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A planet orbits around the closest star to our Solar System — and it may be habitable

Proxima Centauri has an Earth-sized neighbor

An artistic rendering of the surface of Proxima b.
ESO/M. Kornmesser

Proxima Centauri, the closest star to our Solar System, has a planet that may have just the right features to support life on its surface. That makes this world the closest possible residence for alien life outside our Solar System — and perhaps the most exciting exoplanet ever found.

It's perhaps the most exciting exoplanet ever found

The planet, called Proxima b, is at least 1.3 times the size of Earth and is most likely rocky, according to research published today in the journal Nature. The planet is also super close to Proxima Centauri, located around 4.3 million miles away from the star, or just 5 percent of the distance from the Earth to the Sun. As a result, Proxima b’s orbit is a mere 11.2 days — about an eighth the length of Mercury’s orbit.

Proxima b’s unusually close orbit is actually an ideal location for the planet. Proxima Centauri is a cool red dwarf star, much smaller and fainter than our Sun. So its "habitable zone" — the region around a star where temperatures are just right for liquid water to exist on a planet’s surface — is much closer to the star than it is to our Sun. And Proxima b is squarely in this region. That’s important, since liquid water is a key ingredient for life here on Earth; if Proxima b has liquid water, it’s possible the planet could support life, as well. And given how close Proxima b is to us, the planet just became the top candidate in the search for alien life outside of our Solar System. Should we ever want to explore an exoplanet, we’ll probably want to visit this one.

"It’s really exciting because the proximity, it can’t get any better," says Michael Endl, a research scientist at the University of Texas at Austin who worked on the study.

An artistic rendering of Proxima b with its host star Proxima Centauri.
ESO/M. Kornmesser

Astronomers with the European Southern Observatory discovered this planet after continuously observing Proxima Centauri during the first half of 2016 — part of a program called the Pale Red Dot campaign. They didn’t see the planet directly, though. Instead, they figured out the planet existed based on how Proxima Centauri moves. As Proxima b orbits around the star, the tiny world’s gravity tugs on Proxima Centauri, causing the star to wobble. By studying these wobbles, the ESO astronomers were able to learn a lot about the planet they couldn’t see.

Astronomers figured out the planet existed based on how Proxima Centauri moves

Today’s study was inspired by Guillem Anglada-Escudé, an astronomer at Queen Mary, University of London, who found hints in data of a possible planet orbiting Proxima Centauri. Unfortunately that data was inconclusive. To know for sure, Anglada-Escudé and his team observed Proxima Centauri every single night from mid-January through the end of March, using the ESO 3.6-meter telescope in La Silla, Chile. Their main tool was the HARPS spectrograph — an instrument that can pick up small variations in a star’s light, which indicate if the star is moving closer to Earth or father away. And the light fluctuations of Proxima Centauri pointed to the existence of a nearby planet, a finding that was backed up by additional observations from telescopes around the world. "When you’ve got the exact same signal from different instruments, it rules out that this is an instrumental error," says Paul Butler, an astronomer at the Carnegie Institution for Science who worked on the Pale Red Dot campaign. "We’re confident this was real."

Though this planet orbits within Proxima Centauri’s habitable zone, its climate is probably very different from ours. Chances are Proxima b doesn’t have seasons, and since it’s so close to its host star, the world gets blasted with a lot more radiation than we get on Earth. Still, astronomers cannot rule out the possibility of water on the planet’s surface. It’s possible that liquid water is present in regions that are always facing the Sun or along some kind of tropical belt.

The ESO 3.6-meter telescope at the La Silla Observatory in Chile, along with images of Proxima Centauri (bottom right) and nearby double stars Alpha Centauri AB (bottom left).
Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani

Given this potential, the next step is to get a better look at Proxima b. Specifically, astronomers want to know what’s going on in the planet’s atmosphere. The presence of certain gases around the planet could indicate if there really is liquid water — or even biological life — on the surface below. "You want to look for the molecules that Earth has that are supportive of life, like oxygen, water, carbon dioxide, and maybe methane," says Alan Boss, an observational astronomer at the Carnegie Institution for Science, who was not involved with the study. "Those are the so-called bio-signature molecules."

We want to know what’s going on in the planet’s atmosphere

We may be able to get some answers soon if Proxima b is a transiting exoplanet — meaning it passes between Proxima Centauri and Earth. If that’s the case, astronomers could measure how the planet dims Proxima Centauri when passing in front of the star. That could tell us the planet’s radius and density. And astronomers might be able to study the gases around the planet by seeing how the star’s light filters through the atmosphere. But the likelihood that Proxima b is a transiting planet is very small. Plus, Proxima Centauri is a flare star, meaning its brightness changes in unpredictable ways. Any transiting exoplanets will be hard to find.

That’s why the best way to know what’s in Proxima b’s atmosphere is to image the planet directly. And that’s going to be tough. Even though Proxima Centauri is relatively faint, the star’s light easily drowns out the nearby planet, making it hard to see the world from Earth. The only way to directly image Proxima b is to somehow separate the star’s light from the light reflecting off the planet. It’s a daunting task given how close Proxima b is to the star. Future ground and space-based telescopes may be able to pull it off, but those are decades away. And even then, the images they get won’t be super detailed. "We won’t be seeing espresso bars on the beach and aliens waving at us." says Butler.


But the researchers are hopeful that their discovery will motivate others to do follow-up observations and study this planet in greater detail. And Proxima b will no doubt provide extra encouragement to people hoping to visit this star system someday — especially those involved in the Starshot project. That’s the initiative aimed at sending a small spacecraft near Proxima Centauri using a giant laser. Theoretically, such a vehicle could reach the star in just 20 years, rather than tens of thousands of years it would take using modern propulsion. "That’s similar to the time scales of our Solar System probes," says Endl.

Proxima b will no doubt provide extra encouragement to people hoping to visit this star system

Still, Starshot is decades away if it’s even possible. So we may not find out much more about Proxima b for a while. But Butler says the discovery is incredibly exciting given the planet’s potential — and what this world means to the search for extraterrestrial life. "We’re finding more and more super Earths and even things that are closer to Earth mass all the time," says Butler. "There are a lot of stars that have potentially habitable planets." This kind of world orbiting the nearest star in the sky suggests these types of planets are pretty common. And that means Earth may not be the only place in the Universe where life thrives.