Cotton-candy worlds may be steps on planetary evolution ladder

An artist’s impression of the Kepler 51 system. Image: NASA/ESA/STScI

Imagine a world as large as Jupiter with the density of cotton candy. A young star 2,400 light years from Earth features three such puff-ball worlds, relative rarities in the exoplanet catalogue that raise new questions about how planets form and evolve.

First described in 2014, the three planet’s making up the Kepler 51 system already were known to be among the lowest density worlds yet found. An international team of researchers decided to take a closer look with the Hubble Space Telescope, developing new estimates of mass and density.

They found all three planets have a density of less than 0.1 grams per cubic centimetre – roughly identical to that of cotton candy.

“We knew they were low density,” said Libby-Roberts, a graduate student at the University of Colorado at Boulder. “But when you picture a Jupiter-sized ball of cotton candy, that’s really low density.”

Expecting to spectroscopically detect the presence of water molecules and other atmospheric components, the researchers found the atmospheres were, in fact, opaque, as if blanketed by a Titan-like high-altitude haze.


Using computer modelling and other tools, the team theorises the atmospheres are mostly made up of hydrogen and helium with a layer of methane.

“If you hit methane with ultraviolet light, it will form a haze,” Libby-Roberts said. “It’s Titan in a nutshell. … People have been really struggling to find out why this system looks so different than every other system. We’re trying to show that, actually, it does look like some of these other systems.”

The Kepler 51 system is thought to be just 500 million years old. The planets seem to be shedding gas at extreme rates and as the system ages, all three could shrink, losing their current puffiness. In that case, the planets could end up resembling relatively common mini Neptunes

Zachory Berta-Thompson, co-author of a paper describing the research in The Astronomical Journal and an assistant professor at UC-Boulder, said observing the Kepler 51 system in its infancy likely explains why the planets appear so unusual.

“A good bit of their weirdness,” he said, “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.”