E.T., Are You Home?

Astronomy has a long history of supposed sightings of space aliens. In the late 19th century, American astronomer Percival Lowell reported seeing canals on Mars and concluded that intelligent extraterrestrials must've dug them. In 1967, when British researchers spotted a powerful, pulsating source of radio waves, they speculated that it was a beacon from an alien civilization. They even dubbed the source LGM-1, for Little Green Men. Within months, though, the researchers discovered a second pulsating signal and realized it was a natural phenomenon, a rapidly rotating remnant of a dead star. It was renamed the pulsar.

Now astronomers have focused on a new mystery, a strangely behaving F-type star -- slightly larger and hotter than our sun -- that has revived speculations about extraterrestrial intelligence. Known by its formal designation KIC 8462852, the star is located almost 1,500 light-years from Earth. It's one of the more than 100,000 stars observed by the Kepler Space Telescope, the NASA probe that measured the intensity of their light from 2009 to 2013 to see if any planets orbited them. If a planet passes directly in front of a star, it blocks a small fraction of the star's light, enabling scientists to estimate the size of the planet and its orbit. Scientists are still analyzing Kepler's observations, but they've already confirmed the presence of more than a thousand distant planets, including some that might harbor liquid water -- and perhaps life.

Last fall, however, a team of researchers led by Tabetha Boyajian of Yale University found that KIC 8462852 was dramatically different from all the other stars monitored by Kepler. The intensity of its starlight plummeted several times over the four years of observations, dropping 15 percent at one point and 22 percent at another. These stellar brownouts were far more severe than the dimming that an orbiting planet would cause. In other star systems monitored by Kepler, even the biggest planets blocked only 1 percent of the starlight. Still more puzzling, KIC 8462852 -- which became known as Tabby's star, after the lead researcher on the team -- darkened at unpredictable intervals instead of in the regular, clockwork pattern you'd expect to see if one or more planets had obstructed the light.

Boyajian's team considered the possibility that a vast cloud of dust had blocked the starlight, but such clouds are usually seen around young stars, and all the astronomical evidence indicates that Tabby's star is much older. Furthermore, observations of the star by other spacecraft showed no sign of the infrared radiation that would be emitted by the dust grains. A more likely explanation, the researchers concluded, was that a huge swarm of comets zipped past the star as Kepler observed it. A gravitational disturbance, perhaps caused by another star nearby, could've thrown the comets out of their usual orbits and hurled them toward Tabby's star.

But astronomer Bradley Schaefer of Louisiana State University undercut this hypothesis in January when he presented evidence from the archives of the Harvard College Observatory, which collected half a million photographs of the sky from telescopes around the world between 1890 and 1989. Tabby's star appears in hundreds of the glass-plate photos, and Schaefer concluded that its light has faded by about 20 percent over the century of observations. Although a comet swarm might explain the star's recent flickering, it couldn't cause the hundred-year dimming.

The stubborn mystery of Tabby's star has led to the more radical speculations involving extraterrestrials. A team led by Jason Wright of Penn State University had already investigated whether Kepler's data could aid the search for alien intelligence. In 1960 physicist Freeman Dyson theorized that an advanced civilization in a distant star system would be likely to build tremendous power-collecting structures around its star to capture its energy. These megastructures might consist of swarms of giant photoelectric panels, each thousands of miles across, or even a vast shell -- the so-called Dyson Sphere -- that completely encloses the star. A Dyson Sphere would absorb all the star's light and reradiate it as infrared emissions, but a swarm of megastructures would block the starlight in a highly irregular pattern. According to a research paper by Wright's team, Tabby's star has "all of the hallmarks of a Dyson swarm."

The paper dutifully warned that researchers should first investigate all the possible natural causes of the strange dimming before "invoking alien engineering to explain an anomalous astrophysical phenomenon." That warning went unheeded, though, after the news hit the Internet. Stephen Colbert brought up the subject on his late-night talk show, discussing alien megastructures with Neil DeGrasse Tyson, director of the Hayden Planetarium. Colbert displayed a picture of Ringworld, the megastructure imagined in Larry Niven's 1970 science-fiction novel of that name, but Tyson was unconvinced. "Just because you don't understand what you're looking at," he told Colbert, "does not mean it's aliens."

Still, the flickering starlight is a much stronger piece of evidence than a typical UFO sighting or a tale of alien abduction. Scientists are taking the megastructure hypothesis seriously because they can conduct further studies to test the idea. At the Allen Telescope Array in California, astronomers have already pointed their radio dishes at Tabby's star to listen for messages transmitted by extraterrestrials almost 1,500 years ago. The researchers found no unusual radio signals, but because the star is so far away, the Allen Array couldn't detect a signal broadcast from the star system unless it were transmitted with a power of at least four billion megawatts, which is hundreds of times greater than humanity's total power consumption. And it's very possible that an advanced alien civilization would have better ways to communicate than using the wavelengths that our radio telescopes are scanning.

A more definitive answer could come from follow-up observations of the star by ground-based telescopes. Amateur stargazers in the American Association of Variable Star Observers have aimed their telescopes at Tabby's star, hoping to catch a glimpse of another dimming episode. If they do, they'll alert the global community of astronomers, who will then train some of the world's most powerful telescopes on the star. Better yet, the researchers will be able to make detailed spectroscopic observations the next time the star goes dark, and the analysis of the affected wavelengths will tell them a lot more about what's blocking the light. The astronomers may find evidence for an entirely unexpected astrophysical phenomenon that could be dimming Tabby's star.

Or they may find more signs of megastructures. Carl Sagan, the astronomer and science popularizer who often speculated about extraterrestrial intelligence, was famous for saying, "Extraordinary claims require extraordinary evidence." As of now, the evidence for an alien civilization at Tabby's star falls far short of that standard. But it could be a start.

Mark Alpert is a contributing editor at Scientific American and an author of science thrillers. His latest novel, The Orion Plan, is about an alien invasion of New York City.