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Some SETI researchers believe the best way to detect aliens is to search the skies for their laser beams. In the largest survey of its kind, astronomers scanned 5,600 stars in search of these optical signals—and they found…absolutely nothing. Nada. Zilch. Here’s what that means to SETI and the ongoing hunt for alien intelligence.
In a new study accepted for publication at the Astronomical Journal, SETI astronomers Nathaniel Tellis and Geoffrey Marcy from the University of California Berkeley report that they were unable to detect optical signatures of advanced extraterrestrials in over 67,000 individual spectra produced by nearly 5,600 stars in the Milky Way Galaxy. Revealingly, around 2,000 of these stars are suspected of hosting warm, Earth-like planets, suggesting that advanced civilizations either don’t get into the habit of transmitting powerful laser beams across the cosmos, or they simply don’t exist. More practically, it means we should look for optical signals elsewhere, and expand our search to include an assortment of other potential alien signatures. Simply put, we’re not done searching for ET.
That said, the null result is undeniably discouraging. Laser signals would be an effective, cheap, and easy way for an advanced civilization to get our attention. Using technology similar to what we have today, aliens could deliberately transmit artificial infrared, visible, or ultraviolet emissions at our star. These directed signals could attract our attention by being continuous and abnormally powerful, or by containing tell-tale signs of artificiality, such as unexplained pulsing, or a string of binary data expressing some kind of mathematical phenomena (e.g. prime numbers or pi).
Prior to this study, SETI researchers had evaluated around 20,000 stars in search of optical signals at Harvard’s Oak Ridge Observatory, spending about 10 minutes on each object. Clearly, if the laser blinking frequency is longer than that, or if ET’s laser transmission station is temporarily out of service, we’re out of luck. Not surprisingly, nothing interesting has been detected thus far.
In an effort to conduct a more thorough review of the heavens, Tellis and Marcy analyzed a trove of data collected by the Keck 10-meter telescope and its high resolution spectrometer, HIRES, between 2006 and 2016 as part of the California Planet Search (CPS). The 5,600 stars included in the study, the majority of which are located within a distance of 300 million-light years, produced 67,708 individual stellar spectra, averaging 96 spectral signals per star.
“This study leverages the huge amount of data collected by the Keck telescope over decades, mostly as part of planet-hunting projects,” explained Penn State astronomer Jason Wright, who was not involved in the study, in an interview with Gizmodo. “That makes it sensitive to relatively low laser powers from thousands of the most nearby and interesting stars. It’s a great example of how SETI can ‘piggyback’ off of other science, looking for signals that might have been overlooked or thrown out because they were not expected or they look very similar to known sources of noise.”
Armed with this data, the researchers then set about the task of searching for spectral signatures that, in the words of the authors, “would be expected from extraterrestrial optical lasers.” The power of these lasers ranged from 3 kW to 13 MW, which isn’t extreme by any measure. Unlike radio signals, which dissipate over vast distances, laser light manages to hold its integrity as it travels through space. “We may imagine that beings more technologically advanced than humanity would be capable of constructing…laser launchers with power levels at least as high as those detectable here, for any of the 5,600 star systems we surveyed,” explained the researchers in their study.
To analyze this decade’s worth of data, Tellis and Marcy developed an algorithm that was (at least in theory) able to discern a possible alien signal within the natural spectra of a star. If an artificial signal was directed towards Earth,
it would be detectable as an unusually high number of protons compared with the background emissions of the star. The algorithm was configured to flag any occurrence of three consecutive pixels that exceeded the researchers’ thresholds.
“We searched our spectra for ‘brightenings’ of the star, relative to the light it is already emitting, that were both tight in wavelength and in space,” Tellis told Gizmodo. “Finding a signal that matched the instrumental profile of Keck HIRES would have almost unequivocally meant we were seeing laser light, as the normal stellar spectra contain only thermally broadened emission lines. This is one of the key advantages to using Keck, as it has high enough spectral resolution to distinguish the two.”
The thresholds were very liberal, resulting in an initial batch of 5,023 candidates. The researchers manually parsed these results (literally eyeballing the data), pruning the list down further and further as they pinpointed the source of each false positive. The most common sources of these false positives included cosmic rays, gamma rays, radioactivity from the observatory, molecules in Earth’s atmosphere, and emissions from the parent stars. Eventually, Tellis and Marcy had to concede defeat.
“We found no such laser emission coming from the planetary region around any of the 5,600 stars,” conclude the researchers in their study.
This result would seem to strike a blow to the suggestion that advanced civilizations might last for thousands or millions of years, all the while sending hello messages to up-and-coming neighbors. If even a small fraction of the 2,000-or-so systems with potentially Earth-like planets surveyed had technological civilizations who took the time to deliberately beam megawatt-lasers towards us, we should have detected them by now.
“These results put an upper bound on the number of civilizations transmitting lasers at us while we were observing,” said Tellis. “It is only one type of communication, but we believe that for targeted communication, lasers are highly efficient.” That said, he admitted that lasers as a communication medium seem good to us at this time due to our relative youth, and that his team’s strategy relies on serendipity. “We have to ‘catch’ their broadcast,” he said. “Nevertheless, we believe it is a valuable result that the galaxy is apparently not teeming with such bright lasers.”
