiting the star Kepler 51, about 2600 light years away from us and very similar to the Sun. These planets are almost the size of Jupiter but only a hundredth of its mass.
Astronomers believe that they may have formed further away from their star than their current position and then migrated inward. Most likely they have an atmosphere made of low-density hydrogen and helium, an atmosphere that they are losing in space as they get closer and closer to the star.
This same phase could be considered a transitional phase in the evolution of planets of this type. There is nothing like this in our solar system and the first clues of these super swollen planets were found in Kepler-51’s star system in 2012 but it wasn’t until 2014 that it became clear that their densities were so low that they surprised the astronomers themselves.
Now the astronomers have made new measurements and have come to the conclusion that the very low density of these planets is partly a consequence of the young age of the system itself, born just 500 million years ago, a fraction of the 4.5 billion years of our solar system.
According to astronomers, these super-light planets have formed outside the so-called “snow line”, a limit beyond which there are usually only frozen planets. These planets have an atmosphere with such a low density that the same atmospheres are expected to evaporate into space over the next billion years.
The planet closest to the star, Kepler-51b, in a billion years should be a smaller, warmer version of Neptune. The one furthest from the star, Kepler-51b, will continue to be an ultralight planet even if it will shrink a little and lose some atmosphere.
Now researchers intend to determine the chemical composition of the atmosphere of these two planets and in this regard they plan to use NASA’s next space telescope, the James Webb, which with its ability to look infrared could be able to observe beyond the layers of the surface clouds.