Exoplanet hunters hit a milestone today (Aug. 24) with the discovery of the closest alien world to Earth.
Located a mere 4.22 light-years away, the alien planet, dubbed Proxima Centauri b (or just Proxima b), orbits the sun's nearest neighbor, a red dwarf star known as Proxima Centauri.
What makes the discovery even more exciting is that the newfound terrestrial planet is about the size of Earth and lies in Proxima Centauri's "habitable zone" — the range of distances where liquid water could exist on a world's surface. So it's a good candidate for powerful telescopes, or even interstellar spacecraft of the future, to study for signs of alien life. [Proxima b: Closest Earth-Like Planet Discovery in Pictures]
"It's the nearest exoplanet we will ever find," Guillem Anglada-Escudé, an astronomer from Queen Mary University of London who led the research, told reporters today (Aug. 24) during a press conference in Garching, Germany.
Looking for life
Even though the planet lies in the habitable zone, there's no guarantee that Proxima b — which is likely about 1.3 times more massive than Earth — is actually capable of supporting life.
The authors of the new study said they could not yet determine whether the planet has an atmosphere or if there's liquid water on its surface; they also don't know how the world formed. The researchers also emphasized that they did not necessarily find an "Earth-like" planet, but rather an "Earth-size" one.
And that distinction is an important one, said Bruce Macintosh, a professor of physics at Stanford University who was not involved in the research.
"In our solar system, planets [of] that size or smaller are mostly made out of rock, but in other solar systems, we don't really know," Macintosh said. "It could be Earth-like, or it could be a planet like Neptune with a large atmosphere of hydrogen gas, or it could be a 'water world' like [Jupiter's moon] Europa that's mostly water or ice with a smaller rocky core, or some weird and unimaginable combination."
But the discovery team, including Ansgar Reiners, a professor of astrophysics at the University of Göttingen in Germany,saidthat computer modeling work and observations of other star systems suggest that Proxima b likely is a terrestrial planet with a "surface you could stand on."
Still, there are other factors that might work against the possibility of life on Proxima b.
For example, red dwarfs such as Proxima Centauri are quite active stars, and Proxima b's distance from its star is just 5 percent of the distance between the Earth and the sun. So the alien planet is bombarded with anX-ray flux about 100 times greater than the flux that Earth receives from the sun, study scientists said. And it's not clear if Proxima b has a magnetic field like Earth's that would protect the exoplanet from such levels of radiation, which could be harmful to life.
Also, if it turns out the planet has no atmosphere, Proxima b would be quite cold, with surface temperatures of about minus 40 degrees Fahrenheit (minus 40 degrees Celsius), researchers said. The planet would need to have an atmosphere, and therefore a greenhouse effect, to help push temperatures up above freezing. If Proxima b does have an atmosphere, the planet might be able to host liquid water on its surface — a condition thought to be key for the existence of life, Reiner explained during the press conference in Germany. [Alien World 'Proxima b' Around Nearest Star Could Be Earth-Like (Video)]
Getting a closer view
Proxima b was discovered using the European Southern Observatory's (ESO) La Silla Observatory in Chile. The team used La Silla's High Accuracy Radial velocity Planet Searcher instrument, or HARPS, to look for regular "Doppler wobbles," or the gravitational tugs an orbiting planet induces on its parent star.
Other methods of observation are needed to settle the question of whether Proxima b is a good place to look for alien life. It might be possible to observe Proxima b transiting, or appearing to pass in front of its parent star in relation to Earth. By measuring different wavelengths of starlight passing through the atmosphere around Proxima b (if the world has an atmosphere), scientists could learn about the planet’s chemical makeup.
"If it transits, we can learn far more about the planet, including searching its atmospheres for signs of life," said exoplanet expert Sara Seager, an astronomer at the Massachusetts Institute of Technology, who wasn't involved in the new study.
But even if the planet doesn't transit — and there's just a 1.5 percent chance that it does, according to its discoverers — large, ground-based telescopes of the future might be able to study the world by direct imaging.
"It will be possible in the future to take pictures of this system with technology that is not too far away," said Reiners.
Proxima b could be an ideal target for the next generation of observatories, such as the European Extremely Large Telescope, which is now under construction in Chile, researchers said.
These telescopes, Macintosh explained, could "slice" the planet away from the star and maybe measure its spectrum to see if its atmosphere contains life-indicating elements like oxygen.
"That's probably more than a decade off, but having this system right here is a very strong motivator for making sure the extremely large telescopes have planet-imaging capability," Macintosh said.
Before too long, it might even be possible to send an interstellar spacecraft to sail past Proxima Centauri and its planet.
Earlier this year, a new, $100-million private initiative called Breakthrough Starshot launched with the goal of sending a tiny spacecraft on a 20-year journey to reach Alpha Centauri, a binary star system near Proxima Centauri. Some astronomers think all three stars are part of the same system. (Breakthrough Starshot counts astrophysicist Stephen Hawking and Facebook founder Mark Zuckerberg among its board members.)
The main goal of that project is to look for signs of life in Earth's neighboring star system, and "with today's announcement, we now know there's at least one planet that has at least some similarities to Earth," Pete Worden, the leader of the Breakthrough Starshot initiative and the former director of NASA's Ames Research Center, said at today's press conference.
The latest discovery could influence the Starshot mission, and Worden said his team hopes that within a generation it can launch its interstellar nanoprobes. [Proxima Centauri's Alien Planet Closer Than You Think - With Right Spacecraft (Video)]
Is Earth unique?
Scientists are eager to learn about the possibility of life on Proxima b to answer age-old astronomical (and even philosophical) questions, such as, "How common is life in the universe?"
Scientists have discovered more than 3,200 exoplanets to date, but most of them lie hundreds or thousands of light-years from Earth. (That's primarily because NASA's Kepler spacecraft has found about two-thirds of all known alien worlds, and Kepler searched relatively distant stars during its main mission from 2009 through 2013.)
So probing the atmospheres of many potentially life-hosting worlds — such as Kepler-186f, the first Earth-size exoplanet found in its star's habitable zone — isn't feasible, at least in the near future, said Kepler scientist Elisa Quintana, who led the team that found Kepler-186f.
That means it's hard, for now, to understand conditions like the weather, temperatures and surface features of these alien worlds — and to know whether these planets truly have the right conditions for life.
"We can only say that a planet is the right size and in the right orbit for being potentially habitable," Quintana told Space.com.
That's why finding a possibly habitable world so close to Earth is so special: Astronomers have the ability to perform follow-up studies. Indeed, the discovery of Proxima b could have huge potential for helping researchers learn about the habitability of red dwarfs, which are also known as M dwarfs, Quintana said.
"More than 70 percent of the hundreds of billions of stars in our galaxy are M dwarfs," she said. "Learning whether planets like Proxima b can retain an atmosphere — and probing that atmosphere for signs of life — would have huge implications for the abundance of life in the universe."