Tabby's Star is arguably the most mysterious – and controversial – star in our galaxy.
Observed by NASA's Kepler space telescope, the star shot to fame when citizen scientists studying the mission's data noticed the star dim and flicker dramatically between 2011 and 2013. Hypotheses about this weird behavior abound, but not a single one seems to fully explain what's going on. The lack of an obvious answer has fueled speculation that an advanced alien civilization is building a "megastructure" around the star that we call Tabby's Star.
Now, the star has mysteriously dipped in brightness again, raising hopes that the world's astronomers have caught the star in the act and that a clear explanation of the star's true nature may be coming soon.
Kepler's Mysterious Star
Kepler's mission is to search for extrasolar planets – or "exoplanets" – that orbit other stars, and it does this by detecting the very slight dimming of stars as exoplanets pass in front (events known as "transits"). Thousands of alien worlds have been detected during this profound mission, revealing the incredible bounty of planets that exist in our galaxy. In fact, there aren't enough scientists to properly analyze the huge quantities of data that the mission is producing. Enter citizen scientists.
The Planet Hunters crowdsourcing project makes Kepler observations available to hundreds of thousands of participants and significant exoplanet discoveries are being made.
For example, during Kepler's primary mission, one of the targets was KIC 8462852, an average F-type main-sequence star located 1,300 light-years away in the constellation Cygnus. But the Planet Hunters found this star to be very special; it became clear that the brightness variations that KIC 846285 was exhibiting were not your run-of-the-mill exoplanet transit signals – this was an entirely different beast.
The so-called "light-curve" of the star (basically the intensity of starlight that Kepler was detecting over time) was a mess. From 2011 to 2013, there were extreme dips and periods of interference, suggesting that there were many objects in orbit around the star. And some of these objects had to be very large to explain how much starlight they were blocking from view. One dip dimmed the star by an incredible 22 percent. Considering the most massive gas giant exoplanets will dim a star's brightness by a meager 1 percent, this hinted at the extreme nature of the object(s) in orbit about the star.
A paper detailing these results was made available on the arXiv pre-print service in October 2015 (and later accepted for publication in the journal Monthly Notices of the Royal Astronomical Society). The star was nicknamed "Tabby's Star" (or "Boyajian's Star") after astronomer Tabetha S. Boyajian who led the research.
To explain this strange transit signal, astronomers assumed there must be a vast cloud of dust around the star. But this didn't make a whole lot of sense; KIC 8462852 isn't a young star. Dusty rings of debris are usually found around very young stars that are in the process of creating planets.
Researchers then investigated the possibility that the dust might be caused by a chance planetary collision. A collision of this nature, however, would produce a specific heat signature, generating an excess of infrared radiation – but no such signature was discernible by follow-up observations.
What if a huge "swarm" of comets was gravitationally knocked into orbit about KIC 8462852 by a passing star? Might that be enough to cause sufficient dimming? Although this is one of the leading hypotheses that might explain this mystery, other observations of the star have failed to find corroborative evidence that such a swarm is even there.
Lacking an obvious answer, astronomers have been thinking a little more out of the box in hopes of explaining the KIC 8462852 light-curve. After the original Kepler result was announced, Boyajian hinted that "other scenarios" were being investigated and, in a now infamous interview with The Atlantic on Oct. 15, 2015, Penn State astronomer Jason Wright went into some detail about what one of these "other scenarios" could be.
"Aliens should always be the very last hypothesis you consider, but this looked like something you would expect an alien civilization to build," Wright said. Before this interview, Tabby's Star was a scientific curiosity. Now, Tabby's Star is a media sensation and is nicknamed the "Alien Megastructure Star."
Although it is more likely that KIC 8462852's weirdness likely has a natural explanation that has so far been overlooked by the astronomical community and aliens is the least likely explanation, this hypothesis doesn't seem to be going away.
But what kind of alien civilization could build something so big that it blots out the light from an entire star? And why would they want to do such a thing?
In 1964, Soviet astronomer Nikolai Kardashev created the hypothetical "Kardashev scale" that describes the advancement of a civilization as their energy needs increase from a cosmic perspective.
A type I Kardashev civilization, for example, would be advanced enough to harness all the energy that falls on a planet from its parent star. Humanity is thought to be 100-200 years from reaching this goal. A type II civilization would require a lot more energy than this and would need to harness all the energy their star can produce. To do this, a type II civilization might consider building a vast array of solar collectors around their star or even completely enclose it inside a "Dyson Sphere." Type III civilizations would have the mind-boggling ability to harness the energy output of an entire galaxy, though a mid-infrared survey in 2015 concluded that "Kardashev Type III civilizations are either very rare or do not exist in the local Universe."
But could the weirdness of Tabby's Star be the first evidence of a type II civilization?
First described in the 1937 science fiction novel "Star Maker" by Olaf Stapledon and popularized by physicist Freeman Dyson in his 1960 paper "Search for Artificial Stellar Sources of Infrared Radiation," Dyson Spheres are the hypothetical "megastructures" that might be built to encompass an entire star. When looking at the strange dimming events of KIC 8462852, the signal could be interpreted as a Dyson Sphere being constructed. Or it could be evidence of a Dyson Swarm, with many smaller solar energy collectors in orbit about the star.
In addition to the strange transit signals, astronomers have also noted that the star is exhibiting a gradual dimming over the past century, which could be interpreted as a sign of a megastructure being built.
To investigate this possibility, the SETI Institute aimed its powerful Allen Telescope Array (ATA) at Tabby's Star for more than two weeks in November 2015 to listen for any errant radio communications that an advanced extraterrestrial civilization might be transmitting, but no signal was detected.
Until now, astronomers have been left speculating about what could have possibly be behind the weirdness of KIC 8462852, with only past Kepler data to work with. But then, on the morning of May 19, it dipped in brightness again, prompting a flurry of alerts online.
"At about 4 a.m. this morning, I got a phone call from Tabby [Boyajian] saying that Fairborn [Observatory] in Arizona had confirmed that the star was 3 percent dimmer than it normally is and that is enough that we are absolutely confident that this is no statistical fluke," said Wright, during a live webcast on May 19. "We've now got it confirmed at multiple observatories I think."
This first dimming event is now over, but if Tabby's Star is up to its old tricks, there will almost certainly be more. And this time, amateur and professional astronomers are recording the spectrum of starlight that is produced during the dimming events to see if the chemical fingerprint of whatever is passing in front of the star reveals itself.