Astronomers have found the nearest planet outside our Solar System, circling one of the stars of Alpha Centauri just four light-years away.
The planet has at minimum the same mass as Earth, but circles its star far closer than Mercury orbits our Sun.
It is therefore outside the "habitable zone" denoting the possibility of life, as the researchers report in Nature.
However, studies on exoplanets increasingly show that a star with one planet is likely to have several.
At the very least, the work answers the question first posed in ancient times about planets around our nearest stellar neighbours.
The closest star to the Sun is Proxima Centauri, which is believed to be part of a three-star system that includes the brighter stars Alpha Centauri A and B.
The planet was found near Alpha Centauri B by the Harps instrument at the European Southern Observatory's La Silla facility in Chile.
That puts it far closer to Earth than any of the more than 840 confirmed exoplanets
Like a dance between one enormous and one tiny partner, as an exoplanet orbits its much larger host star, its gravity causes the star to move in a small orbit.
Harps and instruments like it measure the subtle change in colour - the redshift or blueshift - of the host star's light as its orbit moves it slightly closer to and further away from Earth.
'Landmark discovery'What has delayed this finding is that because Alpha Centauri is itself a complicated system of stars orbiting one another, the effect of a comparatively tiny planet is difficult to detect.
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