Super-puff’ exoplanets with the density of cotton candy discovered

Astronomers have discovered a number of “super-puff” exoplanets in the Kepler 51 star system that are as dense as cotton candy.

Utilizing data from the Hubble Space Telescope, researchers found fewer than 15 of the planets that are almost as big as Jupiter, but have extremely low density, at less than 100 times the gas giant’s mass or less than 0.1 grams per cubic centimeter of volume.

“They’re very bizarre,” said the study’s lead author, Jessica Libby-Roberts, in a statement.


“This is an extreme example of what’s so cool about exoplanets in general,” said Zachory Berta-Thompson, one of the study’s co-authors, said. “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 three «super-puff» exoplanets in the Kepler 51 system were “straight-up contrary to what we teach in undergraduate classrooms,” Berta-Thompson added.

The Kepler 51 system is approximately 2,400 light-years from Earth and is approximately 500 million years old. A light-year, which measures distance in space, equals 6 trillion miles.

Using the Hubble, the researchers also attempted to look at the planets’ atmospheres, but ran into issues, as the atmospheres were opaque, rather than transparent.

“It definitely sent us scrambling to come up with what could be going on here,” Libby-Roberts continued. “We expected to find water, but we couldn’t observe the signatures of any molecule.”

Kepler 51’s three planets compared to the size of planets from our solar system. (Credit: NASA/ESA/STScI)
Kepler 51’s three planets compared to the size of planets from our solar system. (Credit: NASA/ESA/STScI)
Libby-Roberts and the other researchers theorized that the exoplanets are likely mostly comprised of hydrogen and helium, using computer simulations. It’s also probable that it is covered by a «thick haze made up of methane,» which makes them reminiscent of Saturn’s moon, Titan.

“If you hit methane with ultraviolet light, it will form a haze,” Libby-Roberts said. “It’s Titan in a nutshell.” In June, NASA unveiled a mission that will explore Titan, which could potentially host extraterrestrial life.


The researchers also discovered that the exoplanets are losing gas rapidly, with the innermost of the three exoplanets putting an estimated «tens of billions of tons of material into space every second.» Should that trend continue, these planets could shrink considerably over the next billion years and might wind up looking similar to «mini-Neptune» exoplanets.

“People have been really struggling to find out why this system looks so different than every other system,” Libby-Roberts said. “We’re trying to show that, actually, it does look like some of these other systems.”

“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 concurred.

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