An August 2009 NASA Spitzer space telescope image shows a cloud, known as DR22, bursting with new stars in the Cygnus region of the sky, where the mysterious KIC 8462852 is located. (NASA/AFP via Getty Images)

Remember KIC 8462852, the F-type star in the constellation Cygnus with the odd flickering habit?

You know, the one that had some scientists talking about the incredibly remote possibility of a “swarm of alien megastructures?”

Bet that jogged your memory.

Well, scientists are still pretty much absolutely certain that a vast system of solar panels is not orbiting around this far-off sun, harnessing its energy to power a super-advanced extraterrestrial civilization and blocking the progress of light on its way toward Earth (thus the flickering). Researchers at the Seti (search for extraterrestrial intelligence) Institute have turned their radio telescopes toward the star in hopes of picking up a few alien signals, but so far none have come our way.

“The hypothesis of an alien megastructure around KIC 8462852 is rapidly crumbling apart,” Douglas Vakoch, president of Seti International, said in a statement. “We found no evidence of an advanced civilization beaming intentional laser signals toward Earth.”

[Why NASA’s top scientist is sure that we’ll find signs of alien life in the next decade]

Meanwhile, other far-more-probable explanations for the strange appearance of the star — informally named “Tabby’s Star” after Tabetha Boyajian, who led the team that first reported on it — have been posited. The most popular one was that Tabby’s Star is surrounded by a cluster of comets that blocked the star’s light and caused various levels of dimming as they transited across. At least two studies, including the one from Boyajian’s team, have suggested this as the most likely cause.

But now Louisiana State University astrophysicist Bradley Schaefer says that the comet explanation also falls flat, according to Slate. Which brings us back to the conundrum that got Penn State astronomer Jason Wright talking about the possibility of aliens in the first place: If none of the normal, natural explanations for the odd behavior of Tabby’s Star seem to make sense, what could the abnormal explanation possibly be?

Scientists still aren’t sure. And Schaefer’s study, which he posted on the research-sharing project arXiv, only deepens the mystery.

Remember, Tabby’s Star first came to public attention because of the way observations from the Kepler Telescope showed dramatic dips in its brightness at irregular intervals. Even a Jupiter-size planet transiting across its sun’s surface would alter the brightness by about only 1 percent, and these changes were much more significant.

But nothing about the Kepler observations indicated what might be causing that interference. The star is too mature to be surrounded by the cloud of gas and dust that often blocks young stars’ light, and it’s of a fairly steadfast variety of star — F-type — which are moderately sized, moderately hot and not known for capricious behavior.

[Do we really want to know if we’re not alone in the universe?]

So Schaefer, the Louisiana State scientist, took a look at some other observations. He found that Tabby’s Star has been photographed more than 1,200 times as part of a sky survey that has been conducted at the Harvard College Observatory since 1890. And in the century after the star was first captured on film, it’s dimmed by up to 20 percent. For such a noticeable trend to happen on such a short timescale is “unprecedented” for any F-type star, Schaefer writes.

He measured the brightness of Tabby’s Star using two methods, and both turned up the same result. And similar stars from the Harvard survey don’t show the same kind of behavior, so the changes in brightness can’t be blamed on faulty images.

Now we know that Tabby’s Star is acting bizarrely on two fronts: It’s “flickering” on a day-to-day interval, as demonstrated in the Kepler observations, and it’s dimming over the course of a century, as shown in Schaefer’s analysis of the Harvard images. The basic principles of scientific inquiry force Schaefer to assume that a single phenomenon is causing both behaviors, so he looked into whether the most plausible explanations for the flickering also worked to explain the overall dimming.

The results don’t look good for the comet cluster camp. Schaefer concludes that the dimming trend would require that 648,000 giant comets (each about a tenth of the diameter of Pluto) pass in front of the star in a series of carefully orchestrated transits over the course of the past century. The likelihood of that happening is, well, very, very low. Whether it’s lower than the likelihood of an alien megastructure, Schaefer doesn’t say.

He also argues against another potential explanation for the flickering: that dust circulating around the star is blocking its light. As was the case for the comet-cluster hypothesis, this explanation simply requires too much dust to account for the century-long dimming. It’s not feasible that the same dust could be responsible for both Boyajian’s and Schaefer’s findings.

Curiouser and curiouser.

Slate’s Phil Plait points out that this gradual dimming is something scientists could expect from a Dyson Sphere (the term for the kind of hypothetical structure we assume an advanced alien civilization to would build around their star). Theoretically, as the aliens whose existence remains entirely dubious build their structure, it would block more and more of the star’s light.

But Plait is adamant that he doesn’t actually think a Dyson Sphere is the explanation. He calculated that the aliens would need to build a minimum of 750 billion square kilometers of solar panels to account for the 20 percent drop in their sun’s brightness.

“That’s 1500 times the area of the entire Earth,” Plait writes. “Yikes.”

Schaefer doesn’t think that aliens are responsible for the dimming phenomenon either. But it’s still not clear what could be behind it.

“The comet-family idea was reasonably put forth as the best of the proposals, even while acknowledging that they all were a poor lot,” Schaefer told the New Scientist. “But now we have a refutation of the idea, and indeed, of all published ideas.”

So astronomers are back where they started, with one very weird star and no good explanation for how it got that way. It’s a puzzling place to be in, but they have been here plenty of times before. Scientists can name countless phenomena that were once blamed on aliens and are now considered entirely natural. That’s what science is all about — finding an explanation for the seemingly inexplicable.

The most famous recent example is the discovery of pulsars by Cambridge graduate student Jocelyn Bell and her mentor Antony Hewish. The American Physical Society writes that Bell and Hewish thought that a radio message from an extraterrestrial civilization might explain the “strange bit of scruff” in Bell’s data, but years of research revealed it was actually a new kind of neutron star that swiftly spins and beams out light like a lighthouse. The work on pulsars won Hewish the Nobel Prize in Physics in 1974.

All of which is to say that astronomers are likely to keep their telescopes trained on the mysterious KIC 8462852.