So either advanced alien civilizations don’t behave in this way (e.g. they hide their presence or engage in other activities), or they don’t exist. It’s also possible that technological civilizations are exceptionally rare in the galaxy (both in time and space), greatly limiting the ability of the researchers to detect a signal. As the authors of the new study admit, “We may begin to wonder if arguments along the lines of the so-called Fermi paradox have some merit.” Indeed, the eerie silence of space is getting louder with each new attempt to detect alien intelligence.
Undaunted, the researchers are planning on an expanded search. As part of the $100 million Breakthrough Listen effort, they will now turn their attention to stars that were overlooked in the study, including brown dwarfs and other odd astronomical phenomena. In addition to optical signals, SETI researchers can look for other potential signs of alien intelligence, such as microwave or neutrino emissions, Dysonian megastructures, industrial waste signatures, transiting space habitats, and so on.
If the aliens are out there, we’ll find them. Eventually.
[The Astronomical Journal (preprint available at arXiv)]
It seems like every week, there’s a new contender for Coolest Planet Where There Are Definitely Aliens. For those of us who want to believe, this is an emotionally exhausting cycle, as we’re built up and let down time and again. At the risk of fucking with our fragile hearts even more, it’s worth mentioning that a recently discovered exoplanet 39 lightyears from Earth might actually give the current favorites—Proxima b and the TRAPPIST-1system—a run for their money.
This rookie, known as LHS 1140b, hails from the constellation Cetus (the sea monster). It was initially discovered in 2014 by Harvard astronomers who are part of The MEarth Project, which hunts for exoplanets. The team first spotted the rocky planet using the transit method, meaning they noticed a dip in the light output of its parent star, a red dwarf called LHS 1140, when it passed in front in our line of sight.
“We originally thought it was just something funny going on in the atmosphere,” Harvard astronomer Jason Dittmann, the study’s lead author, told Gizmodo. “It was only about a year later, when I was going back through our data with a machine learning based algorithm…that I pulled this 2014 transit out and flagged it as possibly real.”
Dittmann and his team of researchers followed up on the observation using the European Southern Observatory (ESO) HARPS instrument, which allowed them to confirm the planet’s orbital period, mass, and density. The group’sresearch will be published in Nature on April 20th (nice).
Though it’s much closer to its sun than Earth is to ours, LHS 1140 is a lot cooler than our life-giving buddy. It just so happens that as a result, LHS 1140b lies squarely in the habitable zone, which means that hypothetically, it could support flowing water. Even though it’s only about 1.4 times the size Earth, it’s about seven times as massive, indicating that it is probably a rocky world with a dense iron core. Naturally, researchers are already pumped about the possibility of life there.
“This is the most exciting exoplanet I’ve seen in the past decade,” Dittmann said in a statement. “We could hardly hope for a better target to perform one of the biggest quests in science—searching for evidence of life beyond Earth.”
It’s not just enough to be excited about this planet, though; there’s a bit of exoplanetary competition going on here. Even the researchers are having some fun encouraging the rivalry between LHS 1140b, Proxima b, a potentially Earth-like exoplanet discovered just five light years away in 2016, and the seven Earth-sized planets recently spotted around the ultracool dwarf star TRAPPIST-1.
“The LHS 1140 system might prove to be an even more important target for the future characterization of planets in the habitable zone than Proxima b or TRAPPIST-1,” researchers Xavier Delfosse and Xavier Bonfils said. “This has been a remarkable year for exoplanet discoveries!”
Still, Dittmann thinks that the TRAPPIST-1 system, also located roughly 40 light years away, stands out as a particularly intriguing.
“I really want to emphasize that both our system and TRAPPIST-1 are exciting and both worthy of intense future study,” he told Gizmodo. “LHS 1140 is brighter at optical wavelengths because it’s slightly bigger than the TRAPPIST-1 star. So, when the future 30 meter optical telescopes are built (the Giant Magellan Telecopes and European Extremely Large Telescope), LHS 1140 can feasibly be studied by these telescopes.”
Indeed, we won’t know much more about any of these planets’ habitability until we can observe their atmospheres, which will require more powerful telescopes than today’s state-of-the-art. There are already concerns about the habitability of Proxima b, which, like LHS 1140b, orbits a red dwarf. Some scientists worry that the frequent solar storms from Proxima b’s host star could strip it of its atmosphere, dashing our chances of finding biosignatures there. It’s possible this could be a concern with LHS 1140b, too.
“There’s definitely a concern that high energy radiation from M dwarfs might ‘spoil’ the habitability of their planets,” Dittmann said. “In the case of Proxima b, the star seems to be very active, flaring quite often. This is also true in the case of TRAPPIST-1. In contrast, LHS 1140 is slowly rotating (130 days), and we haven’t seen any flares from the star. We also expect—and hope to check with future data—that the star is very quiet at high energies as well. So, at least in the present day, LHS 1140b finds itself orbiting a very nice, quiet host star.”
Ultimately, time will tell. NASA’s James Webb Telescope (JWT), which is set to launch in October 2018, could provide some of the answers
alien huntersastronomers are desperately seeking. Once it’s completed, JWT will be themost powerful space-based telescope ever deployed—it’ll be used to peer into the atmospheres of all of these planets and more.
When it comes to exoplanetary supremacy, there can only be one. Just kidding, I hope they all have alien babies hiding inside them.