Nasa's new telescope will give our hunt for alien life a major boost

While we haven't yet discovered life beyond planet Earth, our investigations into planets that orbit stars other than our Sun - known as extra-solar planets, or exoplanets - have only just begun. In 2018, we will discover the first exoplanet with atmospheric indications of life, thanks to the launch of Nasa's new space telescope, the Transiting Exoplanet Survey Satellite (Tess), which will begin a two-year survey of more than 200,000 of the closest, brightest stars, in March.

Tess will be looking for reduction in a star's brightness caused by the transit of one or more planets across its face. But many planets are in an orbit that never comes between us and their star. To detect these, we can look for three other kinds of evidence: light reflected by an exoplanet's atmosphere; the gravitational pull that the planet has on its star; and gravitational lensing, the bending of the path of light by an enormous object, as predicted by Einstein.

If an exoplanet is large enough and far enough from its star (such as Jupiter in our own solar system) we can see the star's light reflected by the planet's atmosphere. All planets, whatever their size, exert a gravitational shift on their star and we can detect how it is pulled toward each planet it hosts, through a shift in its light toward the red or blue end of the visual spectrum, as the star moves away and then towards us again. Gravitational lensing allows us to identify the presence of an exoplanet by the momentary brightening of a more distant star behind it. The combination of all four methods means we have a very powerful toolkit to detect exoplanets across a wide range of viewing angles of distant solar systems. But detecting an exoplanet is only half of the story of finding planets that can sustain life. We need to know if the planet's atmosphere contains molecular oxygen and other gases indicative of life as we understand it.

By using spectroscopy to analyse the chemical composition of the atmosphere, we are able to look for the presence of oxygen in the form of ozone. There is only a brief moment to do this, however - when an exoplanet passes in front of its host star. At that point, the star's light passes through its atmosphere, creating a glow around its circumference. If spectroscopy reveals the presence of oxygen, carbon dioxide and water, and the planet resides in what is known as the habitable zone - meaning conditions on its surface are neither too hot nor too cold - this greatly increases the probability that the planet harbours what we'd recognise as life.

The first confirmed detection of an exoplanet was in 1992. Since then, there have been 3,639 exoplanet confirmations in 2,729 planetary systems, many made by the Hubble, Spitzer and Kepler space telescopes. In May 2016, Nasa stated that, of the 1,284 exoplanets discovered by Kepler, roughly 550 could be rocky planets, based upon their size. Of these, nine orbit their stars in the habitable zone. However, no rocky exoplanet has yet been found that is located in the habitable zone and has an atmosphere that contains molecular oxygen, carbon dioxide and water vapour.

There are, at a conservative estimate, 100 billion stars in our own galaxy. With an average 1.6 planets per star observed so far, we can estimate that there are more than ten billion Earth-like, rocky planets in our own galaxy alone. Statistically, this makes the discovery of a life-sustaining planet very likely. We already know that many planets orbiting distant stars might be very much like those we find in our own solar system. All we need to do now is ascertain whether or not they support life. When we do, that will radically alter the understanding of our place in the Universe.

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