The Earth next door "Proxima b": Might be a planet better than Earth.
"There might be a planet better than Earth"
There should be worlds out there so balmy they make Earth look stale, and there are signs of one just four light years away. That's close enough to visit...
“When I was a kid, I was always looking at Alpha Centauri,” says Eduardo Bendek. One of the things he discovered about it while growing up in Chile was that our closest neighboring light had a secret: it is not one star, but two.
More than 30 years later, Benedek, now an astronomer at NASA’S Ames Research Center, suspects that his favorite celestial beacon might just be hiding another, a marvelous secret. There could be a planet orbiting one of the stars. And not just any old space rock. This could be a place so bursting with life that it makes Earth look post-apocalyptic.
And at a mere 4.4 light years away, we might feasibly develop a probe that could visit within decades. That’s precisely what a project backed by Stephen Hawking and billions of dollars now plans to do. We could catch our first glimpse of this bucolic world within a generation.
We are used to thinking small when it comes to alien life. Our list of living worlds has a sole data point, Earth, and even our convivial planet seems to have been a tricky place for life to get started. How could we expect more than a self-replicating bag of biomolecules anywhere else?
That might be too lofty a view of Earth. After all, huge areas of our planet, including the poles and deserts, are rather barren. And whole epochs of time were inhospitable to life.
Proxima b: “The Earth next door”
Red sun in the morning – every day
A planet just 30 percent more massive than Earth orbits in the habitable zone of Proxima Centauri, which is just 4.25 light years away. How Earth-like is it really?
IT’S the planet we’ve all been waiting for. Earlier this month, rumors swirled that astronomers had discovered an Earth-like planet orbiting the closest star to our own, the aptly named Proxima Centauri. Well, the planet’s real, but don’t pack your interstellar bags yet, because this alien world is probably far from homely.
The planet – Proxima b – was discovered by astronomers who spent years looking for signs of the tiny gravitational tug exerted by a planet on its star, after spotting hints of such disruption in 2013. Proxima Centauri is 4.25 light years from Earth, making it slightly closer than the binary star system of Alpha Centauri, which the Proxima star is thought to loosely orbit.
“We’ve been excited for a long time,” says Guillem Anglada-Escudé of the Queen Mary University of London, who led the discovery as part of a project called Pale Red Dot. “We’ve been hunting for this signal and confirmation of the planet for almost four years.”
The team says the planet is likely to be 30 percent more massive than Earth, although it could be bigger than that. It orbits the star at a distance of 7.3 million kilometers – less than 5 percent of the distance between Earth and the sun – making its year last just 11.2 Earth days.
You might think such a tight orbit would scorch the surface of the planet. But Proxima Centauri is a small, red dwarf star and shines much less fiercely than the sun. Standing on the surface of the planet, you’d see the star as a dull red orb, about three times as large as the sun appears from Earth. As a result, the planet sits in its star’s habitable zone, and its surface temperature may be right for it to host liquid water.
The planet is rocky, of a similar mass to Earth, and temperate – all conditions that are promising for life. But Proxima b isn’t a second Earth.
“The similarities end there,” says Anglada-Escudé. Even our knowledge of the surface temperature is fairly uncertain, ranging from a possible -33 °C to the high hundreds, depending on its atmosphere.
That’s just the average temperature. However, Proxima b and its star are probably tidally locked, so the same face of the planet always points towards the star. So one-half of the globe is in perpetual day, the other in the never-ending night. “That’s not very Earth-like,” Anglada-Escudé says.
“The planet is rocky, about the same mass as Earth, and temperate – conditions that are promising for life“Whether life could exist on such a planet also depends on the nature of its atmosphere, which we know nothing about. “The planet’s atmosphere, if it indeed exists, might be something completely different from what we are used to seeing in the solar system,” says Mikko Tuomi of the University of Hertfordshire in Hatfield, UK, who was the first to spot signs of the planet when studying archival data.
“Before we know much more about the atmosphere of the planet and its physical properties, I would be very wary,” says Brice-Olivier Demory at the University of Cambridge.
The atmosphere could be purely carbon dioxide, as Earth’s was before the emergence of life, and with a density that is anything from a Mars-like wisp to the choking clouds of Venus. A dense enough atmosphere would trap heat from the star, and potentially distribute it to Proxima b’s permanent dark side.
“That would make it possible for the planet to retain oceans in their liquid form throughout the planet’s surface,” says Tuomi. “It would thus be a very different place from the Earth, but still ‘Earth-like’ in the sense that life could exist on its surface.”
