These ‘super puff’ planets have the same density as cotton candy

Their impossible fluffiness has researchers scratching their heads.
artist's interpretation of a super puff exoplanet

Improbably fluffy alien worlds may hide behind thick layers of clouds. NASA, ESA, and L. Hustak, J. Olmsted, D. Player and F. Summers (STScI)

With each new alien planet discovered, the astronomical catalog of bizarre worlds and systems swells. “Hot Jupiters” huddle closer to their host stars than Mercury. “Circumbinary” planets circle two stars at once. One “super earth” is blanketed in a layer of water so hot that it acts as both liquid and gas simultaneously. And about a dozen planets (of the thousands discovered so far) seem impossibly light for their gargantuan proportions.

“We knew they were low density,” said Jessica Libby-Roberts, a graduate student at the University of Colorado Boulder who studies these wispy worlds, in a press release. “But when you picture a Jupiter-sized ball of cotton candy—that’s really low density.”

Exoplanet researchers know these lightweights as “super puffs” because of their extremely low average density—less than one tenth of one gram per cubic centimeter. That’s quite similar to the density of cotton candy, and more than a dozen times airier than the gas giant Jupiter. This airiness likely comes from a puffed-up atmosphere extending much farther out into space than the atmospheres of rocky planets or most gas giants. However, this excess fluff poses problems for theories of planetary formation. A 2016 proposal suggested that such worlds were born in icy realms far from their suns, where cooler temperatures could speed the amassing of their oversized atmospheres, before moving inwards to where current candidates have been observed. Now recent research, soon to be published in The Astronomical Journal, uses new Hubble observations to check one prediction of that theory—that the ice accumulated during their formation should persist as water in their atmospheres today. But the search came up short, forcing a re-interpretation of the fluffy worlds.

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

The researchers pointed the Hubble Space Telescope at star Kepler-51 as two of the system’s three super puffs took turns passing in front of it, recording the resulting dimming in various colors of light. Different molecules—such as water—block light in different ways, so if a planet looks bigger or smaller when viewed in certain wavelengths, astronomers can infer the molecular ingredients in its atmosphere.

Kepler-51’s super puff duo, however, appeared the same in each of Hubble’s observations. This monotony could indicate that only bone-dry hydrogen and helium fill the planet’s voluminous atmospheres—but that conclusion would clash with how such worlds likely form. Rather, the researchers suggest that thick grey clouds cloak the planets. Those could block the more varied light from the lower layers, which might contain the water traces they were searching for, from reaching Hubble. Libby-Roberts likens the cloud cover to that of Saturn’s moon Titan, which is entirely enveloped by a yellow haze.

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

If the super puffs truly are as candy-like as they seem (errors in measuring their mass would resolve the mystery in another way, the authors note), they won’t stay fluffy forever. The Kepler worlds orbit so close to their star that its energy must be blowing billions of tons of fluff out into space each second, the researchers calculated using a model. At that rate, one of the planets will naturally mature into a more common type of exoplanet, a “sub-Neptune,” in about five billion years. The other will remain usually light, but lose its super puff status. The Kepler-51 system, the researchers write, may represent a snapshot of mini Neptunes and oversized Earths in their “teenage” years (which for planets means their first billion or so years of life).

Alternatively, other researchers wonder if this class of planet may be nothing but a sweet dream. Earlier this year, the researchers behind that 2016 puff-ball formation theory raised the possibility that the planets could be belching out clouds of dust, inflating their apparent size. Or the long dips observed by Hubble and other telescopes as the planets pass in front of their star could originate from Saturn-like rings, which would similarly exaggerate the worlds’ true sizes. Another team considered this theory in November, and found that it matched some of these mysterious planets, but not others.

To dig deeper into the fluffy worlds, researchers are waiting for the upcoming James Webb Space Telescope. After launch, currently planned for 2021, the instrument’s keen eyes should be able to discern enough detail in the dimming of Kepler-51 and similar host stars to distinguish between alien ring worlds, dust streams, and super puffs.