by Kerry McGowen

Have you ever wondered how scientists hunt for alien life on other planets? What are they looking for?

Before jumping straight into the search for aliens, scientists look for planets that could support alien life in the first place. However, this is no easy task. In the Milky Way alone, which is the galaxy to which our eight-planet solar system belongs, there are a staggering 100 billion planets estimated to exist—these planets are called exoplanets because they are outside of our solar system. With so many exoplanets, how can scientists begin to look?

Instead of looking for green aliens with big eyes, some scientists are scanning the sky. In fact, a research group at Massachusetts Institute of Technology (MIT), is searching for ‘biosignature’ gases with the hope that these may indicate the presence of life. Going one step further, this group has also been studying microbial growth in various atmospheric gases, reporting that Earth’s atmosphere might not be the only composition capable of supporting life.

The qualifications for a planet to be a candidate for alien life

To start, scientists look for exoplanets orbiting a sun-like star, similar to how Earth orbits our sun. In the 1990’s, NASA launched the Kepler Space Telescope, which allowed scientists for the first time to find exoplanets orbiting stars. Even though we only have small glimpses of exoplanets in the Milky Way, scientists estimate that up to 50% of stars might have an orbiting exoplanet, which still leaves billions of candidate exoplanets.

After identifying exoplanets that orbit a sun, scientists must then narrow potential candidates for exoplanets that are also orbiting their suns within a habitable zone. If a planet orbits too close to a sun, the proximity causes extreme heat on the planet’s surface. Conversely, orbiting too far from a sun causes extreme cold temperatures. Thus, a habitable zone is just right: not too close, but not too far either. This allows for moderate temperatures that can sustain liquid water, which as we know from evolution on Earth, is integral for life.

To find out more about an exoplanet candidate, scientists use telescopes to discover what gases make up the distant planets’ atmospheres and whether they are compatible with life. For example, we know that, on Earth, life can exist in an atmosphere primarily made up of nitrogen and oxygen but too much carbon dioxide is dangerous. The atmospheres of exoplanets can be probed using a technique called spectroscopy to detect visible light. Visible light is the light we can see by eye, where the wavelengths of visible light correspond to the different colors of the rainbow (Fig. 1). Wavelengths of light can be absorbed by different substances, and any wavelengths not absorbed are reflected and seen as a visible color. For example, when we look at a green apple we see green because most of the other colors of visible light are absorbed, but green is reflected.

We can use this same principle to detect gas molecules, like oxygen, nitrogen, or hydrogen. Different gases absorb different wavelengths of visible light, so spectroscopy uses this phenomenon to deduce which gases are present by detecting which colors of light are absorbed and which are reflected (Fig. 1).  

Figure 1: Spectroscopy for exoplanet investigation. 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.

How can studying exoplanets’ atmospheres find potentially livable exoplanets?

At first thought, you may think to look only for exoplanets with nitrogen- and oxygen-rich atmospheres like Earth, but scientists are also interested in finding hydrogen-dominated atmospheres. This may seem odd since there are only trace amounts of hydrogen in Earth’s atmosphere, but scientists have good reasons. Hydrogen gas is really light, so it forms a fluffy atmosphere that can be more easily probed for other evidence of life. Think of it this way: hydrogen is light like cotton balls, but heavier gases like nitrogen are dense like marbles. It is much easier to move through and find a tiny object in a room full of cotton balls than marbles.

What is this other evidence of life that scientists are hoping to find in exoplanets’ atmospheres? Well, the answer is actually more gases called ‘biosignature’ gases. Biosignature gases, like ammonia, nitrous oxide, and oxygen are gases that can only be made from past or present organisms. In other words, they are the clues that a living being is living or has lived on that planet. These biosignature gases might make up only a tiny portion of total gases, which would explain why denser, heavier gases would make it harder to detect them.

Figure 2: 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.

While powerful, our current telescopes only allow for a glimpse of exoplanet atmospheres. Given these capabilities, some scientists are interested in identifying planets with hydrogen-dominated atmospheres, a feature that can be gleaned with the current technology. Then, in the future, scientists will use more sophisticated telescopes, like the James Webb Space Telescope set to launch in 2021, to further probe the atmosphere to look for biosignature gases.

However, 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? It turns out that certain microbes can.

What do we know about microbes living in atmospheres different from that of Earth?

A recent study by Seager and colleagues discovered several microbes can grow in 100% pure hydrogen environments. In this study, scientists grew single-celled microbes, E. coli and yeast, in glass bottles with a nutrient broth food source. They changed the available gases in the bottles to reflect possible atmospheric conditions on other planets. In particular, they grew the microbes under pure hydrogen or helium gas, nitrogen-carbon dioxide gas mix, or normal atmospheric gases found on earth.

Interestingly, they found that all conditions supported the life of both microbes tested. These findings show that microbes that do not normally live in these atmospheric conditions can adapt to survive and grow, which supports the notion that other lifeforms could also exist in similar conditions.

However, these results were not all that surprising because we have previously found some microbes surviving in some of the most extreme microenvironments on Earth. It is also important to note that these experiments provided the microbes with a rich food source, so having a hydrogen atmosphere alone likely is not enough to support life. A planet would also potentially need a water source for the exchange of food in the form of chemicals and nutrients. Regardless, this study broadens the known range of conditions that allow for life, which scientists can use to look for indications of microbial alien life.

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?


Kerry McGowen is a 3rd year graduate student in the Biological Sciences in Public Health program at Harvard.

Figures created with BioRender.com

Cover image: “File:Mars zichtbaar vanaf de alpe du hues.jpg” by Daampjeee is licensed under CC BY-SA 4.0

For more information:

  • Read more about Exoplanets here.
  • Check out more information on the James Webb Space Telescope here.
  • Read about the hydrogen gas study here and here.

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