"The biggest question mark for whether this is an Earth-like planet or not is whether there is water. That entirely depends on the formation, on the history of the planet," said astronomer Ansgar Reiners, with the University of Gottingen in Germany.
"You can come up with formation scenarios that end up with an Earth-like atmosphere, that end up with a Venus-like atmosphere, that end up with no atmosphere at all," he said.
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Proxima b also is probably tidally locked to its mother star, with half its surface in permanent darkness and the other half in constant daylight. Outbursts from its parent star probably blast the planet with a barrage of high-energy X-ray and ultraviolet radiation, which might make life a challenge.
Even so, red dwarf stars like Proxima Centauri, the parent star to the newly found Proxima b, have become popular targets in the hunt for habitable worlds beyond the solar system. That is because a planet about the size of Earth in orbit around one of these small stars is proportionally larger and easier to find than similarly sized planets circling stars as big as the sun.
Proxima Centauri, for example, is less than 20 percent the mass of the sun.
Its planet, Proxima b, was discovered after painstaking efforts to understand a slight but regular shift in the wavelengths of light coming from Proxima Centauri, one of a trio of stars in the Alpha Centauri system.
Astronomers discovered that light from Proxima Centauri shifted every 11.2 days, as measured by the HARPS spectrograph on the European Southern Observatory's 3.6-meter telescope in La Silla, Chile, and simultaneously by other telescopes.
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As early as 2013, an international team of scientists began to suspect that Proxima Centauri was being regularly tugged by the gravity of an orbiting planet, but since it is an active star they had to rule out other options, such as stellar flares.
In a paper published in Nature on Wednesday, the team confirmed that the star closest to the sun is indeed home to a potential cousin Earth. It also may have siblings, scientists said.
"We have found a terrestrial planet orbiting Proxima Centauri," said lead author Guillem Anglada-Escude, with Queen Mary University of London.
The discovery gives scientists the closest possible extrasolar planet to try to directly image, though current instruments are not yet good enough yet to separate out light reflecting off Proxima b from light radiating from the parent star.
With even a pixel of light from the planet, scientists can attempt to ferret out whether Proxima b has an atmosphere, water or other chemicals tied to life, such as methane.
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As the closest star to Earth, Proxima Centauri already has received a lot of telescope time, including a cursory scan by astronomers looking for radio signals from potential extraterrestrial civilizations.
The star is only visible from Earth's southern hemisphere, so the SETI Institute has not been able to use its current array of telescopes in California to look for ET on Proxima Centauri, astronomer Seth Shostak told Seeker.com.
A decade ago, the SETI Institute used Australia's Parkes Observatory for a search for extraterrestrials, beginning with a quick look at the sun's nearest neighbors.
"We didn't hear anything, but maybe the Alpha Centaurians were being coy that day," Shostak said.
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