Using Spectra To Find Life Elsewhere In The Galaxy

Since humans first started looking out to the stars, one major question has been on all of our minds – a question so powerful, primal, and controversial, that it often draws global headlines, sparks rigorous scientific debates, and has even spawned a multi-billion-dollar entertainment film sector, well, since films began to be produced.

Are we alone in the Universe?

It’s polarising for society, for science, for politics, for everyone.

Some scientists will argue that due to the sheer scale of the Universe, that we cannot be the only ones – that we should not be so geocentric in our thinking (and attitudes) and consider the statistical probability that life has likely emerged and flourished elsewhere. Other scientists will argue that due to the large set of favourable and variable conditions that Earth was presented with, that maybe we are unique in our own nature.

On the opposite side of the equation, some religious scholars and practices have argued against the idea of the existence of extraterrestrial beings, simply because the teachings and context provided through items like the Bible, do not mention them at all. And then of course there are those to have claimed to have had the first-hand experience, through visitations (in any manifestation), from the others.

In late 2020, the world went into a frenzy with the announcement of a potential biosignature (a molecule that is created by a lifeform – in this case, Phosphine PH₃) on Venus. Was there some microbial life, living in the atmosphere of that hellish planet next door, that was producing a rare molecule known to be generated by lifeforms on Earth? Later studies countered the claim based on a review of how the data was collected and calibrated (in an excellent example of how science works) and the conversation still continues to this date.

Clearly, what is needed to help determine the answer to such a heavy-weighted question is a set of tools that will supply us with absolute evidence, accepted across science communities, that we can use to make a determination. The first step is going to always be confirmation. Then comes the bigger (and much more complicated) step of how, as a planet, we deal with it.

There are basically two methods that will help us find out if lifeforms beyond Earth do exist. The first involves them coming here and knocking on our doorstep, hopefully in a friendly manner. This would indicate that they are an advanced civilisation of beings, technologically capable of traversing the great distances across space and time.

The second is us, using an array of different telescopes and tools here on Earth to observe distant worlds (known as remote sensing) for any particular signs of technosignatures (like radio transmissions, or their own Wi-Fi networks) and biosignatures (chemical fingerprints that indicate lifeforms are present – like an overabundance of Oxygen in their atmospheres). As a recent example, the 2020 report of PH₃ on Venus, is considered a biosignature detection because we know that this molecule is produced by life here on Earth, and not in abundance by natural processes on terrestrial-like planets.

Now, thanks to the work of an international collaboration of scientist led by a Sydney-based team from the University of NSW (UNSW), a resourceful database containing spectral signatures of almost 1,000 molecules has been collated and it could be utilised in searching for biosignatures in planets across the Galaxy

As UNSW School of Chemistry’s Dr Laura McKemmish explains, “when scientists look for evidence of life on other planets, they don’t need to go into space, they can simply point a telescope at the planet in question.”

“To identify life on a planet, we need spectral data,” she says.

“With the right spectral data, light from a planet can tell you what molecular species are in the planet’s atmosphere.”

The team has focused their research on helping narrow down the search for non-intelligent extraterrestrial lifeforms (like microbes) by building this database of spectral signatures for 958 molecules that contain the element Phosphorus – an essential element for life on Earth. 

“Phosphine is a very promising biosignature because it is only produced in tiny concentrations by natural processes. However, if we can’t trace how it is produced or consumed, we can’t answer the question of whether it is unusual chemistry or little green men who are producing phosphine on a planet”, says Dr McKemmish.

To provide insight, Dr McKemmish brought together a large interdisciplinary team to understand how phosphorus behaves chemically, biologically, and geologically and ask how this can be investigated remotely through atmospheric molecular species alone.

“What was great about this study is that it brought together scientists from disparate fields - chemistry, biology, geology, astronomy - to address these fundamental questions around the search for life elsewhere that one field alone could not answer”, commented co-author on the study Associate Professor Brendan Burns.

Dr McKemmish says, “At the start, we looked for which phosphorus-bearing molecules – what we called P-molecules –  are most important in atmospheres but it turns out very little is known. So, we decided to look at a large number of P-molecules that could be found in the gas-phase which would otherwise go undetected by telescopes sensitive to infrared light.”

