We live in a very exciting time: answers to some of the oldest questions humanity has conceived are within our grasp. One of these is whether Earth is the only place that harbours life.

In the last 30 years, the question of whether the Sun is unique in hosting a planetary system has been resoundingly answered: we now know of thousands of exoplanets orbiting other stars.

But can we use telescopes to detect whether any of these distant worlds also harbour life? A promising method is to analyse the gases present in the atmospheres of these planets.

We now know of more than 6,000 exoplanets. With so many now catalogued, there are a number of ways to narrow down which worlds are the most promising for biology. Using the planet’s distance from its host star, for example, astronomers can work out its likely temperature.

Earth is the only planet in the Solar System with liquid water oceans on its surface, so mild temperatures are a possible requirement for a habitable planet. Whether a planet has the correct temperature for liquid water is strongly influenced by the presence and nature of the planet’s atmosphere.

Astonishingly, we can identify molecules present in the atmospheres of exoplanets. Quantum mechanics causes each atmospheric chemical to have its own distinct barcode-like pattern, which it leaves on the light passing through it. By collecting starlight that has been filtered through an exoplanet’s atmosphere, telescopes can see the barcodes of the molecules making up that atmosphere.

To take advantage of this, the planet needs to transit – pass in........

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