One hundred and one years ago this October, a Scottish astronomer named Robert Innes pointed a camera at a grouping of stars near the Southern Cross, the defining feature of the night skies above his adopted Johannesburg. He was looking for a small companion to Alpha Centauri, our closest neighboring star system.
Hunched over glass photographic plates, Innes teased out a signal. Across five years of images, a small, faint star moved, wiggling on the sky. It shifted just as much as Alpha Centauri, suggesting its fate was intertwined with that binary system. But this small star was closer to the sun than Alpha. Innes suggested calling it Proxima Centauri, using the Latin word for “nearest.”
The dim red star soon entered the collective imagination, inspiring dreams of interstellar travel. Gravity has linked the star to the Alpha Centauri system, but our culture of science and storytelling has linked it to the solar system. Today, that link will grow stronger, when an international team of astronomers announces that this nearest of stars also hosts the closest exoplanet, one that might look a whole lot like Earth.
No one will ever find a closer alien world than this. This is it. No other faint, cool stars lurk in the abyss between the Alpha Centauri system and our solar system. In a way, the first discovery of a possibly habitable planet in our backyard is also a final discovery. In the hunt for our cosmic neighbors, this planet is as good as it gets.
For now, the planet is unceremoniously called Proxima Centauri b. It zips around its namesake star every 11.2 days, and is likely locked in place—like the moon, which always shows the same face to Earth. It’s at least 1.3 times as massive as our planet, and based on its likely size, astronomers think it is rocky. Its home star is only .15 percent as bright as the sun, so the planet isn’t as scorched as you might expect, given its tight orbit. Instead, it circles around in a sweet spot that might allow for liquid water on its surface. “It’s in about the same position in the habitable zone of Proxima as the Earth is in the habitable zone around the sun,” says James Kasting, an astronomer at Penn State who was not involved in the new finding.
Viewed from the planet’s surface at midday, Proxima Centauri would be a dark red, and the planet’s atmosphere, if it has one, could make its sky a mixture of purples and oranges instead of blues. If plants have colonized its surface, their leaves might be crimson instead of green. The planet’s discoverers have nicknamed it “Pale Red Dot.”
Though this biographical sketch is intriguing, scientists caution that they don't know exactly what the planet looks like. “At the moment, we have no idea what the makeup of the planet is. Does it have a magnetic field to shield itself? Does it have an atmosphere? We have no idea,” cautions James Jenkins of the University of Chile, another Scotsman working in the Southern Hemisphere and a member of the discovery team.
Still, studies of several hundred other distant worlds can help scientists make some informed guesses. If it is tidally locked the way the moon is, the huge difference in temperatures from day to night could make it difficult to maintain liquid water around the whole planet, Kasting says. It's possible water flows in a ring around the planet's day side; if so, that extra mass could stretch the planet into a slightly oval shape. But it's also possible that a thick atmosphere or a deep ocean could distribute heat around the whole world.
Proxima Centauri showers the planet in radiation that could blow away its atmosphere and water, but it might have formed farther from its star than it is now, at a distance that would have protected the planet during the star's turbulent youth. The presence of water and an atmosphere depends on the planet's history, and that's a story we don't know yet. Scientists disagree about the star's age, but think it is probably around 4.3 billion years old, a few millennia younger than the sun, and the Earth.
Could life have arisen there in that span of time? People who are alive today might live to find out. In the next two decades, a new class of giant telescopes will be built atop barren mountains, where they will begin soaking up starlight. Observatories like the European Extremely Large Telescope and the Thirty Meter Telescope could scrutinize the wee world’s atmosphere, separating the starlight that passes through it to find telltale chemical signs of life.
Or, we could find life when an armada of papery space sails arrives at the planet’s doorstep to take pictures. The $100 million Breakthrough Starshot initiative, an ambitious project funded by Russian billionaire Yuri Milner, aims to use a giant laser to push disc-shaped probes to Alpha Centauri within a generation’s time. The probes would carry cameras with enough megapixels to put Innes’ photographic plates to shame. They would not only be able to see the atmosphere, but maybe look past it, to the surface of this new world, even to its continents, if it has them.
Despite its name, Proxima Centauri is not in close proximity. It’s 25 trillion miles away, more than 270,000 times farther than the sun. Light travels at the ludicrous speed of 186,000 miles per second, but it still takes eight minutes to cross the distance from the sun to Earth. If you looked at Proxima Centauri tonight, you would be seeing photons that escaped its surface sometime in June 2012. Yet in cosmic terms, the star is right next door.
