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November 10, 2013 4:30 pm

We should not be too excited about intergalactic neighbours

Extraterrestrial societies might not relish the prospect of meeting us in any case, writes Anjana Ahuja
It began in 1960 with piecemeal American efforts to scan the skies for unusual radio signals. This week, the increasingly respectable science of alien-hunting notched up a new milestone.
Astronomers crunching data from Nasa’s orbiting Kepler observatory estimate that about one in five Sun-like stars could boast an Earth-like planet. When those statistics are applied to our own galaxy, the Milky Way, the prospect of extraterrestrial neighbours begins to look thrillingly real. There could be 20bn candidate planets on our galactic doorstep, all about Earth’s size and circling their host stars at just the right distance to harbour life. The nearest planetary doppelgänger, according to Erik Petigura, the Berkeley astronomer who published the analysis, could be a mere 12 light years away – close enough for the host star to be seen with the naked eye. Professor Geoffrey Marcy, a colleague of Mr Petigura and a fellow contributor, said the revelation made him feel “tingly inside”.




But it is a long haul to the ultimate goal: discovering whether we really are alone in the universe. We certainly lack company in the solar system; Mars has failed to throw up even a microbe. Still, our life-giving Sun is one star among an estimated septillion (10 to the power of 24) in the cosmos, holding out the statistical probability that we are not unique.
That is why, in 2009, Nasa launched the Kepler spacecraft, containing a fancy light meter that could be pointed at distant stars to record any dimming caused by planets passing across them. In a separate announcement this week, the US space agency said Kepler had clocked up confirmed sightings of more than 3,000 planets orbiting stars. Of these “exoplanets”, 104 are in the “habitable zone”; this means liquid water, seen as a prerequisite of life, can exist on the surface. The habitable zone, with its just-right conditions, is also known as the Goldilocks zone, with planets outside it either too scorched or too frozen.
The next logical step is to scan the atmospheres of these candidate planets for gases, such as oxygen, that would betray the existence of life. That is tough to achieve from Earth; spectral signatures of biomarker gases from other worlds would be hard to read amid the fog of our own vapours. Sending an instrument into space would cancel terrestrial interference but since the parent stars shine 10bn times brighter than their orbiting planets, the challenge of discerning a clean planetary signal becomes astronomical. Nasa was planning such a mission, but the Terrestrial Planet Finder was cancelled in 2011 because of budget cuts.
In any case, an oxygen signature does not confirm life. We could try visiting, but even a journey of 12 light years is prohibitive. One light year is the distance travelled by light in one year; 9.5tn km. Voyager 1, the sole man-made object to have left our solar system, has travelled only 0.002 light years since its 1977 launch. At this speed, the trip would take any earthly messenger at least 276,000 years. It may be that future generations confirm the existence of wormholes – shortcuts in space-time that allow time travel and instant transport across the universe – but they remain for now the stuff of mathematical fantasy.
So our best chance of remote confirmation remains the arrival of an inexplicable radio signal or laser pulse, as predicted 53 years ago, when the first Seti (or search for extraterrestrial intelligence) experiment was conducted in West Virginia. The Arecibo radio telescope in Puerto Rico happens to be in just the right place to “catch” radio signals from some of the exoplanets Kepler has uncovered. This is where Seti efforts will probably congregate.
But what if we do detect an engineered signal from beyond our solar system? It would mark an epochal moment. Our civilisation would need to decide, through the UN, whether to reply. Stephen Hawking, the British physicist, has advised radio silence, in case any neighbours harbour malign intentions and covet Earth as a galactic colony.
Of course, any cosmic company may already know of our existence, through our electromagnetic emissions. An alien society 12 light years away might be enjoying a raft of TV shows first broadcast in 2001, including the first series of The Office. In which case, they have probably decided we earthlings are far too irritating to bother visiting.
The writer is a science commentator

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