Science-fiction writers have been dreaming up alien planets for decades. That other suns should have planets of their own was a reasonable enough speculation. But science had to wait until 1992 for proof that such planets did exist, when two were detected around a pulsar called PSR B1257+12. A steady trickle of discoveries followed. These days that trickle has become a flood. Thanks to a combination of ground-based telescopes and planet-hunting satellites, particularly one called Kepler which was launched in 2009, more than 3,500 such worlds are known.
Unlike their depiction in fiction, reasonably few are much like Earth. Many are gas giants, like Jupiter, whose gravitational pull on their host stars makes them easy to detect. Others orbit dim stars, or super-bright ones. Most are too hot or too cold for life. And almost all are far, far away. Kepler, which has discovered the lion’s share of these planets, is staring at a patch of space covering about 1/400th of the celestial sphere.
From 2017, though, that will change. In December a successor to Kepler, called TESS (for Transiting Exoplanet Survey Satellite), will be launched into orbit. It is designed to survey the entire sky, looking for the sorts of exoplanets that are of most interest to humans—ones that are small (like Earth), rocky (like Earth) and relatively close by.
Over its planned two-year life TESS will scan hundreds of thousands of nearby stars, looking for the telltale dips in their brightness as planets cross them (as seen from Earth). Indeed, Kepler has gathered enough statistical information about how common planets are in the galaxy for astronomers to make a reasonable guess of how many Earth-like planets TESS should find. Assuming Earth’s galactic neighbourhood is average, the new satellite should spot about 3,000 planets in total. About 500 of those should have radii up to twice that of Earth’s; of those, perhaps a few dozen will orbit in the “habitable zone”, where liquid water could exist on their surfaces.
Kepler’s planets are too far away for scientists to learn much about them beyond their rough size and the length of their orbits. Being much closer, the planets TESS turns up will be easier to scrutinise. A new generation of giant telescopes, including the European Extremely Large Telescope, currently being built in Chile, and the James Webb Space Telescope, a successor to the Hubble that is due to be launched in 2018, should have the power to provide intriguing details.
By zeroing in on starlight that has passed through the planets’ atmospheres, scientists hope to learn about the chemical composition of the alien air, including the presence of any chemicals (such as free oxygen or large amounts of methane) that could be telltales of life. If the planets are close enough astronomers may be able to take pictures of them directly. Via painstaking computer modelling, it could even be possible to infer basic geographical features, such as what proportion of a planet’s surface is liquid or solid. Early trials of the technique, using pictures of Earth taken by distant space probes, have been promising.
The ultimate question is whether humans might one day send probes to a nearby exoplanet. In August 2016 scientists announced the discovery of an Earth-like planet orbiting Proxima Centauri, which, at 4.25 light-years away, is the closest star to the Sun. Yuri Milner, a Russian billionaire and physicist (see next story), is already working with Stephen Hawking, a British theoretical physicist, on plans for a tiny, laser-propelled probe that could cover that distance in about 20 years. It is far from certain that they will succeed. But, in the meantime, TESS should give them a comprehensive list of targets. Science fiction is about to get a tiny step closer.
