The current prevailing idea is that extraterrestrial life would manifest itself as a unique and obvious signal, whether natural or technological.
However, a recent study invites us to completely change perspective: instead of tracking isolated clues, we should rather look for global patterns that would appear 'abnormal' across several nearby planets.
The problem with classic biosignatures, such as certain atmospheric gases, is that they can also be produced by non-biological processes. These false positives complicate the search. Similarly, technosignatures rely on risky assumptions about extraterrestrial intelligence.
To circumvent these difficulties, researchers led by Harrison B. Smith and Lana Sinapayen have imagined an "agnostic biosignature." This one does not depend on any particular chemistry but relies on two simple ideas: life can spread between planets, for example via panspermia, and it can modify their environment. By modeling these processes, the team showed that life leaves measurable statistical traces, even if no planet shows an obvious sign.
In their simulation, agents representing life migrate between stellar systems and transform planets. The results indicate that, if life is active, planets close to each other become more similar than what chance would produce. This link between position and appearance constitutes a potential life signal, detectable on a large scale.
By grouping worlds according to their observable properties and their position, researchers can spot clusters where life has likely operated. This approach prioritizes reliability: it reduces false positives, even at the cost of missing some inhabited planets. A valuable advantage when observation time is limited.
The model assumes that life can travel between stars and terraform planets, making them more similar to their world of origin. Ultimately, groups of neighboring planets become abnormally similar.
Credit: Harrison B. Smith
According to Harrison B. Smith, looking for life through its large-scale effects avoids the need for a perfect definition. Lana Sinapayen adds that even a fundamentally different life from ours would leave detectable traces by spreading and modifying worlds. The future of this approach involves better understanding the natural diversity of lifeless planets, to distinguish what is due to biology.
Although the results still rely on simulations, they pave the way for a new class of detection methods. Researchers hope to eventually integrate detailed planetary data and more realistic galactic models. But the idea is launched: life could be spotted not by its exact chemistry, but by the patterns it weaves across the Universe.
Panspermia as a means of dispersing life
Panspermia is a hypothesis according to which life could travel from one planet to another, carried by meteorites, comets, or even dust grains. This idea does not only concern our Solar System: microorganisms could survive an interstellar journey if protected from radiation and vacuum. In the researchers' model, panspermia allows life to colonize new planets, starting from a world of origin. This mechanism creates links between distant worlds.
If life spreads in this way, planets close to each other will have a higher chance of being colonized by the same strain. Over time, they will become more similar to each other, as life modifies their environment. It is this abnormal similarity that can be detected as a life signal, even if the life forms are very different from those on Earth.
Panspermia remains a hypothesis, but it is supported by experiments showing the resistance of certain microorganisms in space. If it is proven true, it would offer a means for life to spread, and thus leave a large-scale signature.