Using data from NASA's Hubble Space Telescope, astronomers have confirmed a new class of planets that have the density of cotton candy.

Because their hydrogen-helium atmospheres balloon out, these planets are nearly as big as Jupiter, but have 100 times less mass. That's why scientists are calling the new planets "super-puffs."

Until recently, astronomers had only known about the three types of planets that we see in our own solar system: terrestrial planets like Earth, ice giants like Neptune, and gas giants like Jupiter. So scientists group the worlds they find around other stars based on these categories.

But on Thursday, NASA confirmed that three exoplanets (the term for worlds that orbit distant stars) fall into the new super-puff grouping. The planets circle a sun-like star 2,600 light-years away. Hubble data revealed their mass and size, but thick clouds prevented researchers from learning more detail about the super-puffs' chemistry.

Scientists at the University of Colorado Boulder described the findings in a new studyset to appear in The Astronomical Journal.

"They're very bizarre," Jessica Libby-Roberts, the graduate student who led the research, said in a press release. "When you picture a Jupiter-sized ball of cotton candythat's really low density."

Nobody knows why the planets' lightweight atmospheres are so bloated.

An artist's illustration shows the three giant planets orbiting the sun-like star Kepler 51 as compared to some of the planets in our solar system. NASA, ESA, and L. Hustak and J. Olmsted (STScI)

The Kepler space telescope found this group of three super-puffs in 2012, and some of the scientists behind the recent discovery figured out just how mysteriously lightweight these planets were in a 2014 study.

The discovery was "straight-up contrary to what we teach in undergraduate classrooms," Zachory Berta-Thompson, an assistant professor of planetary sciences and co-author of the study, said in the release.

The group recently took a closer look at the planets with Hubble, NASA's most powerful space telescope. They used Hubble's infrared lens to peer at star light passing through two of the planets' atmospheres. By measuring the light's intensity, they could determine which chemicals make up the planets' atmospheres.

To their surprise, the researchers couldn't see any chemical signatures. They were totally blocked by thick clouds.

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

Three views of Saturn's moon Titan, taken by the Cassini spacecraft, show its thick haze of methane clouds. NASA/JPL/Space Science Institute

Instead of water, the researchers think the clouds might be made of salt crystals or methane, which would cause a hazy interaction with the telescope's ultraviolet light. That could mean the planets are similar to Saturn's moon Titan, which is clouded in a thick methane atmosphere.

"If you hit methane with ultraviolet light, it will form a haze," Libby-Roberts said. "It's Titan in a nutshell."

Under that methane, lighter gases like hydrogen and helium could be puffing up the planets' atmospheres.

The primary mirror of NASA's James Webb Space Telescope, consisting of 18 hexagonal mirrors, in the clean room of NASA's Goddard Space Flight Center in Greenbelt, Maryland, October 28, 2016. NASA/Chris Gunn

Our best hope of learning more about these super-puffs is NASA's upcoming James Webb Space Telescope, which could peer through the smog and find out what the Kepler-51 planets are truly made of. The new telescope's 21-foot-wide beryllium mirror and new infrared technology make it sensitive to longer wavelengths of infrared light.

The telescope is slated to launch on March 30, 2021.

Both of the planets the researchers observed seemed to be rapidly losing their gases. One was spewing tens of billions of tons into space each second.

If the leakage continued at that rate, the researchers calculated that the planet would shrink to a size smaller than Neptune in 1 billion years, losing all its puffiness in the process.

Small, hot Neptune-like planets are common throughout the Milky Way, unlike these rare super-puffs.

Neptune, photographed by Voyager 2 from 4.4 million miles away, has an atmosphere made of hydrogen, helium, and methane. NASA/JPL

The cotton-candy planets' state of transition could explain why we don't have anything like them in our own solar system. The Kepler-51 star they orbit is relatively young 500 million years old, compared to our 4.6-billion-year-old sun. So its system of planets may not have settled down yet.

"This is an extreme example of what's so cool about exoplanets in general," Berta-Thompson 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."

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NASA has confirmed a new class of huge 'super-puff' planets that have the density of cotton candy - Business Insider