Correction, 22 April: The original version of this article misspelled Dr Quintana's name as "Quintada". Our apologies.
ONE OF the biggest stories in astronomy over the past two decades has been the promotion of exoplanets—planets orbiting stars other than the Sun—from science fiction to reality. Astronomers, and anyone with even the faintest grasp of statistics, had long suspected such planets must exist, but only since the discovery of a planet orbiting a distant pulsar in 1995 have they been able to prove it. These days thousands of such worlds are known.
In a paper just published in Science, Elisa Quintana, an astronomer at NASA, and her colleagues, describe the detection of a particularly special exoplanet. Kepler 186f, to give its quotidian name, appears to be the closest relative to Earth yet discovered. Located about 500 light-years away, in the direction of the constellation Cygnus, Kepler-186f has a radius between 0.97 and 1.25 that of Earth. And it orbits its parent star firmly inside the "habitable zone", in which temperatures are just right for liquid water.
The planet was found by NASA's recently-defunct Kepler space telescope, which stared at hundreds of thousands of distant stars, looking for the tiny dip in brightness produced when a planet crosses in front of its star as seen from Earth. If several such dips are detected, on a predictable schedule, then astronomers can be reasonably sure that they are looking at a planet. By measuring the dip in brightness, they can work out how much of the star's surface is being obstructed. If they know the size of the star—which is intimately related to its brightness—they can infer the size of the planet.
But being Earth-sized and Earth-temperature is not enough to be properly Earth-like. Indeed, exoplanet conneisseurs may be forgiven for being a bit cynical about the epithet "Earth-like". It has been applied to worlds with five times the mass of Earth and nearly twice its radius, or to worlds that skirt the very fringes of the "habitable zone", in which a planet would be neither too hot nor too cold for liquid water to exist on its surface.
And while Dr Quintana can be confident about the new planet's size, much else about is speculative. The paper points out that if the planet has an Earth-like atmosphere, and if it has water, then some of it may be liquid across some of its surface. But although models of planetary evolution suggest that it might well be wet, no-one can know for sure.
It is also possible that the planet is tidally locked to its parent star, in the same way that the Moon's rotation has synchronised with that of the Earth. If so, then one half of the planet would be permanently bathed in sunlight while the other looked out on the blackness of space. If the planet has an atmosphere, then that kind of differential heating would presumably cause permanent, powerful winds to sweep across the surface. Depending on what the planet is made from, it could have far less surface gravity than Earth does, or far more. And in other ways, Kepler-186f is a truly alien world. For one thing, its parent star is a red dwarf, far smaller than the Sun. Someone standing on the surface would see only a third as much sunlight as they see on Earth, and that light would be much ruddier than the illumination that Earth's sun provides.
Finally, no-one can know whether the world is, in fact, inhabited or not. Astronomers have ideas on how they might find out: by analysing the tiny fraction of starlight that passes through a planet's atmosphere can reveal details about its chemical composition. The presence of large quantities of highly-reactive substances like oxygen would suggest that something interesting was going on, as there would be no known non-biological effect that could account for its presence. A few gas giant planets have already had their atmospheres inspected this way. But Kepler-186f is far too small and distant for such an analsysis to be possible today.
Still, Kepler itself was not really designed to answer detailed questions about individual planets. It was designed to detect and characterise large numbers of worlds, giving astronomers statistics about how common the various sorts of planets might be in the galaxy. And the good news for alien-hunters is that, after three years of collecting data, habitable worlds seem abundant. Estimates for the number in the Milky Way alone vary depending on how adventurously "habitable" is defined, but all are in the billions. With several new exoplanet missions planned—notably another NASA mission called the Transiting Exoplanet Survey Satellite, which is designed to monitor the entire sky and is due to launch in 2017, few would bet that Kepler-186f will retain its title of "most Earth-like world" for long.
