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The apparent “puffiness” of some exoplanets could be due to Saturn-like rings, rather than envelopes of gas as was previously thought. That’s the view of astronomers Anthony Piro at the Carnegie Institution for Science and Shreyas Vissapragada at the California Institute of Technology, US, who came to this conclusion after simulating the transits of several “super-puff” exoplanets. Their analysis exposes two such exoplanets as likely candidates for having rings – a finding that could be confirmed after the upcoming launch of the James Webb Space Telescope (JWST).
As the list of known exoplanets expands, astronomers are identifying a growing number of bodies that appear to have remarkably large radii, given their relatively low masses. Nicknamed “super-puffs”, these seemingly ultra-low-density planets are typically anomalously cool, and are found in star systems with widely varying ages – meaning that most of them probably aren’t just young planets that haven’t yet fully formed.
To explain these enigmatic objects, some astronomers have proposed that they are surrounded by thick envelopes of gas. If this were the case, these envelopes could be expected to leave diverse absorption dips in the spectra of starlight passing through them. However, the super-puff spectra observed so far have been frustratingly featureless.
Not so puffy
Piro and Vissaptragada propose a different explanation. In their view, super-puffs aren’t actually puffy, but are instead surrounded by rings. These rings dim the light of the planets’ host stars as they pass between the star and observers on Earth, creating the illusion of exoplanets with far larger radii. They tested this theory by simulating observations of Saturn transiting the Sun, from the perspective of a distant star system. This revealed that Saturn would appear to be half as dense as it actually is if its rings weren’t accounted for.
The duo also simulated the transits of a variety of known super-puffs, aiming to determine whether the transit observations could have been distorted by rings. They found that their hypothesis was consistent with the transits of some exoplanets, but not all of them: given their proximity to their host stars, many of the bodies would need to have heavier, rocky rings instead of ice, which would limit the rings’ radii. In addition, the planets would need to spin fast enough to prevent warping in their rings – but this is often hindered by tidal locking with host stars.
These effects didn’t rule out every planet the duo considered. Of the exoplanets they analysed, Piro and Vissaptragada concluded that Kepler 87c and 177c have the best chance of appearing puffy due to rings. Confirming this will require more accurate photometric techniques than are currently available, but these improved measurements should be within reach of the long-awaited JWST, which is now scheduled for launch in March 2021. If such predictions are confirmed, they could greatly improve astronomers’ understanding of how planetary systems form and evolve.