After all the public excitement surrounding the star KIC 8462852 and its weird transit signal as spotted by NASA's Kepler Space Telescope, the SETI Institute decided to expedite plans to point a powerful radio antenna at the nearby star in the hope of detecting any artificial transmissions emanating from that location. Sadly (or not, depending on how you view the discovery of an intelligent alien civilization living in our cosmic backyard), the first pass drew a blank.
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So, is this "case closed" for the possibility of an alien megastructure around KIC 8462852? Well, not really, but it does make a vanishingly slim chance of aliens even more vanishingly slim.
But before we discuss what SETI has (or, indeed hasn't) found, a quick recap.
Aliens! Or Not In September, astronomers and citizen scientists published a paper describing a "bizarre" transit signal recorded by Kepler, outlining a few possible causes for the phenomenon around KIC 8462852 - informally known as "Tabby's Star." On 2 occasions during Kepler's prime observing run, huge transits were detected around the star.
Usually, Kepler will detect periodic dips in star brightness and the amount of dimming relates to the size of the exoplanet passing in front of the star. Typically, for a small exoplanet, this dip in brightness is of the order of a couple of percent. But the two transit signals detected around Tabby's Star were dramatic. The first transit dipped by 15 percent, but the second transit (that consisted of several objects passing in front of the star over several weeks) dipped to as much as 22 percent. The strength of the transit and its multi-object nature was unprecedented.
Many natural phenomena were explored and the researchers eventually focused on the possibility of a swarm of comets as being the most likely culprit. A nearby star, they argued, may have destabilized comets in Tabby's Star's Oort cloud (a hypothetical region surrounding a star containing countless billions of icy bodies), nudging a huge number of comets toward the star, blocking its light from view.
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This explanation certainly fits many of the transit's characteristics, but during an interview with The Atlantic, astronomer Jason Wright, an astronomer from Penn State University, discussed an alternate avenue of study that he wanted to investigate. On many occasions since, Wright and other astronomers have cautioned against jumping to the alien conclusion - after all, there are other, more likely natural explanations - but when faced with an unprecedented transit signal, why not explore the most extreme possibilities?
As with any speculation about aliens, the slim possibility of this Kepler signal being artificial caused an eruption of interest. While many scientists vented frustration at the frenzy of interest surrounding an unlikely scenario, whether they liked it or not, suddenly everyone was interested in Kepler science and hypotheses of an advanced alien civilization living in our galaxy. In my personal view, so long as the discussion remained planted firmly in the science, and not conspiracy theories and nonsense, this was no bad thing - especially as professional scientists were planning on investigating the possibility themselves.
Allen Telescope Array So, if not comets, what could be causing the bizarre transit signal? Taking the age of our galaxy into consideration, it's not such a stretch to think that if life is common in the Milky Way, perhaps there's a vastly more advanced civilization to our own out there that has the ability to build huge solar energy collectors, say, around their host star. This speculation would certainly fit a Type II Kardashev civilization that has the ability to build huge megastructures around a star (much like a Dyson Sphere) to collect huge quantities of energy. Perhaps Kepler's transit signal was several vast solar collectors passing in front of the star?
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While it's fun to think up all the possible structures an intelligent alien race might build, seeing a star dim a couple of times is hardly proof that aliens are out there. It's not even evidence, especially when Occam's Razor reminds us that there are other explanations out there with far fewer assumptions (i.e. the exocomet explanation). But in the interest of using Tabby's Star as a point of scientific interest and respond to the huge public interest in the star, SETI Institute scientists turned to the their Allen Telescope Array (ATA) to "listen in" on the location.
The ATA is located a few hundred miles north of San Francisco, Calif., in the Cascade Mountains, consisting of 42 6-meter diameter radio antennae. The ATA was aimed at Tabby's Star for 2 weeks and listened out for 2 specific radio signals. The first signal, of a narrow-band 1 Hz bandwidth, might be used as a "hailing signal", according to the SETI Institute, for advanced alien civilizations to announce their presence. The second signal may be of a more broad-band emission that could signify an alien presence through the leakage of beamed propulsion in the star system. Beamed propulsion is one hypothetical method that could be used to power spacecraft in the future, so if there's an alien megastructure around that star, there would likely be spacecraft too, possibly sporting this propulsion method.
"This is the first time we've used the Allen Telescope Array to look for relatively wide-band signals, a type of emission that is generally not considered in SETI searches," said SETI Institute scientist Gerry Harp in a news release.
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But on this first 2 week pass, the ATA didn't detect either type of signal emanating from Tabby's Star:
Analysis of the Array data show no clear evidence for either type of signal between the frequencies of 1 and 10 GHz. This rules out omnidirectional transmitters of approximately 100 times today's total terrestrial energy usage in the case of the narrow-band signals, and ten million times that usage for broad band emissions. -
Slim Possibilities This finding provides an upper limit on our ability to detect these most powerful signals - if these hypothetical aliens possess the energy required to build vast structures around stars, they will certainly have the energy required to generate such powerful beacons. But say if they're not deliberately transmitting? Well, that might explain why we're not hearing the narrowband "hailing" signal. Say if there's little activity around these structures? That might explain why there's little radio "leakage" from the star. What if these are structures left in orbit by a bygone alien civilization? Perhaps they're monuments of an empire that is long gone. A fascinating thought.
Once again, we're speculating about what these hypothetical aliens are or are not, but the SETI investigation certainly rules out certain scenarios.
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"The history of astronomy tells us that every time we thought we had found a phenomenon due to the activities of extraterrestrials, we were wrong," said SETI Institute astronomer Seth Shostak. "But although it's quite likely that this star's strange behavior is due to nature, not aliens, it's only prudent to check such things out."
It's very easy to give every reason why the Kepler signal from Tabby's Star is not aliens - as it most likely isn't - but collecting more data from other observatories taking different approaches certainly isn't a waste of time when on the trail of tracking down the slim possibility of intelligent extraterrestrial life in our galaxy.
Source: SETI Institute