The Sun’s Closest Neighbor Has Its Own Earth-Sized World

“The search for life on Proxima b comes next.”

The closest star to our Sun hosts an Earth-sized planet within its habitable zone, according to exciting research published Thursday in Nature. Dubbed Proxima b, the newly discovered world is about 1.3 times the size of Earth, and orbits the red dwarf star Proxima Centauri, located only 4.2 light years away from our own solar system.

Given its dimensions and distance from its star, Proxima b could theoretically support liquid water on its surface, and perhaps even life. Of course, many follow-up studies will be required to investigate the planet's finer properties—the presence of an atmosphere or protective magnetosphere, for instance—but it's hard to overstate the significance of finding such a promising rocky planet just one stellar door down the cosmic street.

"Many exoplanets have been found and many more will be found, but searching for the closest potential Earth-analogue and succeeding has been the experience of a lifetime for all of us," said Guillem Anglada-Escudé, an exoplanet expert based at Queen Mary, University of London and lead author of the new paper, in a statement.

"Many people's stories and efforts have converged on this discovery. The result is also a tribute to all of them."

Explainer on the results of the new Nature paper. Video: Nature Video/YouTube

Indeed, this thrilling find was hard-earned after over a decade of accumulated observations, generated by dozens of scientists. Where most exoplanets are detected as they pass in front of their stars from our perspective on Earth, causing a very slight dip in stellar brightness, it's not known whether Proxima b ever occults Proxima Centauri at such a favorable angle. In fact, there's only a 1.5 percent chance that it does, so Anglada-Escudé and his co-authors used an old-school method to comb for telltale signs that the planet was there.

The first exoplanets ever discovered were identified by the extremely small gravitational tugs that they exert on their stars, a method called Doppler spectroscopy. In March 2000, researchers using a specialized instrument called the Ultraviolet and Visual Echelle Spectrograph at the European Southern Observatory (ESO) in Chile picked up the first hints that Proxima Centauri was being dragged radially at a velocity of about one meter per second, perhaps by a small planet orbiting the red dwarf star.

For nearly 15 years, these tiny tugs were observed by ESO instruments. But because red dwarfs can be highly active stars that regularly barf out X-ray and ultraviolet flares, astronomers couldn't definitively pin the star's "Doppler wobbles" on the existence of a planet.

This all changed in January 2016, with the launch of the Pale Red Dot campaign. Coordinated by Anglada-Escudé, the goal was to use ESO's High Accuracy Radial velocity Planet Searcher to closely examine Proxima Centauri, and definitively rule out other explanations for its mysterious wobbles.

Within a few months, the team had found that the tugs were occurring over a regular period, regardless of stellar activity. This data strongly suggests that a terrestrial planet is orbiting every 11.2 days, at a distance of 7.5 million kilometres (4.7 million miles), about five percent of the Earth's distance from the Sun.

Size comparison of Proxima b's orbit compared to Mercury's orbit. Image: ESO/M. Kornmesser/G. Coleman

Despite this close orbit, Proxima b is situated well within the habitable zone, because Proxima Centauri is only about 12 percent as massive as the Sun, and much dimmer.

"Once we had established that the wobble wasn't caused by star spots, we knew that that there must be a planet orbiting within a zone where water could exist, which is really exciting," said study co-author John Barnes from The Open University in the UK. "If further research concludes that the conditions of its atmosphere are suitable to support life, this is arguably one of the most important scientific discoveries we will ever make."

Along those lines, the rumor mill has been churning out theories about what this announcement would bring for several days now, and the news will be met with enthusiasm from space nerds around the world.

With several next-generation telescopes coming online within the next decade, our investigations into this potentially habitable world are just beginning. The Breakthrough Starshot mission, headed by entrepreneur Yuri Milner and physicist Stephen Hawking, aims to travel to Proxima Centauri (and its two sister stars, Alpha Centauri A and B) within a generation. Although the Starshot mission faces some daunting technical challenges, it's possible that the 21st century will yield both the discovery of another habitable world, as well as the first interstellar trip to see it up close.

Concept art of Proxima b orbiting Proxima Centauri. Image: ESO/M. Kornmesser

Proxima b is poised to be the subject of enormous scientific scrutiny, as the nearest extrasolar terra incognita ever found. "We hope these findings inspire future generations to keep looking beyond the stars," Anglada-Escudé said.

"The search for life on Proxima b comes next."