Although Proxima Centauri’s dimness provides the planet with a balmy climate, the star is prone to outbursts of harsh X-ray and ultraviolet radiation, which could damage any chance of life on the planet – X-rays hit the surface 400 times more often than those from the sun pummel Earth. A magnetic field and dense atmosphere could shield against the effect of these harmful rays. “The question is how well an atmosphere could deal with that,” says Ignas Snellen of the University of Leiden in the Netherlands. “I think the probability that this planet has life is larger than that of there being life on Mars.”
Pinning down these details might take decades (see “Spying on the neighbors“) because we don’t yet have telescopes powerful enough to see the planet directly.
No more mistakes
Astronomers will still want to turn their scopes towards Proxima Centauri – to confirm that the planet is real, and avoid a repeat of an earlier embarrassment. Despite initial excitement, the claimed discovery in 2012 of a planet orbiting neighboring Alpha Centauri B now looks to have been a mistake.
Tuomi and his colleagues have done everything they can to avoid that happening again. He first saw signs of Proxima b in 2013, when looking at data taken by the Very Large Telescope at Paranal Observatory in Chile between 2003 and 2009. “I spent weeks trying to make the signal go away, trying to show that it was caused by the star’s activity or pure measurement noise rather than a planet,” he says. But the team became increasingly convinced.
To confirm the find, the group examined data from other telescopes and in January this year began the Pale Red Dot campaign, using another instrument in Chile – the HARPS planet searcher at the La Silla Observatory. The observations lasted 60 nights, but the team was confident of a discovery after just 10 nights of data, says Tuomi. “It was as predicted by the previous observations. We knew this was going to become a year to remember for exoplanet science.”
“I think this is a very solid thing,” says Snellen. “For me personally, this is the scientific discovery of the year, maybe of the decade.”
The team also saw signs of a second potential planet around Proxima Centauri, a super-Earth with an orbit of between 60 and 500 days. If such an outer planet exists, it might be possible to observe it, says Tuomi.
The discovery of Proxima b will be a boost for Breakthrough Starshot, an ambitious project announced earlier this year to send a small spacecraft capable of reaching the Alpha Centauri star system in just 20 years. Funded to the tune of $100 million by Russian billionaire Yuri Milner, the mission will need billions more to actually happen. “I’m not sure it will work, but I think it’s worth trying,” says Anglada-Escudé.
“It shows that there is a target we can go and visit,” says Avi Loeb of Harvard University, who heads the project’s scientific advisory board. “We are living in very exciting times now, and we are very fortunate that the nearest star happens to have a habitable planet.”
For now, we can only dream of what awaits us on Proxima b. “When I think about it, I think something like Mars, as it is under a red sun. A planet with polar caps, reddish on the surface, maybe with a thin atmosphere,” says Anglada-Escudé. “But this is pure speculation.”
Spying on the neighbors
The top priority for learning more about Proxima b is determining whether it passes between us and its host star during its orbit. If it performs such a transit, we will be able to tell its size, and perhaps a whole lot more.
“If a transit is found, we can use Hubble to look for features in the atmosphere of the planet,” says Brice-Olivier Demory at the University of Cambridge. “It could be done before the end of the year.”
Analysing light filtered through the planet’s atmosphere during a transit is the only current way to learn about its composition, but for that, the stars must literally align.
Unfortunately, the odds don’t look good – just a few percent chance, given the small size of the star. Searches for transits have always come up short, says Ignas Snellen of the University of Leiden in the Netherlands.
“We are already extremely lucky to find a planet bang in the middle of the habitable zone of our nearest neighbor,” he says. “If it was also transiting, that would be a little bit too much to ask for.”
If there’s no transit, we will have to wait for the construction of telescopes capable of separately resolving the planet and its star. One possibility is the James Webb Space Telescope, NASA’s replacement for Hubble, due to launch in 2018, but Snellen says even that might not be powerful enough. “This will be extremely difficult, and I’m not sure it will be possible.”
Instead, we might need to sit tight for the European Extremely Large Telescope, due in 2024. This mighty 40-metre eye could perhaps gather a single pixel of light from Proxima b, says René Heller of the Max Planck Institute for Solar System Research in G?ttingen, Germany, but watching how this changes over time could reveal a great deal. “We might be able to recover cloud patterns, even separate continents from oceans.”
The desire to know more about Proxima b could also drive the development of other projects. “This planet candidate is our best bet for the next few decades, maybe even forever, to directly image an Earth-mass planet in the habitable zone,” says Heller.
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