The key to detecting life trillions of miles away from Earth

In August, scientists announced they had discovered a whole lot of water on Mars. Using the same seismographic techniques used to probe Earth’s interior, researchers found evidence of enough sub-surface water to cover the Red Planet in a one-mile-deep ocean. But before anyone starts celebrating our wet future on Mars, there was bad news, too: All that water is hidden 12 to 40 miles below the planet’s surface. Reflecting on this good (and bad) news made me think about the search for life far beyond our Solar System and how it also must contend with hidden possibilities. 

When it comes to finding life on an exoplanet many light-years away, it’s becoming clear that we astrobiologists need to think deeply about life’s planet context.

Biosignatures

As I’ve covered before, “biosignatures” are the focus for finding life with telescopes. A biosignature is the imprint of life in the light we receive from an exoplanet. What this really means is we won’t get pictures of individual examples of alien plants or animals. Instead, evidence for life as a collective is what we’ll detect. Biosignatures will be evidence of biospheres. They will be evidence of how the sum of life on an alien world has changed that world. Those changes must also be so powerful they can be detected over trillions of miles of interstellar space. 

Detecting oxygen in an exoplanet’s atmosphere is an often-cited example of a good biosignature. All of Earth’s atmospheric oxygen is the direct result of photosynthetic critters going about their metabolic business. Without photosynthetic life as a whole, we’d have no atmospheric oxygen. That link is what puts oxygen high on the list of potential biosignatures.

False positives

There is, however, a problem with this line of reasoning. There is always the possibility that some “abiotic” process — something that has nothing to do with life — might also produce the signature we’re looking for. Astrobiologists call this a “false positive.” For a while, astronomers were sure that high levels of O₂ in an atmosphere must come from life. Then they found a few ways the gas could build up from purely geophysical mechanisms (for example, light from the Sun splitting H₂O molecules apart). The possibility for false positives means astronomers must dig much deeper into potential biosignature detections before they can claim success in detecting life. That’s where planetary context comes in.

Planets have their own evolutionary processes that occur alongside life. As astrobiology matures, it’s beginning to take these processes seriously. The question we’re asking is straightforward: How does the structure of a planet determine what’s possible for life’s evolution on that world? In other words, how does the planet create a context for understanding whether and how life might form and evolve on it?

Plate tectonics

For a concrete example, let’s consider plate tectonics. The surface conditions of a world are deeply connected to what’s happening below that surface. On Earth, the approximately 20-mile-deep uppermost layer called the crust is broken up into distinct plates. The circulation of the mantle, the layer below the crust, carries those plates around. As the plates move, they collide in some places or slip underneath each other elsewhere on the planet. 

Why does this matter? Many scientists believe that the vigorous resurfacing that comes from plate tectonics may have been crucial to the evolution of complex life on Earth. Even more important, the volcanism that came from those moving plates may have been what kept the planet habitable for life for billions of years. The link to astrobiology comes when we recognize that Earth is the only planet in our solar system with active plate tectonics. That means we might expect many worlds in the galaxy to be without moving crustal plates. Those worlds would then not be good candidates for developing robust biospheres and biosignatures. So, knowing whether a planet had plate tectonics becomes important contextual information in assessing whether a discovered biosignature was real or a false positive. There are also other potential planetary contexts we can look for, like the existence of magnetic fields. 

But getting indicators of planetary context won’t come easy. We are unlikely to see direct evidence for plate tectonics across 100 light-years. But, as we get better at modeling the interiors of planets, we may be able to constrain those worlds with a high probability of developing things like plate tectonics or magnetic fields. Most importantly, we might be able to link those models to measurements we can make, such as a planet’s density or the variety of compounds in its atmosphere. 

The progress in these domains at the boundary of planetary science and astrobiology is beginning to accelerate. The work I am part of at the Center for Matter at Atomic Pressures is pursuing exactly these paths by using giant lasers to understand how material behaves under the pressures one finds deep in planets. That will help with the modeling we and others are doing. Hopefully, by the time astronomers start getting the high-quality data needed for identifying biosignatures, we’ll also be better at identifying features that give us planetary context. 

We won’t be able to make the kind of measurements with telescopes that got NASA that peek at Martian subsurface water. But even across trillions of miles, we may be able to learn enough about what lies below a planet’s surface to help us understand whether there’s life lying on it.