Is Stephen Hawking right about aliens?

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In February 2008, Nasa sent the Beatles song, Across the Universe, across the universe. Pointing the telescopes in its Deep Space Network towards the north star, Polaris, astronomers played out their short cosmic DJ set, hoping that it might be heard by intelligent aliens during its 430-year journey to the star.

The hunt for intelligent species outside Earth may be a staple of literature and film – but it is happening in real life, too. Nasa probes are on the lookout for planets outside our solar system, and astronomers are carefully listening for any messages being beamed through space. How awe-inspiring it would be to get confirmation that we are not alone in the universe, to finally speak to an alien race. Wouldn't it?

Well no, according to the eminent physicist Stephen Hawking. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," Hawking has said in a forthcoming documentary made for the Discovery Channel. He argues that, instead of trying to find and communicate with life in the cosmos, humans would be better off doing everything they can to avoid contact.

Hawking believes that, based on the sheer number of planets that scientists know must exist, we are not the only life-form in the universe. There are, after all, billions and billions of stars in our galaxy alone, with, it is reasonable to expect, an even greater number of planets orbiting them. And it is not unreasonable to expect some of that alien life to be intelligent, and capable of interstellar communication. So, when someone with Hawking's knowledge of the universe advises against contact, it's worth listening, isn't it?...

The biggest and most active hunt for life outside Earth started in 1960, when Frank Drake pointed the Green Bank radio telescope in West Virginia towards the star Tau Ceti. He was looking for anomalous radio signals that could have been sent by intelligent life. Eventually, his idea turned into Seti (standing for Search for Extra Terrestrial Intelligence), which used the downtime on radar telescopes around the world to scour the sky for any signals. For 50 years, however, the sky has been silent....

Scientists discovered the first few extrasolar planets in the early 1990s and, ever since, the numbers have shot up. Today, scientists know of 443 planets orbiting around more than 350 stars. Most are gas giants in the mould of Jupiter, the smallest being Gliese 581, which has a mass of 1.9 Earths. In 2009, Nasa launched the Kepler satellite, a probe specifically designed to look for Earth-like planets.

Future generations of ground-based telescopes, such as the proposed European Extremely Large Telescope (with a 30m main mirror), could be operational by 2030, and would be powerful enough to image the atmospheres of faraway planets, looking for chemical signatures that could indicate life. The Seti Institute also, finally, has a serious piece of kit under construction: the Allen Array (funded by a $11.5m/£7.5m donation from Microsoft co-founder Paul Allen) has, at present, 42 radio antennae, each six metres in diameter, but there are plans, if the Seti Institute can raise another $35m, to have up to 300 radio dishes.

In all the years that Seti has been running, it has managed to look carefully at less than 1,000 star systems. With the full Allen Array, they could look at 1,000 star systems in a couple of years....

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