When We Finally Find Aliens, They Might Smell Terrible

Clara Sousa-Silva | Scientific American
Tuesday, November 19, 2019

A better understanding of what gases to search for in exoplanet atmospheres is key to locating extraterrestrial life

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EAPS Research Scientist and astrobiologist Clara Sousa-Silva hunts for signs of life on other planets. She looks for biosignatures of unusual gases in the atmospheres around planets. One of these is phosphine, Sousa-Silva writes for Scientific American.

There are thousands of different gases that could be evidence of biospheres all over the galaxy. Some, like oxygen, would likely be present in large quantities but can exist without life. Some gases can only be made by living things but require so much energy to form that they are likely to exist in very small amounts. My favorite of these rare gases is phosphine. Phosphine is a fascinating and horrid molecule. It kills in a variety of imaginative ways, all of which have to do with its interference with oxygen metabolism. To us modern, oxygen-loving beings, phosphine is an extremely toxic and outrageously foul-smelling molecule.

This is a molecule that would be around in anoxic environments, like early Earth, but exists in localized places now. "A few of these anoxic environments still exist: swamps, marshlands and the intestines of people, penguins, fish and many other animals. The anaerobic life in these places produces lots of phosphine. They stink of it; they stink of life. Other planets, populated by the kind of life that doesn't rely on oxygen to exist, could also have phosphine as a biosignature."

Sousa-Silva explains that she uses the spectra of these gases to look for life, but first she has to collect the missing spectra, which is difficult in and of itself. Additionally, she notes, science needs some perspective: "We invest so much time and money improving our telescopes and choosing the best planetary targets, but we forget that we may not be ready to interpret the incredible data we will get from these alien atmospheres. We are still lacking fundamental knowledge, not least of which is our understanding of biosignature gases and their spectra."

This work is based on articles from the author (e.g., Sousa-Silva et al, Astrobiology, 2020; Sousa-Silva et al, PCCP, 2019; Sousa-Silva et al, MNRAS, 2014), and collaborations with Janusz Petkowski, William Bains, Sara Seager, Sukrit Ranjan, Sergey Yurchenko, and Jonathan Tennyson.

Story Image: The James Webb Telescope under construction. (Credit: Chris Gunn NASA, Flickr CC BY 2.0)