The database of infrared spectral signatures and results (which have been published in the journal Frontiers in Astronomy and Space Sciences) were produced at an unprecedented rate, by utilising what is known as ‘high-throughput computational quantum chemistry’ (advanced computer algorithms) to make all the signature predictions over a number of weeks. This data is more approximate than the spectral signatures usually used by astronomers (which can take years to generate) but is a crucial starting point that helps prevent misassignments.

Australian scientists played a major role in the study, coming from UNSW, the University of Sydney, University of Technology Sydney, La Trobe University and Australia’s national science agency, CSIRO, and joining researchers from Harvard and Aberystwyth University to produce the database.

It’s been known, since the days of Newton, that visible light can be broken down into smaller components of colour dependent on the wavelengths. From this, the field of spectroscopy emerged and currently has many applications across science, medicine, electronics, remote sensing and more.

By definition, spectroscopy is the study of interactions between matter (like atoms and molecules) and electromagnetic radiation (like photons of light). When applied in an astrophysical context, this technique relates to analysing the spectrum of sources of light (across many different objects) in the Universe.

The tool has proven remarkably useful for astrophysicists, and in general provided context to the Universe we exist in – through the ability to determine a number of factors about the astrophysical light source, such as the chemical composition, temperature, density, mass, luminosity, and motion through the utilisation of Doppler shift measurements.

In particular, by observing the unique spectral signature of elements here on Earth, we can look for these spectral signatures in astrophysical light sources to determine what elements might exist there, and what conditions these elements are experiencing in-situ.

Astrophysical spectroscopy is conducted in several major bands of the electromagnetic (EM) spectrum which involves the usage of different apparatus found across a variety of altitudes around Earth. The primary reason for this is because Earth’s atmosphere blocks or absorbs a lot of the EM spectrum and so instruments need to be placed at higher locations to achieve their best results.

Higher-energy wavelengths, like UV and X-ray sources, will need telescopes and instruments to be above Earth’s blocking atmospheric components – often carried high on balloons or even sent into orbit like the Chandra X-Ray Observatory.

Optical wavelengths pass through our atmosphere easily (it’s the same wavelengths our eyes are tuned into) and so, ground-based optical telescopes can analyse starlight to break it down into its spectral components (if the night is free of clouds). This data can help determine what elements are present in stellar atmospheres, the overall signature of entire galaxies, or determine what makes up the big, beautifully glowing emission nebulae.

In contrast, not all infrared bands can pass through our atmosphere due to a number of different molecules, like water vapour, CO₂, and Ozone particles – all of which play a vital role in ensuring the survivability of life on our planet (e.g., CO₂ allows our atmosphere to retain enough heat to keep the global average temperature above 0-degrees Celsius).

To get around this, astronomers build their ground-based observatories high atop mountains where moisture is located below the cloud line, though this only gives some opportunity to explore the infrared atmospheric windows. To really seize the opportunity to study spectra at these wavelengths, astronomers also utilise airborne observatories, such as SOFIA, or orbiting telescopes – much like the Spitzer Space Telescope, which ended operations in 2020.

Replacing Spitzer (and scheduled to be launched later in 2021) is the much-anticipated James Webb Space Telescope – an infrared orbiting observatory, with a combined 6.5-metre diameter mirror, that will allow scientists to explore astrophysical phenomena (like planetary atmospheres) in unprecedented detail.

And finally, due to their longer wavelengths, radio waves can also utilise large ground-based telescopes, though usage of interferometers at these wavelengths often proves advantageous due to increased resolution to sources in the sky. Radio telescopes have been used to study the different molecules in space that are too cold to produce their own light – such as those which reside within the giant molecular clouds, that eventually go on to become the nurseries of future Solar systems.  

The application of analysing light (all wavelengths) from astrophysical sources, like planets, helps astronomers determine what elements might be present there. For example, the team that outlined the 2020 PH₃ result on Venus used infrared and radio telescopes to determine the signature of the molecule.

Or alternatively, when we are lucky enough to have our telescopes align with a planet passing in front of its host star – any starlight (optical wavelengths) that comes through its atmosphere can be collected and spectroscopically analysed to tell us what elements are there.

