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Proxima Centauri b
An artist’s conception shows Proxima Centauri b orbiting its parent star, a red dwarf, with the two other stars of the Alpha Centauri system in the far background. (Credit: M. Kornmesser / ESO)

The rumors you’ve heard are mostly true: Scientists say the star that’s closest to our solar system has a planet that could be at the right temperature for liquid water and life.

The star is called Proxima Centauri, a red dwarf that’s part of the Alpha Centauri system, 4.2 light-years away. The planet is called Proxima Centauri b, and it’s a terrestrial world whose existence has now been confirmed after 16 years of study.

It’s not yet known whether Proxima b has an atmosphere, or liquid water. But the computer models don’t rule out the possibility. That would make it the closest known exoplanet – and the closest known exoplanet with the potential for life.

As such, it could be the nearest haven for humanity in case things go horribly wrong in our own solar system. And it just so happens that scientists are working on a robotic mission to the Alpha Centauri system: The Breakthrough Starshot initiative received a $100 million kick-start from Russian billionaire Yuri Milner in April.

“We expect either to characterize it, if we get lucky, or maybe visit it in a couple of centuries,” Guillem Anglada-Escude, an astronomer at Queen Mary University of London, told reporters.

Word about Proxima b started leaking out earlier this month, sparked by a report in the German magazine Der Spiegel. Anglada-Escude and his colleagues held off on confirming Der Spiegel’s claims, but today the details came out in the journal Nature.

The planet is at least 30 percent more massive than Earth, and makes one circuit around Proxima Centauri every 11.2 Earth days. Those figures imply that the planet comes as close as 4.6 million miles to its parent star.

If Proxima Centauri were like our sun, the planet would be blazingly hot. But because red dwarfs are much dimmer, scientists say Proxima b should have an Earthlike range of temperatures, from 22 degrees below zero Fahrenheit (-30 degrees Celsius) on its dark side to 86 degrees F (30 degrees C) on its light side.

That would make it possible for water to exist in liquid form, which astrobiologists regard as a key requirement for life.

The scientists’ calculations assume that the planet has an Earthlike, heat-trapping atmosphere, and that the planet’s rotation is tidally locked so that one side is constantly facing the star.

Because the scientists have to make such assumptions without the data to back them up, they also have to hedge their bets as to the planet’s habitability. As a result, the rumors claiming that Proxima b could be inhabited by, say, “Avatar”-like space travelers aren’t supported by the facts.

Proxima Centauri b terrain
An artist’s conception shows a view of the surface of the planet Proxima b, with the red dwarf Proxima Centauri near the horizon. (Credi:t: M. Kornmesser / ESO)

“Do we know anything about the atmosphere or the water? We don’t,” said the University of Göttingen’s Ansgar Reiners, a co-author of the study. “We have no further information about this planet, but of course we can calculate probabilities, whether there is an atmosphere or water. This has been done intensively as well – and from what we know, at least, there is a non-zero probability that there is an atmosphere.”

He said the biggest question is whether the planet has liquid water on its surface, like Earth, or is totally dry. The answer depends on the details of the planet’s formation, and also whether its reservoirs of water were fed by comets and icy asteroids, as was the case for Earth.

Reiners acknowledged that Proxima b receives much more high-energy radiation than Earth does. That comes with the territory for red dwarfs, also known as M-dwarfs, which are the most common stars in our Milky Way galaxy.

The radiation question led Artie Hatzes of the Thuringian State Observatory to lay out some caveats in a commentary also published by Nature today. It’s not yet known whether Proxima b has a protective magnetic field, for example. The X-ray flux from Proxima Centauri may have eroded the planet’s atmosphere, or hindered the development of life, Hatzes said.

“Until we understand what makes a planet habitable, it is better to say that Proxima Centauri b lies in a temperate zone (the right temperature) rather than a habitable zone (the right conditions to support life),” he wrote.

Habitable zone for Proxima Centauri b
An infographic compares Mercury’s orbit (left) with the orbit of the planet around Proxima Centauri (right). Proxima Centauri is smaller and cooler than our sun, and the planet orbits much closer to its star than Mercury. Click on the image for a larger version. (Credit: M. Kornmesser and G. Coleman / ESO)

Anglada-Escude said he and his colleagues think Hatzes’ assessment is too pessimistic. “For sure the difference between Earth and this planet is that this is receiving much more high-energy radiation, but we don’t think it’s a showstopper here,” he said.

Proxima b’s detection demonstrates how exacting the planet-hunting business has become.

The initial rounds of observations were made by two telescopes at the European Southern Observatory in Chile between 2000 and 2014. Those readings pointed to a faint “Doppler wobble” in the spectral signature of the light from Proxima Centauri.

The back-and-forth effect suggested that the gravitational influence of a planet might be pulling the star to and fro, but the effect was so subtle that astronomers couldn’t confirm that a planet existed.

Then, from January to March of this year, the astronomers conducted an intensive 60-night round of “Pale Red Dot” observations, using the ESO’s High Accuracy Radial Velocity Planet Searcher, or HARPS.

When they combined the older data with this year’s Pale Red Dot data, they came up with the hoped-for confirmation. They made additional spectroscopic observations to make sure the effect they were seeing was caused by a planet’s influence and not by stellar activity.

“The significance of the detection goes sky-high,” Anglada-Escude said.

Telescope and stars
This picture combines a view of the southern skies over the ESO 3.6-meter telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower right) and the double star Alpha Centauri AB (lower left) from the Hubble Space Telescope. (Credits: Y. Beletsky / LCO / ESO / ESA / NASA / M. Zamani)

Now the Pale Red Dot campaign is finished, and astronomers will have to use other methods to learn more about Proxima b.

There’s a slight chance that NASA’s James Webb Space Telescope, due for launch in 2018, could gather data about the planet’s atmosphere. But Reiners said a mission customized for Proxima b is more likely to provide insights.

“Physics allows you to actually image the planet itself, and this is certainly something that will be going on over the next years, the next decades,” he said. “We are quite far from it right now.”

This week’s announcement could heighten interest in the Breakthrough Starshot project as well. But because of the distances involved, the Starshot nano-probes won’t get to the Alpha Centauri system for decades, even under the best-case scenario. And using current propulsion technology, it would take tens of thousands of years to send humans to Proxima B.

Does that sound like a long time? In his commentary, Hatzes observed that if life forms exist on Proxima b, there’s no need for us to hurry on their account.

“Interestingly, M-dwarf stars are long-lived, and Proxima Centauri will exist for several hundreds or thousands of times longer than the sun,” he wrote. “Any life on the planet could still be evolving long after our sun has died.”

Anglada-Escude and Reiners are among 31 authors of the paper published in Nature, titled “A Terrestrial Planet Candidate in a Temperate Orbit Around Proxima Centauri.”

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