New Observations Confirm The Existence Of “Cotton Candy” Planets

Artist impression of the Kepler 51 system. NASA, ESA, and J. Libby-Roberts and Z. Berta-Thompson (University of Colorado)

New observations with the Hubble Space Telescope have confirmed that “cotton candy” planets really do exists. Alas no, they are not really made of cotton candy. They are simply bloated young exoplanets the size of Jupiter but with less than one-hundredth of the mass.

These objects, also referred to as "super-puffs", are a very rare and unique class of young planets. Nothing like them exists in our Solar System. They have a very low density (hence the cotton candy link) and a new analysis of three of them around the Sun-like star Kepler 51 (around 2,600 light-years away) suggests that this might be a temporary phase for certain planets. The study will appear in the Astronomical Journal, but is currently available to read on ArXiv.

The planets were first identified in 2014, but these latest observations refine their mass and size, independently confirming that they are indeed super-puffs. The researchers, led by Jessica Libby-Roberts of the University of Colorado, Boulder, estimate all three planets have a density less than 0.1 grams per cubic centimeter of volume, which is almost identical to cotton candy. 

“We knew they were low density,” she told CU Boulder Today. “But when you picture a Jupiter-sized ball of cotton candy – that’s really low density.”

The team was also able to catch the innermost planet, Kepler-51 b and Kepler-51 d, passing in front of the star and that allowed them to study their peculiarly overblown atmospheres. This approach can allow researchers to distinguish specific elemental or molecular components that might pepper their atmosphere, which is overwhelmingly made of hydrogen and helium. But it was not the case for these two planets. They could not see any signatures at all. 

"This was completely unexpected," Libby-Roberts said in a NASA statement. "We had planned on observing large water absorption features, but they just weren't there. We were clouded out!"

The illustration shows how the Super-puffs stack up against the solar system planets. NASAESA, and L. Hustak and J. Olmsted (STScI)

Every proverbial cloud has a silver lining, and the cloud coverage allowed the team new insights into the planets. The team believes that these three planets formed further away from their star, beyond the "snow line" of their star system and subsequently moved inwards. Models suggest that the system is still in its infancy and it will continue to change over the next billion years.

In particular, Kepler-51 b will shrink as more and more of its atmosphere is eroded away into space by the light of the star. It will become a slightly smaller and hotter version of Neptune. Kepler-51 d will also shrink but given that it is further away from its star, it is likely it will remain a puffy outlier.

"This system offers a unique laboratory for testing theories of early planet evolution," said co-author Zach Berta-Thompson of the University of Colorado, Boulder.

The team hopes to observe the planets again once the James Webb Space Telescope is operational. Its capabilities to see further in infrared could allow us to peer through the annoying cloud cover and see what's underneath.

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