But they won’t be the only ones on the hunt. Telescopes can produce much clearer images in space, free from our turbulent atmosphere. The Hubble Space Telescope has been circling Earth since 1990 and has spotted a few planets – and also helped determine what some extra-solar worlds are probably made of. But Hubble’s 2.4m (7.9ft) mirror is too small to see planets smaller than Jupiter, says Matt Mountain, the director of the Space Telescope Science Institute at Nasa. Hubble’s planned successor, the James Webb Space Telescope (JWST) – set to go into orbit around 2018 – is expected to do much more.
Equipped with a 6.5-metre (21ft) diameter mirror, JWST will have a variety of tasks, among them the search for planets orbiting nearby stars. It will work in the infrared spectrum, which will allow it to “probe down to smaller planetary sizes than Hubble, to roughly two-to-three times larger than Earth, more Neptune-scale planets,” says Mountain.
The telescope will aim to find out whether these extra-solar worlds are so-called “super-Earths” – rocky planets that could potentially be habitable – or miniature versions of Neptune, unable to support life. Using an instrument called a coronagraph, JWST will try to determine whether a planet has an atmosphere, and – for the first time – analyse it by examining the spectrum of the light coming from the planet.
Elements and molecules in an atmosphere, such as water and oxygen, have specific signatures in the spectrum, giving us an idea about its composition and the likelihood that there is liquid water present – and hence life, says Avi Loeb, Chair of the Astronomy department at Harvard University.
Spotting water and oxygen would be just the start, though, says Liske. “If you found things like water, oxygen, CO, methane, in the right amounts, that would be a strong hint [of life]. Then of course there would be lots of investigations into whether such a combination of elements could be produced by non-biological processes,” he says. “If we are ever in the situation where we've convinced ourselves that we've found life, then the question would be ‘what kind of life’? If we're lucky we may be able to say something about the exoplanet's surface: is it all water? Any signs of vegetation? That sort of thing could come in principle from either ground or space-based telescopes.”
Sic transit gloria
As exciting as the plans for direct imaging are, most exoplanets are still found using indirect techniques – such as detecting a wobble in the position of the star that indicates it is being pulled slightly towards an orbiting planet, or a method called “transiting”, in which planets are identified by the tiny dip in brightness caused when they pass in front of its star. Transiting was used to great success by Kepler, but the method doesn’t allow us to calculate a planet’s mass – a critical factor in determining its density and hence its rockiness. To determine the mass, another instrument enters the stage – a spectrograph.
One such device, the High Accuracy Radial velocity Planet Searcher (Harps), is mounted on a 3.6m- (11.8ft-) mirror telescope at La Silla, another one of ESO’s observatories in the Atacama. It studies space bodies by recording how a planet’s gravity makes its parent star appear to vibrate as it rotates around it. It can use that vibration to detect new planets, but has also been used to learn more about known exoplanets. It determined that an exoplanet discovered by Corot in September 2009 some 500 light-years away and dubbed Corot-7b had a rocky surface and a mass only five times of our Earth. This would make it a likely habitable candidate if it wasn’t for its proximity to its parent star – the planet lies only 2.5 million km (1.6 million miles) away from it, which is just 1/23rd of the distance from the Sun to Mercury.