Ever wonder how astronomers search for extraterrestrial life on other planets? What are they seeking, exactly?
Scientists research for planets that might host extraterrestrial life before diving right into the hunt for extraterrestrials. This is not a simple task, though.

There are an astounding 100 billion planets predicted to exist in the Milky Way alone, the galaxy to which our eight planet solar system belongs. These planets are called exoplanets since they are outside of our solar system.

An artist’s impression of a habitable exoplanet. Image credit: NASA Ames/SETI Institute/JPL–Caltech

How scientists start looking with so many exoplanets?

They look for “biosignature” gases in the anticipation that they would reveal the existence of life.

Exoplanets that orbit sun-like stars in a manner comparable to how Earth orbits the sun are the first to be found by astronomers. Even though we have only seen a few exoplanets in the Milky Way, scientists believe that up to 50% of stars may be home to at least one exoplanet in orbit, leaving billions of potential exoplanets.

Then they look for exoplanets that are orbiting their suns inside a habitable zone. A habitable zone is therefore just right: not too close, but also not too far. Because of the Earth’s evolution, we know that moderate temperatures are necessary for maintaining liquid water, which is essential for life.

Astronomers use space based telescopes (like Hubble, JWST) to identify the gases that make up the atmospheres of distant planets and determine whether or not they are conducive to life. We know that life can live on Earth in an atmosphere mostly composed of nitrogen and oxygen, but that too much carbon dioxide can be hazardous. With the help of the spectroscopy method, the atmospheres of exoplanets can be studied.

Different gases absorb different wavelengths of light, which can be used to deduce gases present in a distant atmosphere. This figure shows a simplified schematic of the absorption spectrum of visible light, where the black bars represent wavelengths of light that are absorbed by the different gases.

While it could be tempting to exclusively search for exoplanets like Earth that have atmospheres rich in nitrogen and oxygen, researchers are also interested in finding planets with hydrogen-dominated atmospheres.

Given that the atmosphere of the Earth contains so little hydrogen, this may appear strange. However, because hydrogen gas is so light, it creates a fluffy environment that can be more easily searched for other signs of life.

Ammonia, nitrous oxide, and oxygen are examples of biosignature gases, which can only be produced from extinct or living species. They serve as indicators that there is or has been life on that planet. The fact that denser, heavier gases would make it more difficult to detect these biosignature gases suggests that they may only make up a small part of all gases.

 Atmospheric density in the search for life. Planet A (left) has a dense atmosphere (shown in blue) made up of heavy gases, which obscures the biosignature gases (blue circles). In contrast, Planet B’s (right) atmosphere has a less dense atmosphere made up of lighter gases like hydrogen. Light atmospheres can be probed more easily for biosignature gases produced by microbes.
Atmospheric density in the search for life. Planet A (left) has a dense atmosphere (shown in blue) made up of heavy gases, which obscures the biosignature gases (blue circles). In contrast, Planet B’s (right) atmosphere has a less dense atmosphere made up of lighter gases like hydrogen. Light atmospheres can be probed more easily for biosignature gases produced by microbes.

One may think, that what’s the point of probing hydrogen-dominated atmospheres? If we humans cannot survive in hydrogen-dominated atmospheres, can any life live under these harsh atmospheres? We have previously found some microbes surviving in some of the most extreme micro-environments on Earth.

Overall, this information can be used to help narrow the search for extraterrestrial life by focusing on planets with atmospheric gases shown to support microbial life. Perhaps one day in the future, we might not witness the discovery of E.T., but instead, a new species of micro-organism on another planet. That’s still pretty exciting though, isn’t it?


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