In recent years, scientists have made great strides in the search for extraterrestrial life. They have discovered thousands of exoplanets. And some of them are located in the habitable zone of their star, which means that liquid water could exist on their surface and is essential for life as we know it.
The search for life however, is a little more involved than finding planets, measuring their location and distance around the star and figuring out their location within a habitable zone. Astronomers need more information to learn if life may be there. The first thing we need to know is whether or not the planet has an atmosphere and thanks to the Webb Space Telescope, we are starting to get a handle on that data point with every observation it makes using its onboard Near Infrared Spectrometer.
Once astronomers detect an atmosphere, the real fun begins: finding biosignatures. Biosignatures are indicators of the presence of life, traces left behind by living organisms. These indicators can be chemical, biological or physical. An example of a physical biosignature would be things like fossils, and since that requires that we have a physical presence on the planet (we can’t see those from here, not even with JWST), then we are left with chemical and biological indicators.
The best examples of biosignatures that we can detect from so far away are atmospheric gasses and organic molecules.
Certain gases, such as oxygen and methane, can be produced by biological processes. However, they can also be produced by non-biological means. For example, oxygen can be produced by photosynthesis, but it can also be produced by geological activity. The presence of atmospheric gases in a planet’s atmosphere could be a sign of life, but it is important to consider other evidence as well, like organic compounds.
Organic molecules are the building blocks of life. They include amino acids, lipids, carbohydrates, and nucleic acids. Organic molecules can be detected in the atmospheres of exoplanets using a technique called spectroscopy. Spectroscopy involves splitting light into its component wavelengths. Organic molecules absorb light at specific wavelengths, so by looking for dips in the spectrum of an exoplanet, scientists can identify the presence of organic molecules.
JWST is currently the only instrument we have that can detect biosignatures in this way, it looks at absorbed starlight that has passed through the exoplanet’s atmosphere to see what elements were absorbed. Just recently, it reported the possibility of the organic molecule dimethyl sulfide in the planet K2-18b, a compound that on Earth is produced primarily by phytoplankton, which are microscopic organisms that live in the ocean. DMS can also be produced by some other biological processes, but it is not known to be produced by anything other than life.
DMS is a gas, so it can be detected in the atmospheres of exoplanets using spectroscopy. It is also relatively stable in the atmosphere, so it can accumulate to detectable levels. All of this makes dimethyl sulfide an ideal biosignature gas. Other biosignatures that are similar to dimethyl sulfide include: Methanethiol, Dimethyl disulfide, Carbon disulfide and Hydrogen cyanide.
It is important to note that the detection of a single biosignature is not definitive evidence of life. However, the presence of multiple biosignatures in the same place would be very strong evidence. Additionally, if a biosignature is detected in a place where it is unlikely to be produced by non-biological processes, such as in a Martian meteorite, then it is more likely to be a sign of life.
The James Webb Space Telescope and the future Habitable Worlds Observatory are two powerful tools that we have at our disposal to search for life on other planets. Imagine the day when we discover signs of life beyond Earth. It would be a watershed moment in human history, one that would change our understanding of our place in the universe forever.