As we find more and more exoplanets in the Milky Way - numbering in the thousands now - astronomers are discovering some strange objects that don't exist in the Solar System. One such phenomenon are the strangely fluffy "super-puff" planets - the size of gas giants, but way, way less massive.
Exactly how these planets can exist has been a puzzle to astronomers. In extreme cases, a super-puff planet can be less than one percent of the mass of a gas giant of similar size. Now researchers have crunched the numbers and come up with a new explanation: What if they are actually smaller planets with giant rings?
This could solve some of the stranger aspects of super-puff planets, as well as help us to find a feature that has so far proven elusive on exoplanets: planetary rings.
"In principle, rings should be detectable from detailed photometric or spectroscopic changes to transits. The difficulty is that such signals are subtle and difficult to discern in current data," the researchers wrote in their paper.
"There is clearly still a lot we do not know about the rings of exoplanets."
Super-puff exoplanets are detected using the transit method. This is where a telescope studies a star over time, looking for regular dips in the star's light. If dips occur at the same depth and the same length of time between each one, that can be inferred as an exoplanet.
The amount of light the star dips by can then be used to calculate the physical size of said exoplanet. Yet another method can be used to calculate the exoplanet's mass - as planets orbit their stars, they actually exert a gravitational influence of their own, causing the star to wiggle ever so slightly. The amount of this wiggle is determined by the mass of the planet.
In the case of super-puffs, the transit dips return a massively disproportionately large size compared to the mass inferred by the star's wiggle. That got astronomers Anthony Piro of the Carnegie Institution for Science and Shreyas Vissapragada of Caltech wondering what the heck was going on.
First, they started thinking about what kinds of objects could have that large a size, but that low a density. This idea led them to consider planetary rings.
"We started thinking, what if these planets aren't airy like cotton candy at all," Piro said. "What if the super-puffs seem so large because they are actually surrounded by rings?"
Four of the planets in the Solar System have rings - Jupiter, Saturn, Neptune and Uranus. Only Saturn's, however, are large, thick and prominent.
Since so many of the Solar System planets have rings, it stands to reason that many exoplanets would as well. But we're usually looking at these objects from many light-years away, and most of the time can't see the planets directly - so detecting rings seems rather impossible. Or... is it?
"We started to wonder, if you were to look back at us from a distant world, would you recognise Saturn as a ringed planet, or would it appear to be a puffy planet to an alien astronomer?" Vissapragada said.
For the next step, they turned to modelling, to figure out if rings could explain the size of known super-puff planets. The answer was yes, for some of them. But not all.
"These planets tend to orbit in close proximity to their host stars, meaning that the rings would have to be rocky, rather than icy," Piro said. "But rocky ring radii can only be so big, unless the rock is very porous, so not every super-puff would fit these constraints."
There were some other caveats, too. The planet would have to be flattened to a more oval shape to prevent the rings from warping; Saturn is the most flattened planet in the Solar System because of its fast rotation speed. Exoplanets that are tidally locked to their star - meaning their rotation has the same period as their orbit - may be rotating too slowly to create this shape.
Given these constraints, three exoplanets in particular were found to be good candidates for having rings. Kepler 87c and Kepler 117c are both larger than Neptune, but with masses just 6.4 and 7.5 times that of Earth, respectively, making them very low density indeed.
The third, HIP 41378f, was announced as Piro and Vissapragada were finishing their manuscript - and they found it "especially exciting" in the context of their findings, given how neatly it meets all their constraints. In fact, the paper announcing the discovery even mentioned rings as a potential way of explaining the exoplanet's strange properties.
Our current instruments aren't powerful enough to follow up to try to look for rings, but the team believes the James Webb Space Telescope, due to launch next year, will be up for the task. They hope more detailed observations will help figure out the mystery of at least some super-puffs - and finally reveal to us in detail the glorious rings of exoplanets.
"Confirmation of the presence of rocky rings in some cases would not only be an amazing new discovery, but also provide important information about these planets," they wrote in their paper.
The research has been published in The Astronomical Journal.