Closing In on 'Alien Megastructure' Clues

Another study of the dust surrounding infamous Tabby's Star appears to favor the exocomet hypothesis for its very strange transit signal.

After all the kerfuffle surrounding a certain star scrutinized by NASA's Kepler Space Telescope last October, astronomers have been feverishly trying to find a, um, natural explanation for a bizarre transit signal. Though it has turned into something of an epic endeavor, astronomers are zeroing in on the most likely candidate.

You know the story: Kepler is used to detect the transit of exoplanets around other stars. Its strength is that it can detect the very faint dip in starlight of astonishingly small worlds of Earth-like dimensions (and smaller) at tens to hundreds of light-years distant. The Kepler database is crazy-big, so to help astronomers identify transit signals, the crowd-sourcing group Planet Hunters delves into the signals.

One signal, originating from a star designated KIC 846852 - which was eventually nicknamed "Tabby's Star" after discoverer Tabetha S. Boyajian - was dramatic. And by dramatic I mean unprecedented: A series of transit signals caused the star's brightness to dip up to 20 percent. For a mission that deals in transits that usually dim starlight by fractions of a percent, you can see why this signal caused a stir.

Most interestingly, this signal was confirmed to be real, so it wasn't instrumental or analysis error, so astronomers started working on possible explanations. It wasn't long before the "alien" card was pulled and the world's media latched on - was this the first direct evidence of an advanced alien civilization building some kind of "megastructure" around their host star? Could this be the first evidence of a partially built "Dyson Sphere"? Science fiction fans rejoiced! (Tentatively.) I, for one, welcomed our neighboring extraterrestrial architects and their can-do attitude.

But scientists being the buzzkills that they are were already pointing their fingers at other, more plausible explanations - no aliens required.

One idea centered round the possibility of a planetary collision. Although the likelihood of us looking in the right place at the right time to see the exo-smashup was very slim, just because we were lucky, it didn't mean it didn't happen. Another idea focused around a vast swarm of comets blotting out the star from our viewpoint. Indeed, the latter explanation had some meat as there was another nearby star that could have destabilized Tabby's Star's Oort Cloud (the region surrounding a star that is thought to harbor countless billions of ancient icy bodies waiting to be knocked off their gravitational perch and fall through the inner star system).

Now, in research from Harvard-Smithsonian Center for Astrophysics, astronomers have studied Tabby's star using the Submillimeter Array and the James Clerk Maxwell Telescope, located atop Mauna Kea in Hawaii, in an effort to possibly track down dust associated with a planetary collision. The warm dust that would be produced by such an event should glow in emissions at submillimeter and millimeter wavelengths.

They found none.

Though the signal of some warm dust in the system may not be detectable (as the signal is so weak), this null result puts some tough restraints on the amount of dust in the system. There's simply not enough dust there to support the collision hypothesis, but might be consistent with a complete breakup of 30 massive Halley-like comets blocking the starlight from view.

But just because the exocomet idea has been reinforced, it's not easy for astronomers to explain why dozens of comets happened to disintegrate; there is no simple or known mechanism that would do that. Interestingly, this new finding doesn't necessarily disprove the megastructure hypothesis either, but in the interest of Occam's Razor, a cloud of strange comets would certainly be a simpler explanation than an advanced extraterrestrial civilization doing some DIY around a nearby star.

So the message is becoming clear: we're not saying it's not aliens; it's just the exocomet hypothesis is becoming more likely.