For decades, astronomers have scanned the sky looking for planets like our own, rocky worlds that orbit a star at just the right distance to harbour liquid water, and possibly, life.
Scientists on Friday announced the discovery of a planet outside our solar system that is, so far, the closest contender for Earth 2.0.
The exoplanet, named Kepler-186f, is roughly the same size as Earth and orbits its star in the habitable zone – the region around a star within which a planet can sustain liquid water on its surface.
The planet was observed using NASA's planet-hunting space telescope Kepler, which has discovered 961 planets orbiting other stars since it began operating in May 2009.
In an analysis piece published by the journal Science, David Charbonneau, an exoplanet researcher from Harvard University, said Kepler-186f, located more than 500 light years from Earth, was "one of the most significant discoveries" from the space telescope, which stopped collecting data last year after a mechanical failure.
While Kepler has identified dozens of planets that circle their star at just the right distance – so-called goldilocks planets – most were made of gas and far larger than Earth.
While the diameter of Kepler-186f is only 10 per cent bigger than Earth, its mass and composition are not yet known.
"The intensity and spectrum of the star's radiation place Kepler-186f in the stellar habitable zone, implying that if [it] has an Earth-like atmosphere and water at its surface, then some of this water is likely to be in liquid form," NASA Ames Research Centre lead researcher Elisa Quintana said.
Kepler-186f is the outmost of five planets orbiting its star, Kepler-186, a type of stellar object called a red dwarf. These are typically much smaller than the sun and burn more slowly.
"We consider this planet more of an Earth cousin than an Earth twin," said Dr Quintana, who also works at the SETI Institute.
The four inner planets, named Kepler b, c, d and e, were too hot to enter the habitable zone, said the researchers, who also published their findings in the journal Science.
