Could distant aliens be sending out signals telling us they exist? If so, how do we know where to look? Researchers focused on finding extraterrestrial intelligence, or SETI, have devised a new strategy to focus their search.
The strategy applies simple trigonometry to millions of data points, aiming to find potential interstellar beacons that are synchronized with hard-to-miss astronomical phenomena like supernovae.
University of Washington astronomer James Davenport and colleagues set out the plan in a research paper submitted to the arXiv preprint server this month. The idea is also the subject of a talk Davenport will be giving at this week’s Breakthrough Discuss conference in California.
“I think the technique is very simple. It’s about triangles and ellipses, things that are like high school geometry, which is kind of my speed,” Davenport told GeekWire, half-jokingly. “I like simple shapes and things that I can easily calculate.”
The preprint paper, which has not yet been published in a peer-reviewed journal, is based on data from the European Space Agency’s Gaia sky-mapping mission. But Davenport said the technique is tailor-made for the terabytes of astronomical data that will be coming out of the Vera C. Rubin Observatory every night for a few years when it comes online.
Davenport and his SETI colleagues make a few assumptions: First, the aliens must want to communicate, and they must be able to build a means of communication. “The idea from the alien’s point of view would be that maybe you have the technology and the capacity to build some kind of beacon, some kind of lighthouse that you want to shine,” Davenport said. “But it’s very expensive to keep beaming in all directions.”
So when do you turn on the beacon? One strategy would be to synchronize the flare’s flashes with observations of cosmic flares. “It’s like playing Marco Polo,” explained Davenport. “This big thing happened. Someone yells “Marco” and you yell “Polo” or you say “We saw it too. Do you see us?'”
The best recent example of a cosmic flare would be SN 1987A, a supernova explosion that occurred 168,000 light-years away and was spotted on Earth 35 years ago.
The light from the flash of SN 1987A has been spreading in an expanding sphere for more than 168,000 years and will continue to expand into more areas of our celestial neighborhood. If a more distant alien civilization wanted to synchronize its flare flash with the supernova flash, we would see it delayed due to the finite speed of light.
If you know the distance to a particular star, it’s relatively easy to figure out when it’s on the edge of a “SETI ellipsoid,” which is the right time for the flash of the alien beacon and its light from Earth astronomers. But it’s not that easy to keep track of the millions of stars in the ever-expanding ellipsoid.
Two trends in astronomy are making it increasingly easy to monitor SN 1987A’s SETI ellipsoid. One is the move toward large-scale all-sky surveys like Gaia, which measure distances to distant stars with unprecedented precision. The other is the rise of “big data” analysis tools, such as the algorithms being developed at the University of Washington’s DiRAC Institute.
Using such tools, Davenport and his colleagues screened thousands of stars in Gaia’s catalog, all within 326 light-years (100 parsecs) of Earth. “The vast majority of nearby stars are still viable targets for monitoring over time,” they reported.
On average, 734 stars will pass through the SETI ellipsoid annually. “Although this is a large number of targets to monitor each year, for many surveys it is entirely possible,” the researchers say.
When it comes to scanning the skies for synchronized signals, SN 1987A isn’t the only game in town: Other SETI ellipsoids can be plotted for a variety of astronomical phenomena, including galactic novae, gamma-ray bursts, and neutron star mergers.
Identifying what appears to be a synchronized signal would only be the first step in investigating a specific target.
“The concern is that we end up with a scenario like the ‘wow signal’ where you have this really interesting signal and no repeat or other follow up to show you what it is or if it will ever repeat .” Davenport said. “That’s definitely a problem.”
A seemingly synchronized flash may well turn out to be a cosmic fluke, potentially involving mysterious anomalies such as dimming and brightening in a star system known as KIC 8462852, or “Tabby’s Star.” A few years ago, some astronomers suggested that the phenomenon could be attributed to an extraterrestrial megastructure, but now the leading hypothesis is that a dust cloud was the cause. “It’s still a very interesting object because we still don’t know what the dust is,” Davenport said.
Davenport has recruited students to develop ways to use big data more efficiently for SETI. “There are a lot of tricks we can use and we can write them down as algorithms and put them in databases, computers and big machines and then run them,” he said.
In addition to SETI ellipsoids, such algorithms could focus on the so-called Earth transit zone, a band of the night sky where extraterrestrial astronomers could theoretically see Earth passing our own host star. In addition to analyzing the Gaia database, Davenport and his SETI colleagues were able to view observations from NASA’s Zwicky Transient Facility and Transiting Exoplanet Survey Satellite (TESS), as well as the data to be generated by the Rubin Observatory’s LSST survey.
Davenport acknowledged that the ellipsoid search strategy was a long road — which is why it’s important to use existing data for the long term, perhaps centuries.
“We don’t know what another civilization thinks is the right way to build a lighthouse,” Davenport said. “We don’t know what would make sense for them or what would be conspicuous. Instead, let’s make the most of the data we have, as we invest a lot of time, energy and money into developing that data for a variety of other reasons.”
In addition to Davenport, the authors of Searching the SETI Ellipsoid with Gaia include Barbara Cabrales, Sofia Sheikh, Steve Croft, Andrew PV Siemion, Daniel Giles and Ann Marie Cody.