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- The nearest habitable world beyond our Solar System might be right on our doorstep - astronomically speaking.
- Scientists say their investigations of the closest star, Proxima Centauri, show it to have an Earth-sized planet orbiting about it.
- What is more, this rocky globe is moving in a zone that would make liquid water on its surface a possibility.
- Proxima is 40 trillion km away and would take a spacecraft using current technology thousands of years to reach.
- What makes a planet "potentially habitable"?
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- Media captionProxima b is the closest known, potentially habitable planet
- Nonetheless, the discovery of a planet potentially favourable to life in our cosmic neighbourhood is likely to fire the imagination.
- "For sure, to go there right now is science fiction, but people are thinking about it and it's no longer just an academic exercise to imagine we could send a probe there one day," said Guillem Anglada-Escudé whose "Pale Red Dot" team reports the existence of the new world in the journal Nature.
- Artwork lightsailImage copyrightSPL
- Image caption
- Artwork: New propulsion ideas include "lightsails" powered by lasers
- Earlier this year, the billionaire venture capitalist Yuri Milner said he was investing $100m in studies to develop tiny spacecraft that could be propelled across the galaxy by lasers.
- These would travel at perhaps 20% of the speed of light, shortening the journey to a star like Proxima Centauri to mere decades.
- Just how "habitable" this particular planet really is, one has to say is pure speculation for the time being.
- The Queen Mary University of London researcher and his group concede they still have much work to do to extend their observations.
- Simply identifying the world, catalogued as "Proxima b", was a considerable challenge.
- It was made possible through the use of an ultra-precise instrument called HARPS.
- This spectrograph, attached to a 3.6m telescope in Chile, detects the very slight wobble induced in a star when circled by a gravitationally bound planet.
- Its data suggests Proxima b has a minimum mass 1.3 times that of Earth and orbits at a distance of about 7.5 million km from the star, taking 11.2 days to complete one revolution.
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- Media captionGuillem Anglada-Escudé: "It gives us a target; we have a reason to go now"
- La SillaImage copyrightESO
- Image caption
- The HARPS instrument is attached to a 3.6m telescope at the La Silla observatory in Chile
- The distance between the star and its planet is considerably smaller than Earth's separation from the Sun (149 million km). But Proxima Centauri is what is termed a red dwarf star. It is much reduced in size and dimmer compared with our Sun, and so a planet can be nearer and still enjoy conditions that are potentially as benign as those on Earth.
- "This planet is at 5% of the Earth's distance from the Sun. However, Proxima is 1,000 times fainter than the Sun. So the flux - the energy - reaching Proxima b is about 70% of what the Earth receives. It's like taking Earth a bit further away, but it's comparable," explained Dr Anglada-Escudé.
- Whether the temperatures on Proxima b are favourable for life to exist is going to depend on the presence of an atmosphere.
- An envelope of greenhouse gases would warm surface conditions and provide sufficient pressure to keep water - essential for biology - in a liquid state.
- But even with the limited information we currently have, scientists are excited by the news.
- "I think it's the most important exoplanet discovery there will ever be - how can you ever trump something that could be habitable orbiting around the very closest star to the Sun?" commented Dr Carole Haswell from the Open University.
- "When I was a kid, it wasn't clear there were any other planets that we could walk around on and find liquid water on - so I think it's absolutely thrilling," she told BBC News.
- Graphic comparing size of stars
- Proxima Centauri is a red dwarf; it is much cooler and smaller than our Sun
- It is marginally closer but probably gravitationally bound to Alpha Centauri A & B
- To the naked eye, Alpha Centauri A & B appear as a single star on the sky
- Sirius A is the brightest and most easily recognisable star in the night sky
- Jupiter is our biggest planet. If 80 times more massive, it would turn into a star
- Researchers are presently looking to see if the planet crosses the face of Proxima Centauri as viewed from Earth - an event referred to as a "transit".
- This kind of backlit observation could confirm not just the existence of an atmosphere but reveal perhaps something of its chemical properties.
- Indeed, researchers have long talked about using transits to try to detect the signatures of life on planets that are too far away to be visited by spacecraft.
- If there are lifeforms on Proxima b - even simple microbes - they may find the going rather tough, however.
- Red dwarfs are very active. They tend to throw out big flares that would bombard a nearby planet with energetic particles. The X-ray emission is much more intense as well.
- Even so, these kinds of stars are now the subject of great interest in the search for Earth-like planets simply because they are so abundant in the galaxy.
- One priority for the future will be to get a direct image of the planet.
- This should be possible with the European Extremely Large Telescope now under construction in Chile.
- It is being given a 39m-wide primary mirror and state-of-the-art instrumentation precisely to do this kind of observation.
- "A planet around even a wimpy star like Proxima Centauri is going to be more than a billion times fainter than the star itself. So, what you do is block out the light from the star using a special device and that allows you then to go deeper into the star's surroundings," explained Cambridge University's Prof Gerry Gilmore.
- "This is one of the E-ELT's design goals. There's also a Nasa mission under development called W-First. It will have a high-resolution coronagraphic mode which again is designed for the same purpose."
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