Pale blue dot, meet pale red dot.
Astronomers have spent decades scouring the skies looking for Earth-sized planets around distant stars. And now they've found one, sitting smack in the habitable zone of our nearest stellar neighbor.
Proxima b, described Wednesday in the journal Nature, could be one of the first planets where humans might find life outside our solar system.
"It's the closest star. It has a potentially habitable world. I just think it's amazing," said Cornell astrophysicist Lisa Kaltenegger, who was not involved in the discovery. "This is just such a great, exciting time to live in because we'll figure out how we fit into all of this — and hopefully, also, if we're alone in the universe."
The discovery comes four months after the announcement of Breakthrough Starshot, an initiative to build and send tiny spacecraft to the nearest star system within the coming decades. Now the project has a tantalizing planetary target.
"The technology today is sufficient to begin thinking about these things," said Pete Worden, Breakthrough Starshot's executive director and the former head of NASA's Ames Research Center. "We are really excited, and, to use the U.S. term, pumped, about this discovery. We're on our way."
Proxima b orbits Proxima Centauri, the third wheel to the binary star pair known as Alpha Centauri AB. As its name suggests, Proxima Centauri is the closest star to our solar system, sitting a relatively close 4.2 light-years away. But as an M-dwarf — a dim, red, low-mass star — it can't be seen from Earth with the naked eye. It has just 12% of the sun's mass and 0.15% of its luminosity.
The newly discovered planet is estimated to hold at least 1.3 Earth masses, which means it’s probably a rocky world like our own. And though it lies a mere 4.3 million miles or so from the surface of its star — about nine times closer than Mercury is to the sun — Proxima Centauri is so dim that water, theoretically, could remain stable on the planet’s surface, assuming there’s a protective atmosphere.
Scientists discovered Proxima b thanks to what's known as the radial velocity method, which takes advantage of the Doppler effect. As a planet moves around its star, it tugs just a little bit, causing the star to wobble back and forth. When the wobble brings the star closer to us, the light reaching us is squeezed, making it bluer. When the star is pulled slightly away from us, the light is stretched, making it redder. That color shift allows astronomers to determine the mass of the planet tugging on it.
In some ways, this is simpler to do with M dwarfs; because of their size, they're more easily pulled this way and that by their planets. And since Proxima b completes an entire orbit in just 11.2 Earth days, the tugging should be easy to pick out. Plus, as the star closest to us, Proxima Centauri is one of the best-studied red dwarfs to date.
And yet it took years to find the planet. That's partly because M dwarfs are very noisy, variable stars whose activity can drown out the Doppler signal. Observations made years earlier revealed hints of the planet but could not decisively prove its existence.
"The data collected for this research [span] 16 years," said study coauthor Pedro Amado, an astrophysicist at the Institute of Astrophysics of Andalusia in Granada, Spain. "The first datasets did not show anything, but as our analysis technique improved and we added data from more precise instruments, a periodic signal started to show up."
This year, a team of astronomers known as the Pale Red Dot campaign studied Proxima Centauri with the European Southern Observatory's HARPS instrument, a spectrograph installed on the 3.8-meter telescope at La Silla in Chile. Using other telescopes, they monitored the star's brightness to make sure that its own variability wasn't producing the tantalizing exoplanetary signal.
They found that, at times, Proxima Centauri was moving toward or away from Earth at roughly 3 mph — a typical human walking pace. An unseen planet, they realized, must be tugging on this star.
"All the pieces together is what allows us to be very, very sure that we have it right this time," said study leader Guillem Anglada-Escuse, a planet hunter at the Queen Mary University of London.
As it happens, there was another signal mixed into the data — one that might hint at the existence of a larger, more distant planet circling Proxima Centauri.
Could life exist on Proxima b? There are several unknowns that make it impossible to say right now, according to scientists. The planet is tidally locked to Proxima Centauri, so one side may permanently face the star while the other remains shrouded in darkness. But if there is an atmosphere, it should redistribute heat across the surface, the researchers said.
As an M dwarf, Proxima Centauri is prone to frequent flares and bursts of X-rays that would send down 400 times the X-ray flux that Earth receives from the sun, according to the study. Those X-rays could eat away at the atmosphere, even if one exists.
And it's also not clear whether water could have survived on the planet over the eons. The answer depends on how violent the star was in the past and where the planet originated — both of which remain a mystery.
"This is the biggest question mark for the question of whether it's an Earth-like planet or not," said study coauthor Ansgar Reiners of the University of Goettingen in Germany. "This will be subject to further studies."
Fortunately, Proxima Centauri and its planetary companion are so close that it should be relatively easy to start probing these questions. Some teams already have started.
If there were life on this planet, it probably survived either underground or deep within its hypothetical oceans, said Kaltenegger, the director of Cornell's Carl Sagan Institute, which is dedicated to the search for habitable worlds. But there's a chance that organisms evolved to handle the extreme radiation that may reach the surface, she added, perhaps by using biofluorescence.
The discovery signals a shift in the hunt for exoplanets, from broad surveys such as NASA's Kepler and K2 missions toward in-depth profiles of individual planets, said MIT astrophysicist Sara Seager, who was not involved in the work.
"Exoplanets," she said, are "the gift that keeps on giving."
There's only a 1.5% chance that Proxima b's transiting across its star's surface is visible from Earth, which means researchers will probably not be able to study its atmosphere for a while. But as ever more-powerful telescopes come online, it may be possible to take images of this nearby star system.
Plans to visit Proxima b remain an exceedingly distant prospect. With current technology, it would probably take tens of thousands of years to get there — and more than four years just to send a message back.
In April, physicist Stephen Hawking and Russian billionaire Yuri Milner announced the $100-million Breakthrough Starshot program that aims to build nanosatellites capable of accelerating to 20% of the speed of light. At that rate, it would take roughly 20 years to reach our nearest neighbors — and it could take much longer for that technology to be designed and built.
In the meantime, scientists will probably have their hands full studying Proxima b from Earth.
"Our picture of the galactic neighborhood has changed," Reiners said. "We have a new neighbor, and there will likely be a whole branch of science aiming to understand its nature."
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7:20 p.m.: This article was revised throughout for additional details and for clarity.
2:55 p.m.: This article was updated throughout.
11:50 a.m.: This article was updated with a comment from Pete Worden.
10:40 a.m.: This article was updated with a comment from astrophysicist Pedro Amado and additional information from other members of the research team.