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Mars has not been forgotten, and the more interesting moons of Saturn and Jupiter remaining intriguing candidates in the hunt for extra-terrestrial life. But new plans are now underway to extend the search way beyond the boundaries of our solar system. If, and it's a big if, all goes according to plan, telescopes of unprecedented resolution will soon be launched into space to begin their detective work.
Nasa's Terrestrial Planet Finder, planned for the end of the next decade, will look for Earth-sized planets in the habitable zones of distant solar systems. The European Space Agency have similar ambitions for their Darwin mission, and the two projects may yet be combined into one collaborative effort.
Either way, none of this will be easy. Distant planets are extremely difficult to detect because the light they produce is swamped by the much more intense light coming from their own sun. It's a bit like trying to spot the glow of a lighthouse keeper's cigarette from a thousand kilometres away. Fortunately, bright lights can be dimmed by looking in the infrared spectrum, rather than at visible wavelengths – a technique that reduces the brightness differential and clears the picture. Spectral analysis of infra-red radiation will also enable astronomers to identify a planet's atmospheric gases. Evidence of oxygen, carbon dioxide and methane could be crucial signs of life.
Meanwhile, back on Earth, the SETI (Search for Extraterrestrial Intelligence) Institute in concert with the University of California, Berkeley, is building one of the biggest radio telescopes ever seen. The Allen Telescope Array (ATA) will eventually consist of 350 dishes, each measuring six metres across. The telescopes will be so powerful that they should be able to pick up extraterrestrial signals from a million solar systems – about a thousand times more than have been examined to date. If an extraterrestrial has his radio on, then he will be heard.
Of course, Earth has been transmitting its own signals for a number of years now. Space is full of old episodes of Ground Force and the Dukes of Hazard, travelling at lightning speed into distant galaxies. One can only guess what alien life will make of it all when it eventually drops on their doorstep. Thankfully, we can count on the vastness of space to ensure that it will probably be thousands of years before these transmissions are ever received. And who knows where we'll be then?
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