It's the planet we’ve all been waiting for. Earlier this month, rumours 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 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 per cent more massive than Earth, although it could be bigger than that. It orbits the star at a distance of 7.3 million kilometres – less than 5 per cent 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 never-ending night. “That’s not very Earth-like,” Anglada-Escudé says.
Guessing the atmosphere
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, 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 neighbouring 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.”