Beyond our solar system, there’s a whole selection of planets unlike anything we see around the Sun. In the cosmic candy shop, there are Ring Pops (less magnificent versions of Saturn), Hot Tamales (Jupiter-like planets close to their stars and blazing hot), and even cotton candy planets (big puffy gas giants that actually have the density of the cotton candy you eat!).
These extremely low density cotton candy exoplanets—known as super-puffs—are a big puzzle for astronomers trying to figure out how planets form. And, according to new research, some of them might not actually be so puffy after all.
“It’s very hard to form planets this fluffy,” says Yale astronomer Tiger Lu, lead author on the new work. “The standard idea of gas planet formation is that planets start as rocky cores, which then gravitationally attract and accrete gas from their surroundings…It turns out that in this standard model, it’s almost impossible to accrete enough gas to create the gas to rock ratio necessary for these cotton-candy like worlds.”
Super-puffs close to their stars are likely the result of the star heating up the planet’s atmosphere and fluffing it up. But there are a few strange super-puffs too far from their stars for this heating to work. One example is the planet HIP-41378 f, a gas giant with a three year orbit around an almost Sun-like star about 350 light years away from Earth. “None of the accepted puffing mechanisms are even remotely possible” for this world, explains Lu.
It’s important to note that the supposed ultra-low densities of cotton candy worlds come from measurements of transit depth, the amount of light that a planet blocks when it passes in front of its star. This quantity is proportional to how big the planet is—a bigger planet blocks out more light, and vice versa. But what if the planet isn’t actually that big, and there’s something else in the way blocking out that light?
“Both rings and moons are common among sufficiently large planets in our Solar System,” explains Yoni Brande, a University of Kansas astronomer not involved in the new study. “So why shouldn’t we try and see if they can explain some of the weird exoplanet data we’ve already got?”
Lu’s work shows that HIP-41378 f has been kicked around by gravity, tilting it on its side and conveniently making the rings visible to us in a way that would make the planet appear bigger when it transits. “If the planet were not tilted, we would view these thin rings edge-on and barely be able to see them,” says Lu.
Interestingly, planets really close to their stars couldn’t be tilted in the same way, meaning those particular super-puffs are likely explained by the more traditional ideas of heating instead of rings. “So, the further away a super-puff is from its host star the more likely the ring hypothesis is,” adds Lu.
These rings wouldn’t be quite as magnificent as Saturn’s iconic bands, though. To explain the observations of HIP 41378 f, you’d only need about half the size of Saturn’s rings. Plus, Saturn’s rings are icy, which makes them sparkle to create the recognizable ringed planet we know and love. HIP 41378 f, on the other hand, is in a much warmer part of its cosmic neighborhood, making its rings more likely rocky and therefore less bright and exciting.
As with most areas of astronomy, the James Webb Space Telescope (JWST) is likely to make a huge impact here. The authors propose that JWST could actually test the hypothesis that this particular super-puff is actually a ringed, not-so-puffy planet in the coming years. And there are more super-puff observations in the works around other stars, too, to help unravel the mystery of these weird exoplanets. “While I don’t think any observations of HIP 41378 f are yet planned with JWST, some other similar super-puff planets have already been observed and those results should be announced fairly soon,” adds Brande.