The search for alien life or, similarly, the search for habitable planets, are some of the most sought-after research questions in astronomy and astrobiology.
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Just about anyone would love to know whether the planets and aliens similar to those we watch and read about in science fiction are real. But how do we go about tackling that question?
The answer lies in the concept of biosignatures.
These are observable features of a planet, such as its atmospheric composition, that cannot be explained by some non-biological process.
Of course, we need to restrict our search for life as we know it. If there were silicon-based lifeforms out there - in other words, robots - we really wouldn't know what we were looking for.
To identify a biosignature, first we ideally need the spectrum of a planet.
Think of these as "space rainbows", where we take the light reflected from a planet and split it up into all of the colours of a rainbow.
However, some of the colours will be missing, and these correspond to different molecules or atoms in the planet's atmosphere that absorb very particular colours of light.
For example, a spectral feature that can be considered a biosignature is the so-called "vegetation red edge", where chlorophyll in plants (the chemical that allows plants to absorb sunlight and makes them green) changes the amount of light reflected between visible and infrared light from 5 per cent to 50 per cent.
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If we can detect a similar jump in reflectivity on other planets, this is potentially a sign that there is a lot of vegetation on that world.
The astronomer Carl Sagan did this with the Galileo mission. He pointed its camera at Earth and was able to determine (without assuming anything) that there was a high probability of life living on the planet.
Another biosignature is the presence of oxygen, carbon dioxide and methane in the atmosphere. These are all present in the Earth's atmosphere, with the latter two being the main contributors to climate change.
While detecting each one on their own is not a very strong sign for habitability as non-biological processes can make these gasses, detecting them simultaneously is one of the strongest biosignature pairs we know of. This is because these chemicals together indicate the type of disruption to an atmosphere that biological life is known to make here on Earth.
Phosphine is another chemical that has been used to suggest life may have existed on Venus
Of course, with all of these we have to be careful to ensure we don't pick up false signals or incorrectly attribute some chemical that could be produced by non-living processes.
We also have to consider that a planet's atmosphere can change substantially over time. Earth had much, much less oxygen in its atmosphere very early on in its formation, and only developed the modern atmosphere due to life's processes. It's possible that we may find planets where life is only just getting started.
Regardless, the hunt for habitable worlds is alive and well, and I hope that one day astronomers will be able to announce that those science fiction worlds are actually science fact.
- Jonah Hansen is a PhD student specialising in space interferometry at Mount Stromlo Observatory, at the Australian National University.
