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.
The planet Proxima b, described in the journal Nature, could be one of the first planets outside our solar system where humans could go searching for life.
“It’s the closest star, it has a potentially habitable world – I just think it’s amazing,” said Cornell astrophysicist Lisa Kaltenegger, director of Carl Sagan Institute, who was not involved in the study. “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.”
Proxima b circles Proxima Centauri, the stellar third wheel to the binary pair of stars known as Alpha Centauri AB. It’s the closest star to our solar system, sitting just 4.2 light-years away, but as an M-dwarf – a dim, red, low-mass star – it can’t be seen with the naked eye.
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 while it circles roughly 4.3 million miles from the surface of its star (taking just 11.2 Earth days to complete a revolution) Proxima Centauri is small and dim, with just 12 percent of the sun’s mass and 0.15 percent of its luminosity. At that distance, it’s still temperate enough for water, theoretically, to remain stable on the 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 on the star just a little bit, causing it 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. By using that color shift to track that wobble, astronomers can determine the mass of the planet tugging on it.
In some ways, this is simpler to do with M dwarfs; because they’re much smaller than stars like our sun, they’re also more easily pulled this way and that by their planets. And since Proxima b is so close to the star, it circles very frequently and should be easy to pick out. That’s not to mention that Proxima Centauri, as the closest star to us, is arguably the best-studied red dwarf to date.
And yet it took years to find the planet. That’s in large part because M dwarfs are very noisy, variable stars, whose stellar activity can drown out that telltale Doppler signal. Earlier observations made years before revealed hints of the planet but could not decisively prove its existence.
Earlier this year, a team of astronomers known as the Pale Red Dot campaign studied Proxima Centauri using the European Southern Observatory’s HARPS instrument, a spectrograph installed on the 3.8-meter telescope at La Silla in Chile. Using other telescopes, including the ASH2 telescope at the San Pedro de Atacama Celestial Explorations Observatory and the Las Cumbres Observatory Global Telescope Network, they monitored the star’s brightness to make sure that its variability wasn’t producing the tantalizing exoplanetary signal.
They found that, at times, Proxima Centauri was moving toward and away from Earth at about 3 miles per hour – 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,” Guillem Anglada-Escude of the Queen Mary University of London said at a news briefing on his team’s findings.
As it turns out, there was also another signal mixed into the data – one that might potentially 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, scientists said. The planet is tidally locked to Proxima Centauri; one side of the planet may permanently face the star while the other half remains shrouded in darkness. But that shouldn’t be a deal-breaker for the existence of life: If there is an atmosphere, it should redistribute heat across the surface, the researchers said.
But as an M dwarf, Proxima Centauri is also far more mercurial a star than our own, 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. (In the news briefing, the study authors said they’d been conservative and overstated the risk; it was more likely on the order of 100 times.) And X-rays could eat away at the atmosphere, even if it does exist.
And it’s also not clear whether water could have truly survived on the planet over the eons; it depends on how violent the star was in the past, and where the planet actually 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 co-author Ansgar Reiners of the University of Gottingen. “Whether there’s water or not, we do not know; that entirely depends on the formation, on the history of the planet. And this will be subject to further studies.”
The good news: Proxima Centauri is so close that it should be relatively easier to start probing these questions. Some teams have already started.
If life were on this planet, it probably survived either underground or deep within its hypothetical oceans, Kaltenegger said. But perhaps it could have eventually evolved to handle the extreme radiation that may reach the surface, she added, perhaps by using biofluorescence (an idea she and a colleague flesh out in a paper posted to arXiv).
Others expressed mixtures of caution and hope.
“Personally I’m holding out for the Earth twin,” Sara Seager, an MIT astrophysicist who was not involved in the study, said in reference to the search for an Earth-like planet around a sunlike star.
The next step is to try and catch the planet transiting across the star’s surface, she added. Seager is one of the scientists working on TESS, NASA’s Transiting Exoplanet Survey Satellite, which will launch in 2017 to hunt for nearby exoplanets.
But in some ways the discovery of Proxima b heralds a new era, Seager added – one that moves from vast surveys like NASA’s Kepler spacecraft (as well as TESS) and toward individual, in-depth profiles of particularly interesting planets.
TESS may help identify many more interesting red dwarfs, which are the most common type of star found near our solar system. In fact, they’re so plentiful that some scientists say these cool, dim stars might actually have a greater chance of hosting a life-friendly world than sunlike stars (though again, it’s unclear what their variable nature may have on planets).
In the meantime, Proxima b presents astronomers with an ideal opportunity to study one of these stars (and the planet that circles it). There’s only a 1.5 percent chance that the planet transits in front of Proxima Centauri, which means researchers will probably not be able to study its atmosphere for a while. But as ever-more-powerful telescopes being to come online, it may be possible to actually take images of this nearby star system.
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