In 1960, physicist Freeman Dyson proposed that a sufficiently advanced civilization could build a sphere around a star and would be able to capture all the star's radiant energy (Earthbound observers would see it re-radiated it as infrared light, so the star would look like a giant heat source).
But Tabby's star is clearly not surrounded by a solid sphere, since we can see the star's light. The kind of alien megastructure that one might imagine around this star would be a swarm of small bodies arranged in a spherical formation, or perhaps some large object or set of objects that regularly passes in front of the star.
The former is called a Dyson swarm, and would be easier to build than a sphere. If the star was surrounded by a Dyson swarm, each satellite would have to be at least asteroid-size, and you'd need many thousands of them. If they are orbiting at distances comparable to our own solar system's inner planets, their periods, or the time it takes them to make one revolution around their host star, would be anywhere from a few months to a few years, following Kepler's laws of planetary motion.
Could Tabby's star be surrounded by a ring-shaped structure a la Larry Niven's "Ringworld" novels? Such a ring, if built around the sun, would have to be about 93 million miles (149 million km) in radius, or about 584 million miles (940 million km) in circumference. That's so large that no known material could survive the stresses; Niven had to come up with a fictional material called scrith.
Astrophysicist Katie Mack,
told the BBC that you'd need something that was bound more strongly than ordinary molecular bonds. Niven's books posit a Ringworld that is about 1 million miles (1.6 million km) wide, large enough that it would block the light from its parent star if it was in the line of sight of an observer. But the pattern of eclipsing light from Tabby's star doesn't seem to fit what you'd see with a ring; the dimming wouldn't show the irregularities that it does with such a structure, according to Mack.
One could imagine a vast Dyson ring, though, with miles-wide satellites placed at intervals around Tabby's star. But in this case, too, to be at all visible, the satellites would need to be orders of magnitude larger than any space habitat humans have ever attempted to build or colonize, according to Mack — even the ISS is only 368 feet (108 meters) across, and that's far too small to be visible even from as "close" as Mars, let alone light years distant.
The smallest exoplanet that Kepler has found so far is
larger than Earth's moon. Called Kepler 37b, it's also a lot closer to Earth than Tabby's star is, only 215 light-years distant. If Tabby's star is surrounded by satellites, they would have to be even bigger than the
Death Star from "Star Wars," which is stated in various franchise-related sources as being 60 miles (100 kilomters) across. [
'Star Wars' Tech: 8 Sci-Fi Inventions and Their Real-Life Counterparts]
So why don't scientists think aliens are the explanation for the dimming of Tabby's star? One reason is that an alien megastructure would give off radiation in the infrared in a specific way. Any object that gets lit by a nearby star reflects some light and absorbs the rest, and that absorbed light gets re-emitted in a longer wavelength. Basically, objects warm up. In a talk at the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, in August 2016, astronomer Jason Wright of Penn State University said studies of the star's light have shown no sign of such "waste heat" with Tabby's star.
The other issue is that such megastructures would be difficult to keep stable. A swarm of satellites orbiting would eventually settle into a disk-like arrangement absent active controls, because any object orbiting another or rotating experiences some centripetal force – this is why the Earth is slightly flattened.
The same is true for Dyson rings and swarms. Futurist Anders Sandberg notes on his site outlining Dyson sphere principles that one structure that might stay stable is a Dyson bubble, made of gigantic, miles-square satellites that stay in place because of radiation pressure, in huge solar sails. But to stay in place, those satellites would have to be stationary with respect to the parent star, so they wouldn't cause anomalous dimming over time as was seen with Tabby's star.
