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Given our decades-long lack of success in finding hard evidence for an extraterrestrial civilization, it hardly comes as a surprise that a recent campaign studying the seven-planet TRAPPIST-1 system came up without a detection. 28 hours of scanning with the Allen Telescope Array by scientists at the SETI Institute and Penn State University produced about 11,000 candidate signals for further analysis, subsequently narrowed down to 2,264 of higher interest. None proved to be evidence for non-human intelligence, but the campaign is interesting in its own right. Let’s dig into it.
The unique configuration of the TRAPPIST-1 planets allowed the scientists involved to use planet-planet occultations (PPOs). A cool M-dwarf star, TRAPPIST-1 brings with it the features that make such stars optimal for detecting exoplanets. The relative mass and size of the planets and star mean that if we’re looking for rocky terrestrial-class worlds, we’re more likely to find and characterize them than around other kinds of star. True, they’re also orbiting a class of star that is dim, but another beauty of TRAPPIST-1 is that it’s only 40 light years out, and we see its seven planets virtually edge-on.
Planets e, f and g can be squeezed into the star’s habitable zone (liquid water on the surface) if we tweak our numbers for possible atmospheres. The edge-on vantage means that planets can pass in front of each other from our viewpoint, with the additional advantage that this well-studied system has planetary orbits that are sharply defined. This raises intriguing possibilities when you consider our own space activities. The Deep Space Network sends powerful signals to communicate with distant craft like the Voyagers, signals wide enough to propagate beyond them and into deep space. The right kind of receiver, if by chance aligned with them, might make a detection, producing evidence for a technology by the nature of its signal.
At TRAPPIST-1, there are seven planet-planet occultations, with two of them involving a potential transmission-source planet within the star’s habitable zone. But we have to consider that transmissions between planets might not be this limited, for radio traffic could move through relays placed for communications purposes on worlds that are uninhabitable. This would obviously be traffic never intended for interstellar reception, the kind of ongoing activity that marks a society communicating with itself, but perhaps leaving a technosignature that would reach the Earth through the width of its beam.
Image: A look from above at the communications line of sight between two worlds in the TRAPPIST-1 system, illustrating the PPO method used in this study. Credit: SETI Institute/Zayna Sheikh.
The possibilities of a detection using this PPO technique vary, of course, with the orbital parameters of the planets in any given system. We must also account for the drift rates produced by orbital motion. The paper explains the recent search’s technique this way:
…it is assumed that the TRAPPIST-1 planets are tidally locked due to their proximity to their host star and will have a negligible rotational contribution to the drift rate of a transmitter on their surfaces. Additionally, their orbital parameters are well constrained, making it possible to calculate the drift rate contributions from their orbital motion. Satellite transmitters in circular orbit around each planet could produce much higher drift rates, up to an additional ∼45 nHz on top of the contribution from the planet’s orbit around the star. However, we have chosen to limit our scope to analogues of our deep space communications, the strongest of which are surface transmitters to deep space probes.
The seven planet-planet occultations studied during the 28 hours of observation ranged from 8.6 minutes to 99.4 minutes. And it turns out that widening that window of observation through simulations produces numerous PPOs with a similarly large range of duration, making this strategy still more interesting. The animation below shows the TRAPPIST-1 system in motion and the possible communications opportunities. Credit: Tusay et al., citation below.
Animation: This is Figure 10 from the paper, the caption of which reads: Simulated potential PPO events during our observations. Online viewers will see a concatenated video of the orbital configuration of the system during each of the observations, including any potential PPO events that we found to occur during those windows. A still image of a PPO event during the observation on Oct 29, 2022 is included where the animation is not accessible. The top panel shows a bird’s-eye view of the system with planet radii scaled up for better viewing. The distances and beam sizes are to scale, assuming a beam created with a 3.4m dish at 3.3 GHz (the maximum frequency observed during this particular session) from the surface of planet g aimed at planet e. The bottom panel shows the edge-on view with planet sizes scaled with distance, showing how much of the beam spills over the planet toward the direction of Earth in the negative z-direction. The red dashed lines in the illustrated beam is the inner angle blocked by the occulting planet, e. The blue dashed lines show the outer angle of the beam that would spill over the planet. The window for this event lasted roughly 95 minutes. Credit: Tusay et al.
Is this method the longest of longshots? SETI itself might be described that way, depending on your views of life in the cosmos. But our steadily growing capabilities at signal detection can’t be ruled out when we consider the possibilities. From the paper:
The analysis of the observations presented here demonstrates that precise characterization of ideal systems, like TRAPPIST-1, enabling orbital dynamical modeling and prediction of PPO events offer practical application for leaked emission searches. This provides SETI a powerful new observational tool and search strategy. As signal detection and RFI mitigation pipelines improve, the inclusion of PPOs to provide narrow search windows may make it more feasible to increase time resolution and sensitivity at higher drift rates.
What beckons most strongly about technosignatures is that they assume no intent (which in any case would be unguessable) on the part of a hypothetical alien civilization. We would essentially be eavesdropping on their activities. Grad student Nick Tusay (Pennsylvania State), lead author of the paper on this work, adds this: “[W]ith better equipment, like the upcoming Square Kilometer Array (SKA), we might soon be able to detect signals from an alien civilization communicating with its spacecraft.” And that would be a SETI detection for the ages.
The paper is Tusay et al., “A Radio Technosignature Search of TRAPPIST-1 with the Allen Telescope Array,” currently available as a preprint.
