As the number of confirmed exoplanets grows (we just passed the 5,000 mark) attention is turning toward determining more about these planets orbiting other stars. The most compelling question, of course, is whether or not they are capable of containing life, and if they actually do have life. If a planet did sport life around a star tens or hundreds of light years away, how would we know? There are two basic approaches – looking for technosignatures and biosignatures. A technosignature is a sign of advanced technology, like heat radiating from a massive structure. But this also requires an advanced technological civilization, not just life. Biosignatures are just signs of life, even simple bacteria.

There are two primary biosignatures that scientists are currently interested in, oxygen and methane. These two gases have in common that they are produced by the biochemical activity of life (at least on Earth) and they are highly reactive and therefore short-lived in an atmosphere. Therefore if they exist they are being constantly replenished, and one potential source is life.

For completeness you may remember the possible detection of phosphene in the upper atmosphere of Venus. Phosphene is another potential biosignature, raising the possibility of critters living in the clouds of Venus. But later analysis concluded that the detection was likely a mistake, the signal coming from another more common source not indicative of life.

All of these potential biosignatures have another thing in common – they can be released by non-living sources. This makes them problematic as biosignatures. Just detecting oxygen is not enough, we can’t pop the corks yet. Further analysis is necessary to determine the likelihood of a biological vs non-biological source for any of these putative biosignatures, and that is where recent work has been focusing. A 2018 paper, for example, examines the characteristics of an oxygen signature that will help us rule out a “false positive” biosignature using spectrographic, photometric, and time-dependent analysis.

A recent study published in PNAS focuses on methane as a biosignature. Methane is perhaps the most likely to produce false positives because it can be released by common geological activity, such as volcanoes. Essentially the study authors concluded that you cannot just look at the presence or absence of a single biosignature, but have to look at the biosignature in the context of other environmental factors. For example, if the methane detected is being released by volcanic activity then there should also be a certain amount of carbon monoxide (CO). Life, however, tends to metabolize CO and produce CO2. Therefore if methane exists with high levels of CO2 and low levels of CO then that is a more compelling picture of life. If CO2 is low and CO is high then that favors a geological source. This does not hold, however, for planets with a very high water content.

Therefore looking at multiple chemicals in an exoplanet’s atmosphere is likely to give us a more complete picture and minimize false positive biosignatures. However, there remains a significant weakness of this approach as described so far – it is using the Earth as our one example. We know how oxygen and methane behave on Earth, but that knowledge cannot necessarily be extrapolated to exoplanets (or even our neighbors, like Mars). We therefore also have to consider as many planetary factors as possible, the geochemistry of the planet and its interaction with its parent star. A rocky planet orbiting close to a red dwarf star may have a different methane cycle than what we see on Earth.

These types of studies are just getting started, and it will likely take years or decades of research to refine our analysis techniques to more accurately use biosignatures to determine if an exoplanet likely has life. Unfortunately, there is currently no way to confirm if our analysis is correct. We cannot go to these planets and determine more directly if our methods are correct or not. This is why it would be very interesting if we detect life on Mars or Venus because we can compare their biosignatures with direct detection. But still that is a limited data set. We may ultimately find that we developed elaborate criteria for biosignatures that don’t hold up once we can travel to exoplanets and explore them directly.

I do think, however, that we can get to relatively high levels of confidence with extensive modeling and research on Earth. The laws of physics will be the same on exoplanets, and that will constrain what environmental conditions can exist there. Further, as or technology advances we will be able to gather greater details about exoplanets. That is where the James Webb telescope comes in. It will be able to detect methane in the atmospheres of exoplanets with much higher precision (not so much oxygen). That is why, in fact, the researchers focused on methane, to prepare to data coming in from James Webb. Later in this century we may have better instruments still. It’s likely our instruments for observing exoplanets will have a long time to improve before we can actually travel to any exoplanets.

All of this means that we may very well one day be able to say with a high level of confidence that we have found life on an exoplanet light years away.