As we discover more and more exoplanets in the Milky Way – they already number in the thousands – astronomers are discovering some strange objects not found in the solar system.
One such phenomenon is strangely 'fluffy' planets – the size of a gas giant, but much less massive.
How exactly these planets could exist has been a mystery to astronomers. In extreme cases, the mass of such a planet can be less than one percent of the mass of a gas giant of similar size.
Now the researchers have come up with a new explanation: what if they are in fact smaller planets with giant rings?
This could solve some of the weird aspects of such planets, and also help us find a feature that has so far proved elusive on exoplanets: planetary rings.
'In principle, rings should be detected by detailed photometric or spectroscopic changes in transit. The difficulty lies in the fact that such signals are elusive and difficult to discern in the current data, ”the researchers write in their paper.
'Obviously, there is still a lot we don't know about exoplanet rings.'
Super-chubby exoplanets are detected using the transit method. It is here that the telescope studies the star over time, looking for regular dips in the star's light. If the dips occur at the same depth and the same time interval between them, we can conclude – an exoplanet.
The amount of light that the star lets through can then be used to calculate the physical size of the specified exoplanet. Another method can be used to calculate the mass of an exoplanet – when planets orbit their stars, they actually exert their own gravitational influence, causing the star to wiggle slightly. The amount of this wiggle is determined by the mass of the planet.
Since many planets in the solar system have rings, it is obvious that many exoplanets also have rings. But we usually look at these objects many light years away, and most of the time we cannot see the planets directly – so detecting rings seems impossible. Or not?
'We started to wonder if you looked back at us from a distant world, would you recognize Saturn as a planet with rings or would it be a plump planet for an alien astronomer?' asks Shreyasa Vissapragada of Caltech.
As a next step, astronomers turned to simulations to see if the rings could explain the size of known super-puffy planets. The answer was yes, for some of them. But not all.
“These planets tend to orbit in close proximity to their stars, which means the rings should be rocky rather than icy,” Piro said. “But the radii of the rocky rings can be so large, unless the rock is very porous, so not every planet will meet these limits.”
The planet must be flattened to a more oval shape so that the rings do not deform; Saturn is the flattest planet in the solar system due to its high rotational speed. Exoplanets that are tidally attached to their star – that is, their rotation has the same period as their orbit – may rotate too slowly to create this shape.
Given these limitations, some exoplanets have proven to be good candidates for rings. Kepler 87c and Kepler 117c are larger than Neptune, but with masses of only 6.4 and 7.5 times the mass of Earth, respectively, making them really low density.
“Confirmation of the presence of rocky rings in some cases will not only be an amazing new discovery, but also provide important information about these planets,” the scientists wrote in their article.
The study was published in the Astronomical Journal.
Sources: Photo: Robin Dienel / Carnegie Institution for Science
