Agnostic biosignatures: the unlikely group of molecules that could be the key to finding life on other planets

Since the first confirmed discovery of a planet outside our solar system in 1992, our view of the universe has rapidly expanded. As of March 2020, we are aware of over 4000 exoplanets. Both within and beyond our solar system there is a wealth of unexplored history, some of which is potentially biological. But where do we start when looking for life? Many space scientists propose that evidence for living organisms can be found in atmospheres. The evidence they’re searching for are biosignatures – molecules that could be indicators of current or past life.

At the present time, scientists are trying to detect chemicals such as oxygen, carbon dioxide and methane. These molecules are commonly produced during life-based processes such as respiration and photosynthesis. It is thought that if we can detect them, they could be an indication of current, or past, life forms. For example, Curiosity—the rover exploring Mars—has been able to gauge the changing methane concentration in the atmosphere, at one point measuring an unexplained tenfold spike.

There is one big problem with these biosignatures: false positives. Whilst the methane detected on Mars may have been produced by microbes, it could also have been produced by a reaction between the organic molecules, formed by falling comet dust on the surface, and UV rays. Determining the source is difficult. Traditional biosignatures, such as oxygen, can be formed in many different ways and so detecting them does not instantly equate to the existence of life. For instance, CO2 rich atmospheres can produce detectable concentrations of oxygen through photolysis. This is a process where carbon dioxide is broken down by light, but is not an indication of living organisms.

Dr Clara Sousa-Silva, an astrochemist at MIT, suggests the key could be in agnostic biosignatures – molecules that are not obviously connected to life processes on earth. Sousa-Silva’s approach is to ask, ‘What if life is not what we know?’. She asks us not to assume that life on other planets is similar to our own. Her particular interest is in the molecule phosphine (PH3). Initially, this seems like an unlikely choice for a biosignature as it is highly flammable, toxic, and difficult to make. Indeed, it seems to be produced by processes that are particularly ‘anti-life’. Phosphine gas is a byproduct of methamphetamine production and is used in chemical warfare.

However, Sousa-Silva points out that phosphine is found all over the world in anaerobic environments such as rice fields. Therefore, it could be a useful signifier of life on exoplanets with anoxic (oxygen absent) atmospheres. What makes phosphine attractive is that it has no known abiotic false positives – we don’t know of any non-biological processes that produce it. Another benefit is that phosphine has particular characteristic spectral features that make it easier to identify in an exoplanet’s atmosphere.

It is unlikely we will find life with the detection of a single molecule, however. Sousa-Silva suggests that finding life is a game of probability and we must therefore consider as many molecular components as possible in a given atmosphere. The head of her research group, Sara Seager, and colleagues have identified a staggering 16,367 molecules that could be potential biosignatures. But how do we use this data to look for life on other planets?

Firstly, these molecules need to be detected and then analysed through a form of spectroscopy such as infrared. With a molecule like phosphine this can be difficult, because it is often found in small quantities. Appropriate detection is therefore a matter of both improving instrumentation and looking for similar molecules in higher concentrations. For example, isoprene (C5H8) has been proposed as an alternative. On earth, isoprene is produced in large quantities by many organisms – including bacteria that have the potential to thrive in anaerobic conditions – but is rapidly destroyed by our atmosphere. Therefore, it could be a good biosignature in oxygen-free environments.

Sousa-Silva suggests that the main limiting factor to this research is the amount of spectral data we have. To identify these chemicals, we need reference spectra to compare them to. At the moment, out of the 16,367 potential biosignatures, we have spectral data (of any quality) for less than 4% of them. The main reason for this is the lack of manpower. The process of producing reference spectra is time consuming. The molecule first must be synthesised, which can be very difficult if there is currently no known synthetic route. Then, measuring accurate molecular spectra experimentally is expensive, dangerous if toxic and sometimes completely impossible – for example, when the lifetime of a molecule is very short (some compounds exist for only nanoseconds).

Her solution to this is ATMoS, or Approximate Theoretical Molecular Spectra, a programme that approximates spectral data for thousands of molecules. It does so through the use of existing experimental databases and a computational model that uses the similarities and differences between molecules to predict new spectra. Whilst the predictions are not yet as accurate as creating spectra experimentally, this approach can produce the main defining spectral features of a molecule. Consequently, it has great potential to help streamline the process of identifying biosignatures in a complex atmosphere.

There is still research to be done to determine whether the detection of these molecules will ultimately prove useful. However, agnostic biosignatures show real promise and help us to rethink how we approach the search for life on other planets. To see whether they are instrumental for finding life forms, we will have to await a future space mission.