Sahin Torlakcik, a Turkish high school student, just solved one of SETI's most frustrating problems: With nearly a billion stars in our galaxy and only so much telescope time available, which ones are actually worth listening to? The answer, published in the peer-reviewed journal Publications of the Astronomical Society of the Pacific, is the Torlakcik Catalog—a filtering system that cuts through the cosmic noise by identifying which stars are fundamentally unlikely to host complex life, and which ones might actually be worth our attention.
The challenge facing SETI researchers has always been practical rather than philosophical. Most programs point their dishes at the nearest and brightest stars, reasoning that closer sources produce stronger signals. But proximity alone is a poor guide to habitability. Some stars burn through their fuel so rapidly they'd incinerate any developing biosphere before it had time to evolve. Others are chemically impoverished in ways that make planet formation nearly impossible. Listening to those stars isn't just unlikely to succeed—it might be a complete waste of precious observation time.
Torlakcik's solution was elegantly simple: Instead of trying to find the best candidates, systematically identify the worst ones and remove them from consideration. Using data from the Gaia space telescope's archive of nearly 1.75 million stars, he applied seven scientifically grounded filtering criteria. Stars more than one and a half times the mass of our sun burn through their fuel too fast—and since complex life on Earth took roughly 3 billion years just to evolve past single-celled organisms, massive stars simply don't live long enough. By the same logic, stars younger than 3 billion years are eliminated. The model also excludes stars with insufficient iron and heavy metals, which are essential for forming rocky planets. Binary star systems, where gravitational dynamics can destabilize planetary orbits entirely, are filtered out. So are red dwarfs prone to dangerous stellar flares and chromospheric activity that would bombard any nearby planets with harmful radiation.
The results are striking: This filtering process excludes roughly 55 percent of stars from the search, leaving 777,835 high-priority candidates for harboring complex life. Age and metallicity do the heaviest lifting, each responsible for eliminating around 29 percent of the total. The retained population is dominated by older, chemically rich K dwarfs and quiet M dwarfs—stars far more likely to host stable, habitable worlds.
One methodological choice reveals Torlakcik's sophistication with the data's limitations. Rather than applying a hard cutoff at 3 billion years, which would discard any star with an uncertain age estimate that might possibly be younger, he applied the threshold to the upper age bound instead. This seemingly modest decision has an enormous practical consequence: It preserves 355,086 stars that a blunter approach would have eliminated—roughly one-third of the final candidate list.
The catalog was cross-matched against the Breakthrough Listen program, currently the most prominent radio SETI survey operating. Strikingly, more than half of Breakthrough Listen's existing targets would be flagged for exclusion under these habitability criteria, primarily due to low metallicity. This isn't a criticism of Breakthrough Listen—it reflects two genuinely different approaches. One optimizes for detectability; the other prioritizes plausibility. Together, they create a shortlist of stars that are both close enough to hear and promising enough to deserve our attention.
Perhaps most impressively, the catalog and its filtering tools are freely available online for any telescope program worldwide to apply to their own stellar data. Not bad for a high school project.