Biosignatures

Sara Seager, Drake Deming. Exoplanet Atmospheres. Annu. Rev. Astron. Astrophys. 2010. 48:631–72

One of the ultimate goals in the search for exoplanets is to find life outside of our solar system. This search is conducted by first hunting for planets, then determining whether or not they may be habitable based on their measured characteristics (such as orbital distance), and finally taking a transmission spectrum of the planet during a transit to model the planetary and atmospheric compostion. To determine if the planet's atmosphere contains any biosignatures, which are chemical species that may indicate the presence of life, telescopes first measure the spectra of light radiated and reflected from the target planet. These spectra are then analysed to identify specific peaks in the infrared and visible regions. In the infrared region for example, peaks can be seen which indicate molecules such as carbon dioxide, water, ozone, methane, ammonia and nitrous oxide.

Once the transmission spectra are analyzed to identify any peaks, the next step is to attempt to determine whether life may be present. One method is to look for peaks that indicate molecules or compounds that should not be present naturally, or at least cannot remain in equilibrium in an atmosphere for long periods of time. Methane, nitrous oxide and ammonia are produced on Earth primarily by bacteria. Oxygen (and ultimately ozone) is produced by photosynthetic life and composes approximately 20% of Earth's atmosphere. The peaks for these molecules in Earth's spectrum are shown in the image to the right. Since these molecules are a byproduct of life on earth, and are not produced abiotically in large quantities, they are considered to be biosignatures, or signs of life. Another reason why molecules such as oxygen and methane are considered to be reliable biosignatures is that even if they can be produced naturally in low quantities, they are unstable over long periods of time in the atmosphere; if these compounds are detected the atmosphere of an exoplanet, it is likely that there is a source continually producing them in high quantities, and the only such source currently known is life. In addition to the detection of molecules in the atmosphere, infrared measurements can also give clues about the planet’s surface temperature, and whether or not it is capable of supporting liquid water.

Sara Seager, Matthew Schrenk, and William Bains. An Astrophysical View of Earth-Based Metabolic Biosignature Gases. ASTROBIOLOGY Volume 12, Number 1, 2012

More on Earth's Biosignatures

On Earth, there are a large number of chemical biproducts produced by the many forms of life. But, only a small subset of these are classified as biosignatures. This is because biosignatures must first of all be gaseous, otherwise they could not be detected in transmission spectra. Of those gaseous byproducts, those that are known to be produced naturally through geology or photochemistry must also be eliminated. Of the remaining chemicals, those that are soluble in water or easily broken down are less likely to be found. The final set of all remaining biosignature candidates are oxygen (O2), ozone (O3), and nitrous oxide (N2O). Scientists also look for water (H2O) and methane, however they can also be produced naturally and so are not considered strong biosignatures. This hierarchy of biosignature classification is illustrated in the image to the right.

The table below also summarizes some of the pros and cons of different biosignature candidates on Earth.

Sara Seager, Matthew Schrenk, and William Bains. An Astrophysical View of Earth-Based Metabolic Biosignature Gases. ASTROBIOLOGY Volume 12, Number 1, 2012

Reference material:

Sara Seager, Matthew Schrenk, and William Bains. An Astrophysical View of Earth-Based Metabolic Biosignature Gases. ASTROBIOLOGY Volume 12, Number 1, 2012

Sara Seager, Drake Deming. Exoplanet Atmospheres. Annu. Rev. Astron. Astrophys. 2010. 48:631–72