Tabby's star, which undergoes a chaotic pattern of dimming, has attracted a lot of attention due to suggestions that it might host an orbiting megastructure constructed by aliens. But it's not the only star with an odd pattern of dimming. Astronomers have now taken a close look at one called PDS 110 and come up with a possible explanation for its pattern: a giant planet with rings that orbits outside the plane of most of the other material in the system.

While it's a pretty tenuous explanation, the good news is that we should be able to get more data soon. The planet's next passage is expected to be in September, and the dimming should be clear enough to be visible to amateur astronomers.

Deep dive into the archives

PDS 110 lies a bit over 1,100 light years from Earth. It's similar to our Sun in terms of mass, but it's much, much younger; estimates are that it's less than 10 million years old. At that age, it's still expected to have a large disk of dust and gas that may be forming planets. But imaging of PDS 110 shows no sign of the dust, suggesting that the disk is at an angle where it doesn't obscure any of the star from Earth's perspective.

Since planets form in the same plane as the disk, we wouldn't expect any of them to transit between the star and Earth. That would preclude detecting them by tracking a regular dimming of the star's light as the planet orbits.

Yet something is dimming the light of PDS 100 in a rather significant way. The eclipse lasts for 30 days, and roughly 30 percent of the star's light is blocked as it happens.

Researchers first saw this dimming in data taken by the WASP telescope in 2011, leading them to dive into the archives to search for other telescopes that might have imaged the star. They found data taken by the INTEGRAL space telescope, which looked at PDS 110 14 different times. And the All-Sky Automated Survey had observed the star on and off for nearly a decade. But neither of these showed anything like the 30-percent dip in brightness.

They finally hit pay dirt when they checked data from the KELT-South telescope (KELT stands for the Kilodegree Extremely Little Telescope, obviously). KELT happened to catch a second eclipse with an extremely similar depth and duration. The authors estimated that something was orbiting the star every 808 days, give or take a few. That's a bit over 2.2 years, which would place the object a bit outside of Mars' orbit.

(With all the observations we've made of PDS 110, it was simply bad luck that only two of them covered one of these eclipses. You can practically hear the authors gritting their teeth as they write "Despite 25 seasons of data across 15 years and five surveys, all other predicted eclipses lie in observing gaps.")

What is this thing?

What the observations don't obviously indicate is what we're looking at. Given that PDS 110 is likely to have a large, dust-rich disk, the simplest explanation would be an area of high-density dust in that disk. Except there's no indication we can see the disk at all at other times. Any high-density area, therefore, would have to extend pretty far outside the plane of the disk—far enough that the gravitational pull of the disk would shear it out very rapidly. As a result, the authors don't think that's a very good explanation. Similar problems exist for some alternate explanations, like debris from a collision within the disk.

So, they consider a planet instead. But the drop in light from the star doesn't look like it was caused by a planet. Planets don't let any light travel through them, so the loss of light from the star bottoms out suddenly and remains at this bottom for nearly the entire length of the transit. PDS 110, by contrast, shows a gentle downward slope, which indicates that whatever is passing in front of the star is partly transparent. This rules out both a planet and a dim companion star. Instead, it looks more like the sort of thin collection of dust that was ruled out by the considerations above.

The team, therefore, returns to that explanation, except this time adds something to keep the material together: "The simplest way of concentrating material is with the gravitational attraction of a massive body." In other words, there's a large planet orbiting the star and a disk of material orbiting the planet. In other words, we may be watching a super-Saturn pass in front of the star.

The authors calculate that it would have rings that extend 0.3 Astronomical Units—a bit shy of the distance from the Sun to Mercury. Holding together something that big requires a correspondingly big planet. It has to be at least twice the size of Jupiter, and it could be as much as 70 times Jupiter's mass, right on the border of being a brown dwarf star.

There are some problems with this idea. For one, the planet itself would need to have been pushed out of the planet-forming disk through interactions with other planets. And its rings have to be rotating in a plane that's perpendicular to that of the star's disk. It's not clear how common either of those are, but they can happen (Uranus has rings perpendicular to the rest of our own Solar System, for example). And there is a precedent for a planet with rings outside our Solar System: 1SWASP J1407.

What we need then at this point is more data. Conveniently, some will be on the way soon. If the team is right, the next transit will occur from September 9 through 30, later this year. While it will only be visible for a few hours each night from ground-based telescopes, you can bet the researchers have applied for observation time on some space-based hardware.

Monthly Notices of the Royal Astronomical Society, 2017. DOI: 10.1093/mnras/stx1249 (About DOIs).