The Earth Next Door
Astronomers find an exoplanet that could be habitable—and it’s as close to us as it could possibly be
By Lee Billings on August 24, 2016
It was just over 20 years ago—a blink of a cosmic eye—that astronomers found the first planets orbiting stars other than our Sun. All these new worlds were gas-shrouded giants like Jupiter or Saturn and utterly inhospitable to life as we know it—but for years each discovery was dutifully reported as front-page news, while scientists and the public alike dreamed of a day when we would find a habitable world. An Earth-like place with plentiful surface water, neither frozen nor vaporized but in the liquid state so essential to life. Back then the safe bet was to guess that the discovery of such a planet would only come after many decades, and that when a promising new world’s misty shores materialized on the other side of our telescopes, it would prove too faraway and faint to study in any detail.
Evidently the safe bet was wrong. On Wednesday astronomers made the kind of announcement that can only occur once in human history: the discovery of the nearest potentially habitable world beyond our solar system. This world may be rocky like ours and whirls in a temperate orbit around the Sun’s closest stellar neighbor, the red dwarf star Proxima Centauri just over four light-years away. Their findings are reported in a study in the journal Nature.
Although technically still considered a “candidate” planet awaiting verification, most astronomers consulted for this story believe the world to be there. Scarcely more than the planet’s orbital period and approximate mass are known, but that is enough to send shivers down spines. Proxima Centauri shines with only about a thousandth of our Sun’s luminosity, meaning any life-friendly planets must huddle close. The newfound world, christened “Proxima b” by scientists, resides in an 11.2-day orbit where water—and thus the kind of life we understand—could conceivably exist. And it is likely to be little more than one-third heavier than Earth, suggesting it offers a solid surface upon which seas and oceans could pool. In a feat of discovery that could reshape the history of science and human dreams of interstellar futures, our species has uncovered a potentially habitable planet right next door.
“Succeeding in the search for the nearest terrestrial planet beyond the solar system has been an experience of a lifetime, and has drawn on the dedication and passion of a number of international researchers,” says the study’s lead author Guillem Anglada-Escudé, an astronomer at Queen Mary University of London who spearheaded the observations. “We hope these findings inspire future generations to look beyond the stars. The search for life on Proxima b comes next.”
For some, Proxima b is a fitting capstone to the astronomical revolution that began when the first exoplanets were found. “For more than 20 years the history of exoplanets has been defined by studying stars tens to hundreds of light-years away, when the Holy Grail—a small, rocky, potentially habitable planet—was just waiting to be discovered around our closest neighbor,” says astronomer Debra Fischer, a veteran planet hunter at Yale University who has led independent surveys of the Alpha Centauri system. “When we launch our first robotic explorers to stars beyond the solar system, we know where we should send them!”
Caleb Scharf, director of astrobiology research at Columbia University, says the the new planet represents “a tremendously important psychological moment for the field, as well as for our species. Discovering who lives in the house next door can change perspectives and priorities—and that's what Proxima b will do.”
Although it is barely more than four light-years away, Proxima Centauri is too faint to be seen with the naked eye. It drifts at the outskirts of the twin Sunlike stars Alpha Centauri A and B, forming a stellar trio that appears as a single gleaming point in the southern constellation of Centaurus. The tiny star is fated to slowly slip further away from us on the Milky Way’s celestial currents, but will remain the closest one bearing a planet for perhaps the next 40,000 years.
“For the first time, we have an exoplanet within our reach that could be a host to biological organisms,” says study co-author Mikko Tuomi, an astronomer at the University of Hertfordshire. “And that makes Proxima b not only one of the most fascinating discoveries astronomers have made but also one of the most important that can be made.”
Even so, it is a discovery that almost didn’t happen. “People seem to think we just found the planet. But no, we have believed it was there for years,” Anglada-Escudé says. “We just had to build an argument to convince others it exists.”
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