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Infrared astronomy

Although the planet orbits at a distance that would permit liquid water, other factors might render it unlivable. It might be tidally locked — meaning that the same hemisphere always faces the star, which scorches one side of the planet while the other remains cool. The active star might occasionally zap the planet with destructive X-ray flares. And it’s unclear whether the planet has a protective, life-friendly atmosphere.

Proxima itself belongs to the triple-star system Alpha Centauri. In 2012, a Nature paper reported that an Earth-mass planet orbited another member of that stellar trio, Alpha Centauri B2. That result has now mostly been dismissed34, but exoplanet specialists say the Proxima claim is more likely to hold up.

“People call me Mr Sceptical, and I think this result is more robust,” says Artie Hatzes, an astronomer at the Thuringian State Observatory in Tautenburg, Germany.

False alarm

This time, the combination of new observations and older measurements dating back to 2000 increases confidence in the finding, Anglada-Escudé’s team argues. “It’s stayed there robustly in phase and amplitude over a very long time,” says team member Michael Endl, an astronomer at the University of Texas at Austin. “That’s a telltale sign of a planet.” The data even contain hints that a second planet may exist, orbiting Proxima somewhere between every 100 and 400 days.

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The researchers now hope to learn whether the Proxima planet’s pass across the face of its star can be seen from Earth. The chances are low, but such a ‘transit’ could reveal details of the planet, such as whether it has an atmosphere. A team led by Kipping has been independently looking for transits around Proxima, and is frantically crunching its data in search of any signal.

The discovery of the Proxima planet comes at a time of growing scientific interest in small planets around dwarf stars, says Steinn Sigurdsson, an astrophysicist at Pennsylvania State University in University Park. NASA’s Kepler space telescope has shown that rocky planets are common around such stars, which themselves are the most common type of star in the Galaxy. “This is a total vindication of that strategy,” he says.