How to Test the Cosmic Zoo Hypothesis

Future space missions should be able to detect a complex biosphere on an exoplanet.

Is there complex life on this exoplanet?

In a new publication in the journal Nature Astronomy, William Bains of Cambridge University and I propose that it’s time to consider search strategies for complex life—and not just simple life—on exoplanets. While the search to date has focused mostly on finding extraterrestrial microbes, we make the argument that space missions are on the horizon that could, in principle, detect the signatures of more advanced life, such as animal and plant life, as predicted by our Cosmic Zoo hypothesis. Our point is that we should prepare for it.

That may seem like a tall order when we have yet to detect microbial life elsewhere in our own Solar System, or even agree on how to detect it. But consider how an alien observer might detect complex life on Earth. One clue would be the effects of rain forests on the terrestrial atmosphere. Rain forests affect global cloud distribution and regional precipitation patterns, and have a net cooling effect over the land they cover. They also release large amounts of volatile organic compounds, which can be detected remotely. More generally, extensive land (as opposed to ocean) life on a planet could point to the existence of complex organisms, as it is hard to see how this could be accomplished by microbes only. Life on land is hard. Species often have to deal with a lack of water and high levels of ultraviolet radiation. Only multicellular, macroscopic life eventually evolved adaptation strategies to settle Earth’s dry land for good.

No current telescope has enough sensitivity to detect these signs of complex life on a distant exoplanet. But future space-based telescopes such as the Wide Field Infrared Survey Telescope, or ground-based telescopes such as the Giant Magellan Telescope, the Thirty Meter Telescope, and the European Extremely Large Telescope—all planned for the early 2020s—could take the first step of distinguishing between land and ocean areas on exoplanets. Farther in the future, exoplanet imaging missions such as the Large UV/Optical/IR Surveyor, or LUVOIR and the Habitable Exoplanet Imaging Mission (HabEx) should be able to measure regional cloud systems on the closest exoplanets, as well as cooling effects associated with rain forests and the volatile organic compounds they produce.

Proving the presence of life would still be difficult, because sophisticated global climate models would be needed to compare these observations with those on a lifeless planet. But in principle, this is not beyond our reach. We’ll need researchers with expertise in ecology, evolutionary biology, geochemistry and other sciences to be involved in the missions right from the start, and design them to hone in on the search for complex life.

We’ll also need to pay attention to the “middle layer” of biological complexity, between microbial life on one end and technologically advanced life on the other. That may eventually help us answer a fundamental question in astrobiology: whether we live on a rare planet in a lonely Universe, or in a Cosmic Zoo.

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