We are at the dawn of a new era in planetary science. Already thousands of planets around other stars have been confirmed, and we expect to be taking atmospheric measurements that will give us evidence of the first potentially habitable exoplanets before 2030.
But will we know how to read the signs?
The journal Astrobiology recently published a special collection of papers on exoplanet “biosignatures,” put together by Nancy Kiang from the NASA Goddard Space Flight Center and colleagues. The papers resulted from a 2016 workshop titled “Exoplanet Biosignatures Workshop Without Walls.”
It’s clear from the papers that scientists are making progress in identifying biosignatures—specific markers that are the result of life processes on a specific planet or moon. But it’s also clear that major challenges remain. For example, Victoria Meadows from the University of Washington and colleagues discuss the problems with using molecular oxygen as a potential biosignature. Oxygen is not an unambiguous indicator of biology, because there are several planetary mechanisms that can generate abundant molecular oxygen without life being present. Even on Earth, atmospheric oxygen was only abundant for a relatively short time early in our planet’s history, whereas life has been present for much longer.
Thus, environmental context is critical, a notion emphasized in a paper by David Catling (also from the University of Washington) and colleagues. Along with knowing an exoplanet’s mass and radius, we need certain climate parameters in order to have enough context to avoid false positives when evaluating evidence of life. Sara Walker from Arizona State University and colleagues go even further, arguing that we also need to better understand the mechanisms driving life’s emergence and the factors that influence evolutionary history to place constraints on the likelihood of life existing on a specific exoplanet.
Based on what we know about our own planet, life seems to persist once it arises, assuming a planet stays habitable. Extremophilic organisms that can adapt to a variety of harsh environments, ranging from dry and cold deserts to acidic mines to hydrothermal vents, support this idea. But the environmental conditions that allow life to originate in the first place are likely to be much more constrained. And to assess those conditions for an exoplanet, we would have to know the planet’s history. Did that particular world once have conditions favorable to the origin of life? Unfortunately, we don’t even fully understand the history of life on Earth!
In other words, finding reliable biosignatures for exoplanets presents us with challenges on top of challenges. But these are the kinds of questions we’ll need to answer to find out whether we live in a lonely universe or a cosmic zoo.