by Ruth Titz-Weider (DLR), May 2024
What if there was life on a planet outside our solar system? What evidence could there be? What could be measured and what could indicate life?
Here are a few examples of molecules that have made it into the headlines in the search for extraterrestrial life: water, phosphine, methane and dimethylsulfide.
Water H2O
Water is essential for life as we know it. If water is found on a celestial body, things can get interesting.
A single drop of water, from filterzentrale.com
It can also be geological structures formed by water. One example of this is our neighbour planet Mars. On Mars, Giovanni Schiaparelli (1835 – 1910) observed linear structures – canali in Italian – which he believed to be naturally formed, linear basins through which water may have spread on the otherwise dry surface. This gave rise to many speculations, including Martians, encouraged by the mistranslation of the English term canal as an artificial structure.
Microbial life could have existed on early Mars. There is evidence that water was once present there in a liquid state. Today, water ice can be found, which thaws drop by drop under certain conditions and depending on the season, and trickles down the steep Martian slopes. It is said to originate from hydrous salts, perchlorates, which can bind water molecules in their interior. However, it is not clear exactly where the water comes from.
More precise knowledge about the presence of water will be gained for Mars, as there are many missions to our neighbour planet, e.g. the Mars rover Perseverance, which landed as part of the Mars 2020 mission and is searching directly and indirectly for life. Something like this is not feasible for planets outside our solar system. The distance to these planets is at best a few light years and this distance only indicates the time it takes for a signal to or from the exoplanet and is beyond the reach of ground or air sampling.
However, water vapour has been detected in the atmospheres of several exoplanets, e.g. through measurements of GJ 9827 d with the Hubble telescope. But this does not mean that there are oceans or lakes or ponds on the surface of the planet, and certainly not that microorganisms are romping around there. Even if life as we know it cannot exist without water, water is not a good biomarker.
Phosphine PH3
In penguin colonies, phosphine is found in significant quantities in the faeces of the animals (guano). Image source: M. Murphy via Wikimedia Commons
A molecule called phosphine was the focus of scientific and interested public attention in 2020. A British research group published the measurement of phosphine in the upper cloud layer of our neighbour planet Venus. Phosphine is a molecule that is produced by organic weathering processes, i.e. a biomarker, because other, abiotic production – that is, formation without the involvement of living organisms – can be ruled out. The media were quick to sensationalise the discovery of life on Venus. But there was a headwind: a new analysis of the measurement data and new measurements with other telescopes showed significantly less phosphine or none at all.
In addition to hydrogen, phosphine contains phosphorus, the element that is processed in all important building blocks of life, e.g. in DNA, deoxyribonucleic acid, the carrier of genetic information. This is another reason why phosphine is considered one of the most promising biomarkers.
Methane CH4
Large quantities of the greenhouse gas methane escape from rice fields. Image source
Methane is the simplest organic molecule and a very effective and long-lasting greenhouse gas. While the better known greenhouse gas carbon dioxide occurs at a volume fraction of about 0.04 % or 400 ppm, the value for methane is 200 times smaller.
On earth, around 95% of methane is of biological origin. It can be produced either anaerobically (without molecular oxygen) by microorganisms or aerobically (with molecular oxygen) by phytoplankton, plants or fungi. Rice cultivation and the digestive organs of cattle and sheep release further methane into the atmosphere. These human activities – agriculture and livestock farming – account for around 70% of methane emissions. But even without humans, there would be methane on earth due to microorganisms.
The only way to detect methane in the atmosphere of an exoplanet is to analyse the light that is received from the star when the planet passes in front of it. These transit measurements at different wavelengths can be used to deduce the existence of certain molecules in the planet’s atmosphere. This is hardly possible from the ground, but the James Webb Space Telescope (JWST) has instruments that make such measurements possible.
Spectrum of the atmosphere of the planet K2-18 b, measured with the James Webb Space Telescope. The molecules that can be assigned to the measured values at different wavelengths are labelled. Detail of an image from NASA, ESA, CSA, Ralf Crawford (STScI), Joseph Olmsted (STScI) here.
Using the spectrometers in the near-infrared wavelength range of the JWST, methane, carbon dioxide and possibly dimethyl sulfide – see next section – were found on the exoplanet K2-18 b in 2023. It lies in the life-friendly, habitable zone of its star and is 8.6 times heavier than the Earth with a radius 2.5 times that of the Earth. This is the category of super-Earths that we are not familiar with from our solar system, but are common among exoplanets.
But habitable zone does not necessarily mean that there is life and spectral lines of certain molecules that occur in biological processes do not yet tell us what the exact conditions on the planet are like. K2-18 b may be a planet that is covered by an ocean and has an extensive hydrogen-rich atmosphere. Another group of researchers believes the data is best explained by a mini-Neptune made of gas that has no solid surface.
DIMETHYLSULFIDE CH3-S-CH3
Image source: https://wallup.net/sea-nature-waves/
In the JWST measurements of the planetary atmosphere of K2-18b, lines indicating the molecule dimethyl sulfide may have been discovered alongside carbon dioxide and methane. On Earth, it is only produced by biological processes, largely through the metabolism of phytoplankton in the world’s oceans. The detection of this molecule on K2-18b still needs to be confirmed by further measurements with JWST.
The simultaneous measurements of molecules such as carbon dioxide, methane and possibly dimethyl sulfide on K2-18b expand our knowledge of the structure of exoplanet atmospheres, as they must be able to be consistently integrated without contradiction into the models that are intended to explain the physical, chemical and biological interactions between the atmosphere and the surface.
