Around 400 years ago, the Copernican revolution irreparably shifted our view of the universe. The Earth, which had hitherto been placed at the centre of the cosmos, was demoted to being just another planet, wandering around the Sun with only the Moon for company.

As Newton completed his theory of gravitation and the scientific revolution flourished, it became accepted that many of the lights visible in the night sky were in fact stars just like our Sun, separated from the solar system by immeasurably vast distances. Ever since, the idea that these stars might play host to planets of their own, and that these planets might support life, has inspired generations of scientists and thinkers alike.

However, compared to the stars they orbit, these ‘exoplanets’ (any planet orbiting a star other than our Sun) are so small and dim that detecting their existence presents an enormous challenge. It is extremely rare that an exoplanet can be directly imaged – the main techniques used to infer an exoplanet’s existence instead rely on the planet’s ability to perturb the behaviour of its host star. The instrumental sensitivity required to reliably detect these perturbations is incredibly high and, up until the final years of the 20th century, had proved impossible to do.

The breakthrough moment came in October 1995, when PhD student Didier Queloz (now a member of the Astrophysics group here in Cambridge) discovered a Jupiter-sized planet orbiting the star 51 Pegasi, located 51 light-years from Earth in the constellation of Pegasus. Since then, advances in observational techniques and equipment have allowed several thousand exoplanets to be discovered. It is now believed that there is on average one exoplanet for every star in the sky, with a sizeable proportion having multiple planets like our solar system.  

Since its launch in 2009, the Kepler space telescope has become the undisputed ‘king’ of all exoplanet-hunting activity, having discovered over 70 per cent of all currently known exoplanets.  It uses a technique called ‘transit photometry’ to watch for small decreases in the light output of a star, which may be caused by a planet transiting across the star’s surface and blocking a portion of its light. Because there are several other effects which could cause this dimming, a succession of follow-up observations using both Kepler and ground-based observatories must be performed before the existence of an exoplanet can be confirmed. For stars that host multiple planets, the combined effects of different transits and gravitational interactions between planets can make this an extremely difficult task. 

On 10th May 2016, the team in charge of the Kepler mission announced the discovery of 1,284 new verified exoplanets, increasing the total number discovered to nearly 3,500. Even more excitingly, nine of these are believed to be rocky, near-Earth-sized planets located in the ‘habitable zone’ of their parent stars. This zone is loosely defined as the distance from a star in which an exoplanet receives the same amount of energy as the Earth does from the Sun. It is in this region that liquid water has the best chance of existing on a planet’s surface, and hence gives the best chance for the existence of life as we understand it.

The discovery of planets such as these raises a plethora of tantalisingly difficult questions. Can these planets support life? Are they perhaps doing so at this very moment? Could humans survive there for an extended period? And how many such planets exist? Is it possible that, even as you read this article, the Earth is being verified as a new exoplanet by another form of intelligent beings, who are asking themselves exactly the same questions?

The answers to such questions will come from a range of sources. The topic of extrasolar life requires a vast interdisciplinary approach, and is a fantastic example of how different areas of science are increasingly coming together to answer some of the biggest questions possible. From an astrophysical point of view, the next couple of years will see the introduction of new telescopes such as the James Webb Space Telescope (the successor to Hubble) and the TESS (Transiting Exoplanet Survey Satellite) telescope, which will offer unprecedented resolution for exoplanet detection.

Current detection methods and equipment such as the Kepler telescope are inherently biased towards detecting large Jupiter-sized planets close to their parent stars; models of planetary formation, on the other hand, suggest that Earth-sized rocky planets should be just as common (if not more so) than their large counterparts. It is currently believed that one in five Sun-like stars should have an Earth-like planet orbiting in their habitable zone. These new telescopes may help confirm that there are billions of potentially habitable planets in our galaxy alone.

From a biochemical standpoint, the conditions required for life to flourish on extrasolar planets are still poorly determined, and the likelihood that life will exist, even given favourable planetary conditions, is almost completely unknown. As the quality of data increases, it will be possible to tell whether exoplanets contain significant amounts of oxygen, water vapour or carbon dioxide, and how these relate to conditions on the planet’s surface. Advances may also allow exoplanets to be characterised in terms of their geological activity, with more research needed to ascertain how this might impacts a planet’s habitability, or lack thereof.

It is certainly conceivable that within a generation we will start locating exoplanets whose atmospheres show traces of oxygen or organic materials, exoplanets whose location and size are so well-constrained that their surface temperature profiles are known, and exoplanets that not only appear habitable, but are also (on a cosmic scale) just a stone’s throw away – perhaps less than 15 light years distant. If so, it will be time to start asking the next big question: How do we get there?