Hundreds of potentially habitable planets have been found outside the Solar System, orbiting nearby stars. It could be possible to narrow down this search for extraterrestrial life by looking at a planet's atmosphere and studying seasonal changes.

It is highly unlikely that humans are going to visit exoplanets anytime soon, so the search for life should begin by looking at their atmospheres, says a new study, reports Phys.Org. Called biosignatures, these are likened to fingerprints that biological life is likely to leave in the atmosphere. Researchers believe that they can be measured using advanced, next-gen telescopes looking for the composition of gases in planets several light years away.

However, the report also points out that biosignatures based on single measurements can be inaccurate and even misleading. So to make this an effective method to look for alien life, researchers are developing the first ever quantitative framework to detect dynamic biosignatures that are based on Earth's atmospheric seasons and how it changes every year, notes the report. It is being done with funding from the NASA Astrobiology Institute and by scientists at the University of California, Riverside's Alternative Earths Astrobiology Center.

Alien life
If there are seasonal changes in exoplanets, that could mean there is life out thereCreative Commons

"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," said lead author is Stephanie Olson. "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."

Weather on Earth and beyond 

Earth, on its tilted axis, receives different levels of sunlight in different parts of the year, this leads to visible changes in the weather and the length of days in the surface. However, the changes in weather also has an effect on atmospheric composition, notes the report. The northern hemisphere, for example, has a majority of the planet's vegetation, so in the summer, the northern hemisphere has a measurably lower level of carbon dioxide and higher level of oxygen.

"Atmospheric seasonality is a promising biosignature because it is biologically modulated on Earth and is likely to occur on other inhabited worlds," said Olson. "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."

Apart from axis tilt like on Earth, extremely elliptical orbits could also cause seasons to change on exoplanets, notes the report. Using an imaging technique called spectroscopy, it could be possible to model fluctuations of gases like oxygen in these alien planets.

"It's really important that we accurately model these kinds of scenarios now, so the space and ground-based telescopes of the future can be designed to identify the most promising biosignatures," said Edward Schwieterman, a NASA Postdoctoral Program fellow at UCR. "In the case of ozone, we would need telescopes to include ultraviolet capabilities to easily detect it."

"Both oxygen and methane are promising biosignatures, but there are ways they can be produced without life," he added.

The paper, titled "Atmospheric Seasonality As An Exoplanet Biosignature," was first published in the  Astrophysical Journal Letters.