Secrets of Bizarre 'Cotton Candy' Exoplanets Which Have Strangely Low Densities Revealed by Hubble Telescope

Astronomers have uncovered fascinating new insights into a bizarre set of "super-puff" exoplanets which are similar in density to cotton candy.

The three planets—which were discovered in 2012 orbiting the Sun-like star Kepler 51 around 2,400 light-years away—have the lowest densities of any known exoplanets.

"They're very bizarre," said Libby-Roberts from the University of Colorado Boulder, leader of a new study looking into the properties of the planets that's set to be published in the Astronomical Journal.

When scientists first determined the extraordinarily low densities of the planets in 2014, many were surprised by the results.

Now, Libby-Roberts and colleagues have come up with new estimates for the mass and densities of the three planets using observations with the Hubble Space Telescope, independently confirming their "puffy" nature.

These planets are roughly the size of Jupiter, however, they are around a hundred times lighter with masses only several times that of the Earth's. In fact, the team worked out that the planets had densities of less than 0.1 grams per cubic centimeter of volume—which is almost the same as that of cotton candy, according to Libby-Roberts.

"We knew they were low density," she said. "But when you picture a Jupiter-sized ball of cotton candy—that's really low density."

The team suggest that the planets are likely made up of hydrogen and helium although they are surrounded by a thick haze of methane. This atmosphere may be similar to that of Saturn's moon Titan, according to the researchers.

Finally, the astronomers say that the the planets appear to be evaporating at a rapid pace because they are located close to their star, meaning they could shrink significantly over the next billion years.

Kepler 51 system
This illustration depicts the Sun-like star Kepler 51, and three giant planets that NASA's Kepler space telescope discovered in 2012-2014. These planets are all roughly the size of Jupiter but a tiny fraction of its mass. NASA, ESA, and L. Hustak, J. Olmsted, D. Player and F. Summers STScI

Super-puff planets are extremely rare—less than 15 have been documented so far—and nothing like them exists in our solar system.

"This is an extreme example of what's so cool about exoplanets in general," Zachory Berta-Thompson, a co-author of the research, said in a statement. "They give us an opportunity to study worlds that are very different than ours, but they also place the planets in our own solar system into a larger context."

The researchers suggest that the planets may represent a brief transitory phase in planet evolution—a consequence of the young age of the Kepler 51 star system, which is only around 500 million years old compared to our 4.6-billion-year-old sun.

"A good bit of their weirdness is coming from the fact that we're seeing them at a time in their development where we've rarely gotten the chance to observe planets," Berta Thompson said.