We have discovered almost 4,000 exoplanets, but the question remains: How many of those worlds possibly harbor life? As we slowly begin to study the atmospheres of the most promising worlds, scientists are trying to work out what might be the most glaring hallmarks of life that we can spot from so far away. And now the presence of seasons might help us out.
According to a study published in The Astrophysical Journal Letters, atmospheric composition varies with the seasons as long as there is life. For example, the Northern Hemisphere has more landmass and more vegetation, so during the summer there’s less carbon dioxide and more oxygen in the atmosphere. Something similar, the paper suggests, might be witnessed on distant exoplanets.
"Atmospheric seasonality is a promising biosignature because it is biologically modulated on Earth and is likely to occur on other inhabited worlds," lead author Stephanie Olson, a graduate student at the University of California Riverside, said in a statement. "Inferring life based on seasonality wouldn't require a detailed understanding of alien biochemistry because it arises as a biological response to seasonal changes in the environment, rather than as a consequence of a specific biological activity that might be unique to the Earth."
The team modeled the changes of several molecules that are important for life, such as oxygen, carbon monoxide, and methane. These are formed both by biological and inorganic processes, so seasonal changes could be useful to work out their origin.
"A potentially powerful way to assess exoplanets for inhabitation would be to observe their atmospheres throughout their orbits to see if we can detect changes in these biosignature gases over the course of a year," she said. "In some circumstances, such changes would be difficult to explain without life and may even allow us to make progress towards characterizing, rather than simply recognizing, life on an exoplanet."
The models suggested that seasonal ozone variations could be easy to spot even if oxygen levels are low. This could be important in young planets where complex life might not have evolved yet.
"We are particularly excited about the prospect of characterizing oxygen fluctuations at the low levels we would expect to find on an early version of Earth," senior author Professor Timothy Lyons added. "Seasonal variations as revealed by ozone would be most readily detectable on a planet like Earth was billions of years ago, when most life was still microscopic and ocean dwelling."
With the new planet-hunter telescope TESS starting its science mission, and the James Webb Space Telescope following in a couple of years, it is important to work out what are the best ways to look for life outside the Solar System.