The search for life outside our solar system just got a whole lot closer.
Scientists have discovered a small, probably rocky planet orbiting our nearest stellar neighbour, Proxima Centauri. It is a tantalisingly close 4.2 light years away.
That planet, dubbed Proxima b, is 1.3 times the mass of Earth and orbits its star inside the so-called "Goldilocks zone", where it can theoretically support liquid water.
It orbits Proxima Centauri every 11.2 Earth days and it is 5 per cent the distance from its much-cooler red dwarf star as we are from our sun.
"This has just moved from the abstract," said Paul Butler, co-author of the study published today in Nature.
Since the 1990s, scientists have identified more than 3000 extra-solar planets, or exoplanets, around distant stars.
"But this star is different," said Dr Butler, who works at the Carnegie Institution of Washington. "It is prominent in the public imagination. This is the best case we have of a potentially habitable planet and none will ever be found nearer than this."
He said until now, exoplanet research has been based on maths that most people don't understand.
"But what Proxima does is act like a neon sign: right here, folks, this is not fuzzy or vague, it's the very nearest star in the sky."
Dr Butler said this discovery adds to a body of evidence that suggests about 30 per cent of all stars have a planet orbiting in the habitable zone.
"This isn't a rigid figure," he said. "It could be 20 per cent or up to 50 per cent."
With about 100 billion stars in the galaxy, even a conservative estimate means there are probably billions of planets that could harbour life.
"This is the jackpot for astronomers and futuristic dreamers alike. It's really an extraordinary discovery," said Professor Peter Tuthill, an astronomer at the University of Sydney who was unconnected with the study.
Dr Butler said the whole point of the science he is doing is to provide information for the next generation.
"I think of what I'm doing as like the early ocean voyagers that found the Americas and Australia. The first role was simply to map what they had found so people could follow up. And that's exactly what I'm trying to do.
"I want to build a map of the nearest stars that can point the way for the next generation."
In 2012, researchers announced a planet around Alpha Centauri, the next nearest star. But that was debunked last year. Is Dr Butler sure of their discovery?
"It's really unlikely that two totally different telescopes using two totally different techniques could have the same systematic errors. That's really implausible." The Alpha Centauri announcement relied on one data set.
"This is data from two telescopes over a number of years. It is reasonably compelling evidence," said Professor Chris Tinney at the University of NSW. Professor Tinney, while not connected to this study, has worked with Dr Butler.
None of the detected exoplanets have been directly observed. However, detecting and measuring the velocity of a star's wobble indicates the mass of any object orbiting it.
The speed of the wobble is determined by measuring Doppler shifts in light reaching Earth. When the star is moving ever so slightly towards us, the light is shifted to the blue end of the spectrum, when it wobbles slightly away from us, measured light shifts towards the red.
Southern skies over the ESO telescope in Chile with images of the stars Proxima Centauri and Alpha Centauri AB. Photo: ESO/ESA/NASA
"Any talk about life on Proxima b is pure speculation," said Dr Butler, speaking to Fairfax Media from the Magellan telescope at Las Campanas, Chile.
"We won't have a clue about life until we take the next step, which is direct imaging of the planet allowing us to study the atmosphere ... but that is realistically 20 years away."
That, however, is a lot quicker than sending a probe. The fastest spacecraft humans have sent into space is the New Horizons mission to Pluto. It was sent from Earth at some 45 kilometres a second and flew past Pluto at a still impressive 14 kilometres a second.
However, even at the upper speed it would take almost 30,000 years for a probe to reach Proxima b.
This image of the sky around the bright star Alpha Centauri AB also shows the much fainter red dwarf star, Proxima Centauri, the closest star to our solar system. Photo: ESO/ESA/NASA
While Proxima b is a bit like the third rock from our sun, the star it orbits is very unlike our own.
Proxima Centauri is a red dwarf "M-class" star. It is much smaller and cooler than our sun, a "G-class" star. At about 3000 degrees, it has about half the surface temperature of the sun and is just 12 per cent the sun's mass.
Its luminosity – the amount of energy it emits – is just 0.15 per cent that of our sun. That explains why it cannot be seen with the naked eye and was not discovered until 1915.
Astronomers estimate that as many as 75 per cent of stars in the galaxy are M-class red dwarfs. This means studying Proxima Centauri and its planet will give us an understanding of many of the potential star systems in the Milky Way.
Writing in Nature, Artie Hatzes of the Thuringian State Observatory said: "Circling a star at the right distance is no guarantee that the planet has liquid water or even an atmosphere that can support life."
The small distance from Proxima b to its star means that, in the same way our moon always faces one side to Earth, this planet is most likely to be "tidally locked" and would have a permanently bright side and a permanently dark side.
Further, the magnetic field of Proxima Centauri is 600 times greater than the sun's and, as the researchers point out, the planet suffers "X-ray fluxes that are approximately 400 times that experienced by Earth".
"This could be bad news if you want to keep a biosphere going," said Professor Tuthill at Sydney University.
However, UNSW's Professor Tinney said: "It's not obvious that this precludes life. We don't know if they are killer blows. Our understanding so far involves a huge range of assumptions. There is so much we don't know."
While the conditions for life seem different to those on Earth, any aliens will have much longer to evolve. Stars like our sun die after 10 billion years. Red dwarfs like Proxima Centauri have life spans of trillions of years.
