Not so long ago, scientists astronomers discovered that in the star system of Kepler 51 are three completely unusual exoplanets. The mass of these exoplanets is several times the mass of the Earth, but the smallest of them exceeds the size of Neptune. This allowed scientists to suggest that the tiny rocky core of each of these planets is surrounded by a very thick layer of “fluff”, gaseous matter with a very low density, which allowed these planets to be attributed to a relatively new class of “super-fluffy” exoplanets.

To obtain information on the parameters of fluffy exoplanets, scientists recorded three transit cases for the planets Kepler 51b and Kepler 51d using the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope. This information, combined with information collected at one time by the Kepler space telescope, allowed scientists to calculate the average density of matter of each of the planets. And this density was lower than 0.1 grams per cubic centimeter.

Kepler 51 system's planets

The planets Kepler 51b, 51c and 51d, whose size is comparable to the size of Jupiter, revolve around the star Kepler 51 with a period of 45, 85 and 130 days, respectively. The star Kepler 51 is relatively young, its age is about 500 million years, but it is very different from other planets of its age. It has a very low brightness, which in this case made it very difficult to measure exoplanet parameters.

But, having made a series of very complex calculations, astronomers still determined the mass and size of each of the three planets. And it turned out that these planets not only have a low density of matter, they are planets with the lowest density of matter among all the planets known today. Unfortunately, even the most accurate astronomical instruments are not able to determine the color of these exoplanets. Therefore, we absolutely do not know whether they look like balls of cotton candy, whose density is comparable to the density of the matter of these planets, or similar to something else?

“The new three planets of the Kepler 51 system are the planets with the lowest density at the moment, according to NASA exoplanet archives,” the researchers write, “and you yourself can verify this by going to the appropriate section of the California Institute of Technology’s website.”