Although planetary rings are extremely common in our solar system—every gas giant circling our sun has one—they’ve proved harder to spot around worlds orbiting other stars. That’s a shame, because studies of ring systems around younger worlds could help clarify what the giant planets of our nearly five-billion-year-old solar system looked like in their first few million years.
More than two decades of planet hunting have revealed just one ringed exoplanet—a super-size version of Saturn that researchers have only just begun to study using very large telescopes. But now they may have have found a second super-Saturn half-hidden in a disk of gas and dust surrounding a young star, a world readily observable even with backyard telescopes.
A few years ago, astronomers affiliated with the Wide-Angle Search for Planets (WASP) survey spotted an unusual feature in the shadowy haze around the star called PDS 110. For nearly two years the puzzling detection sat on the desk of WASP team member Hugh Osborn, a graduate student at the University of Warwick in England who first noticed it. “I wasn’t really sure what it could be,” Osborn says. Then, at a conference years later, another astronomer noted the same blip had appeared in data on PDS 110’s disk from the Kilodegree Extremely Little Telescope (KELT) survey, entirely independent Osborn’s original detection. At that point, “it became clear it was a bit more interesting than I originally thought,” he says. A paper detailing the research has been accepted for publication in Monthly Notices of the Royal Astronomical Society.
Separated by more than 800 days, the observations were nearly identical. Both revealed a strange, 25-day dimming of the star—something far too long to be explained as the shadow of a planet passing across the star’s face as seen from Earth. Osborn and his colleagues postulated the unusual signal might be a ring system around a previously unseen companion moving through the disk of gas and dust leftover from the star’s formation. Those rings would stretch about 50 million kilometers across (that is, nearly 200 times wider than Saturn’s rings, which are some 280,000 kilometers across). Such a large ring system, Osborn and his teammates estimated, could only be held in place by a massive central object—potentially a gas-giant planet even larger than Jupiter. Alternatively, the unseen companion could be a brown dwarf, an object midway in mass between a planet and a star.
So far only one other exoplanet has been found sporting rings. Called J1407 b, it is a gas giant on a decade-long orbit around a distant star; astronomers surmised its sprawling, super-Saturnian ring system based only on a single observation from 2012, and will have to wait until the 2020s to glimpse it again. PDS 110’s ringed companion has a much shorter orbit, giving astronomers more chances to see and study its shadow, but so far they have not capitalized on those opportunities.
That is now changing, according to Osborn, as the ringed world’s orbit should bring it back into view in September 2017. Even a midsize, store-bought telescope should be able to detect the deep shadow of the rings backlit by the star, allowing amateur astronomers to observe and study the system. Whatever their source, a high-quality third set of observations should provide astronomers the confidence they need to confirm there is in fact a shadowy something embedded in the disk and periodically blocking the star’s light. “Once is not enough to convince anyone,” says Joel Kastner, an astronomer studying young stars at Rochester Institute of Technology who was not involved with the research. Even two sightings could be unrelated. Three, on the other hand, is unlikely to be a statistical fluke. “If you see the same shape and depth in the dimmings three times, and the gaps between the dimmings are the same, then you certainly have a very strong case for periodicity.”
Osborn agrees. “We can’t strictly say it’s periodic until we see another eclipse.”
Matthew Kenworthy, a planet hunter at Leiden Observatory who worked with Osborn and was part of the team that identified J1407 b, thinks the initial observations of PDS 110 hold promise for greater discoveries to come. “I’m very excited because I think it’s another ring system,” he says.
Unlike Saturn’s rings, which are in nearly the same plane as the planet’s orbit, the putative rings around PDS 110’s companion would be tilted more perpendicularly like those of Uranus, poking out above the circumstellar disk. Osborn says such a distortion could be the result of interactions with another, unseen planet.
Geoffroy Lesur, an astronomer at the Institute of Planetology and Astrophysics of Grenoble in France who studies young circumstellar disks and was not part of the work, thinks Osborn and his team may be on the right track. But he is not sure they’ve spotted a ring. “It’s quite convincing, because it’s the only explanation that fits all the data they have,” he says. Although he agrees the companion probably has material surrounding it, he is not sure a ring system would remain stable within the star’s disk, however. As the world passes through the disk twice on every orbit, the disk’s material surrounding the star should pull on any rings of gas and dust around the planet, distorting them. Lesur thinks the result is more likely to be a shroud of debris instead of a ring. “It’s not going to be a nice [set of rings] like Saturn,” he says. “It’s more like a cocoon around the planet.”
Kenworthy calls such a cocoon “absolutely possible,” but argues it would lend a different shape to the shadowy blip around PDS 110 than what has been observed. It could be instead that a world orbiting within PDS 110’s disk could clear a gap there, creating space in which rings could persist undisturbed by the surrounding debris. Alternatively, if the rings are sufficiently massive, they could simply plow through the disk relatively unscathed.
It is also possible the shadowy blip around the star has nothing to do with a planet but is instead one or more clumps of debris falling back into the circumstellar disk. Such disks tend to be lumpy rather than smooth, filled with turbulent flows that send material arcing out of the disk only to be pulled back in by gravity. Debris flung out in this manner can coalesce into clumps about the size of the observed blip, Lesur and Kastner say. A particularly long-lived clump—or two independent clumps coincidentally erupting into view at just the right time—could then explain the blip’s observed repetition.
But Kenworthy says, whereas such scenarios are possible they are unlikely to produce two independent-yet-identical signals so far apart in time. Although rings would be gravitationally held in place by their planet, the loose clumps would only be weakly bound and should suffer dramatic changes over their orbits. “It’s difficult to see how this could hold the same shape over 800 days to give the same eclipse shape,” he says.
Structures such as vortices within the disk could also theoretically eclipse the star but the team says—and Lesur agrees—such arrangements would be far larger than anticipated to explain the blip. “They have a pretty convincing argument,” Lesur says.
By closely monitoring the shape of the shadows within and around PDS 110’s disk, the detailed September observations should help distinguish rings from clumps and could even reveal structures and gaps within the rings themselves. If the rings are confirmed, “that will be tremendously exciting,” Kenworthy says. “We can then plan for the next eclipse with more detailed experiments that can determine the type of materials in the rings.”
The first few million years after they formed, Saturn and Jupiter may have had enormous rings that were somehow stripped away, coalescing into moons or falling onto the planets. Observing ringed worlds around young stars can help scientists better understand what might have happened in the early solar system.
“What I think we’re seeing with these giant ring systems,” Kenworthy says, “is the very early stage of moon formation like when the solar system was incredibly young.”