Phosphorus is a major element that is present in all life on Earth, playing an important role in cellular metabolism, storage of genetic information, formation of cell membranes and cell regulation. Its ability to create complex organic molecules with unique functionality make this element a prime target for the search for astrobiological life elsewhere in the Universe. 

Whilst Phosphorus is found throughout the galaxy around objects like comets, cool stars and circumstellar regions in a number of different molecular structures, it is the molecules detected within planetary atmospheres that are of most interest to scientists as this could be representative of environments where life could emerge. 

On large planets - like the Gas Giants of our system that have atmospheres that are rich in hydrogen, the most common P-molecule is PH₃, which is formed in the hot, deeper layers within the planet than delivered to the top of the atmosphere (where it can be detected) through convection processes. 

This is different from the terrestrial (rocky) planets, which have oxidising atmospheres that can reduce the capability of P-molecules like PH₃ to remain stable. On Earth, the high concentration of O₂ causes this oxidisation, whilst on Venus, the high value of sulfuric acid does the job. 

As such, any PH₃ detected in Earth’s atmosphere is associated exclusively with life, derived through either human-based activity (like agriculture) or through anaerobic ecosystems like marshlands and lake sediments. 

These and other ways of formation and destruction of P-molecules are discussed in the paper and why a diverse group of geologist, biologists and chemists are important to add to the team.  It also explains why detecting Phosphorus in the atmospheres of planets beyond Earth is important, and why this new database of spectral signatures will play an important role in these future studies. 

This new database now becomes a useful resource for astronomers who are studying distant light sources to be able to compare the spectral signatures of Phosphorus-based molecules to those remotely sensed with their telescopes and instruments – thus allowing a more efficient and rigorous scientific process of confirming or ruling out what the detection provides.

“Though this new dataset doesn’t yet have the accuracy to enable new detections, it can help prevent misassignments by highlighting the potential for multiple molecular species having similar spectral barcodes – for example, at low resolution with some telescopes, water and alcohol could be indistinguishable,” said Dr McKemmish.

Often it is thought that detection is the only method of confirming a biosignature being present out at a distant planet, but equally as important is ruling out the presence of certain signatures which are required for life as we know it. A good example of this is water – a molecule that most life needs. If water is detected then this means there could be a chance for life to flourish, and inversely, if the conditions present never allowed water to exist or remain, then life as we know it is unlikely to have developed.

“The data can also be used to rank how easy a molecule is to detect. For example, counter-intuitively, alien astronomers looking at Earth would find it much easier to detect 0.04 percent CO₂ in our atmosphere than the 20 percent O₂. This is because CO₂ absorbs light much more strongly than O₂ – this is actually what causes the greenhouse effect on Earth.”

“The only way we're going to be able to look at exoplanets and if there's life there is to use spectral data collected by telescopes – that is astronomer’s one and only tool,” says Dr McKemmish.

“Our paper provides a novel scientific approach to following up the detection of potential biosignatures and has relevance to the study of astrochemistry within and outside the Solar System,” says Dr McKemmish. “Future studies will rapidly improve the accuracy of the data and expand the range of molecules considered, paving the way for its use in future detections and identifications of molecules.”

By now combining this data set with additional studies that involve telescopes that search across a variety of wavelengths, such as those that will soon commence using the James Webb Space Telescope (infrared bands) or in the future, the Square Kilometre Array (radio bands), the search for life elsewhere becomes much more detailed, and approachable from a number of different perspectives, as discussed by fellow co-author and CSIRO astronomer Dr Chenoa Tremblay.

“This information has come at a critical time in astronomy,” she says.

“A new infrared telescope called the James Webb Space Telescope is due to launch later this year and it will be far more sensitive and cover more wavelengths than its predecessors like the Herschel Space Observatory. We will need this information at a very rapid rate to identify new molecules in the data.”

She says although the team’s work was focused on the vibrational motions of molecules detected with telescopes sensitive to infrared light, they are currently working to extend the technique to the radio wavelengths as well. 

“This will be important for current and new telescopes like the upcoming Square Kilometre Array to be built in Western Australia.”