“Until this discovery, everything has been sort of statistical,” says Paul Butler of the Carnegie Institution for Science, a member of the team that discovered the new planet. Butler has been looking for exoplanets for three decades. During that time, astronomers have expanded our lonely nine-planet system to a cosmos chockablock with worlds.
A number of instruments—including NASA’s Kepler Space Telescope—have been used to show that most stars have planets, and statistical analysis suggests some 30 percent of them are possibly livable, at Goldilocks distances just right for water to stay liquid and, just maybe, for life to burble in it. But without propulsion systems ripped from the pages of science fiction, most of these stars are too far for humans to ever reach, even with robotic probes.
“With this discovery, you go from fuzzy mathematical statistics to a flashing neon sign: The very nearest star has a potentially habitable planet,” Butler told me.
To find the planet, the Proxima team used the radial velocity or “wobble” method, a way of detecting a faint spectral shift caused by a planet’s gravity. If the planet tugs the star ever so slightly toward Earth, its light will look bluer; if it pushes the star a tiny bit away from Earth, its light will look redder. The HARPS spectrograph, mounted on a 3.6-meter telescope at La Silla Observatory in Chile, is the best instrument in the world for measuring these faint color changes, which is why the Proxima team used it.
All the way back in 2013, Guillem Anglada-Escudé of Queen Mary University London said he noticed promising signs of a wobble. He decided to launch the Pale Red Dot campaign to find the planet once and for all. Anglada- Escudé and his team analyzed several years of existing HARPS data, plus a Proxima archive from a different spectrograph. Every clear night from Jan. 18 to April 1 of this year, they turned HARPS toward Proxima about a half hour before sunrise. Then they combined the new and old data, which revealed a crystal-clear wobble, showing a planet orbiting every 11.2 days.
It’s a much more convincing signal than a previous putative planet at this star system. In 2012, a Swiss team announced a blockbuster finding of a planet orbiting the smaller sunlike star, Alpha Centauri B. But during follow-up observations, the signal evaporated, and with it astronomers’ hopes of a nearby world.
This time, the planet-hunters knew they’d found the real deal after only two weeks of HARPS time, says team member Cristina Rodríguez-López of the Institute of Astrophysics of Andalusia in Spain.
“Guillem sent us what was coming out, and I wrote him and said ‘I would like to print this and put it as a poster in my office,’” she says. “You could see it so clearly.”
That clarity is despite a lot of noise from the host star. Though Proxima is dim, it’s constantly firing off X-rays and stellar flares, activity that can easily be mistaken for a wobble, says Mike Endl, an astronomer at the University of Texas at Austin. He started studying Proxima in 2000, and his data set was one of the keys to proving the planet exists.
“When we started observing Proxima, there were only a few exoplanets known. We live in a completely different time now,” he says. I asked Endl if he ever thought it would be possible, when he completed his Ph.D two decades ago, that we would have not only a bumper crop of exoplanets, but a realistic chance of sending probes to one around the nearest star.
“No. No. No. No, I never thought that would be possible in my lifetime,” he said. Now, he thinks Proxima has a few planets that might be worth visiting.
The team noticed a second signal, deep within the data, but they can’t tell yet whether it’s a planet. Its wobble is somewhere between 60 and 500 days, so further observations will help pin it down, says Jenkins, the Scottish astronomer.
“Small planets seem to come along with more brothers and sisters,” he says. “Small stars come quite packed with planets, so that motivates us to really pin down this signal.”
As the Pale Red Dot team members sift through more data, astronomers around the world will race to double-check their findings, and examine the star with different telescopes and spectrographs. The community is collectively holding its breath for a transit, a view of the planet crossing in front of the star. There’s only a 1.5 percent chance the Proxima Centauri system’s geometry is arranged in such a way that we could see this. But if so, we might be able to look at its atmosphere.
“That’s where the game becomes really exciting,” Butler says. “That’s when we go from ‘potentially habitable’ to something that has life.”
For centuries, if not millennia, thinkers of various stripes have wondered whether other stars played host to planets. But it took us until 1995 to find one, and another 20 years to find more than a handful. In one sense, this is a discovery that spans centuries, and as such it’s a fitting preface to our actual exploration of these worlds, which may take just as long.
Visiting the Proxima planet, with a probe or in person, would be a multi-generational effort. Decades from now, stargazers might consider the discovery of Proxima Centauri b a turning point that transformed the way we look at the stars. No longer an anonymous collection of glitterings in the night sky, the stars were now a destination, beckoning us to a future we’d only dreamt about.
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