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If you are going to look for intelligent life beyond Earth, there are few better candidates than the TRAPPIST-1 star system. It isn't a perfect choice. Red dwarf stars like TRAPPIST-1 are notorious for emitting flares and hard X-rays in their youth, but the system is just 40 light-years away and has seven Earth-sized worlds.

Three of them are in the potentially habitable zone of the star. They are clustered closely enough to experience tidal forces and thus be geologically active. If intelligent life arises easily in the cosmos, then there's a good chance it exists in the TRAPPIST-1 system.

But finding evidence of intelligent life on a distant planet is difficult. Unless Mr. Mxyzptlk or the Great Gazoo want to talk about your car's extended car warranty, any signal we detect will likely be subtle, similar to the stray radio signals we emit from Earth.

So the challenge is to distinguish actual signals from aliens, known as technosignatures, from the naturally occurring emissions of stars and planets. Recently, a team used the Allen Telescope Array to capture 28 hours of TRAPPIST-1 signals in an effort to find the elusive aliens.

The study began with a few assumptions. The biggest one was to presume that if TRAPPIST-1 has an intelligent civilization it is likely to spread across more than one world. Given how compact the system is, that isn't too outlandish. Getting from one world to another wouldn't be much more difficult than it is for us to get to the moon.

With that assumption, the team then assumed that the worlds would transmit radio messages with each other. Since the signals would need to transverse interplanetary distances, they would be the strongest and most clear technosignatures in the system.

So the team focused on signals during a planet-planet occultation (PPO). That is when two planets line up from our vantage point. During a PPO, any signal sent from the far planet to the closer planet would spill over and eventually reach us.

The findings are on the arXiv preprint server.

With 28 hours of observation data in hand, the team filtered out more than 11,000 candidate signals鈥攕ignals that were stronger than the expected range for natural signals. Then, using computer models of the system, they determined 7 possible PPO events and further narrowed things down to about 2,200 potential signals occurring during a PPO window. From there they went on to determine whether any of those signals were statistically unusual enough to suggest an intelligent origin. The answer to that was sadly no.

Alas, if there are aliens in the TRAPPIST-1 system, we haven't found them yet. But the result shouldn't minimize this study. It is the longest continuous survey of the system to date, which is pretty cool. And it's kind of amazing that we've reached the point where we're able to do this study. We are actively searching for known exoplanets in detail.