Science

Is anybody out there?

How do we know whether there is life on other planets, asks JOHN HOLDEN

Wed Feb 16 2011 - 00:00

How do we know whether there is life on other planets, asks JOHN HOLDEN

ISN'T IT funny just how many alien sightings there have been in middle America? How come no one ever sees ET in Bray or Carrick-on-Shannon? Extraterrestrial Highway in Nevada is where the two characters played by Simon Pegg and Nick Frost in the new movie Paul(pictured) encounter an alien.

There are too many movies about alien contact to even try to name them. Some have plots that have been drawn from thin air but others are based on famous hoaxes. That does not stop the search for life beyond Earth. While we are yet to find concrete evidence of extraterrestrials, the past decade has been extraordinary for new planet discoveries.

“When I was doing my undergraduate degree in the early 1990s, there were nine planets that we knew of,” says Peter Gallagher, physics and astrophysics course director at Trinity College Dublin. “Now we know of over 500 planets orbiting different stars.”

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Just this month NASA’s Kepler space telescope found six new planets huddled around a sun-like star. The discovery of new planets precurses the search for life.

“The quest is to find Earth-like planets,” says Gallagher. “If we’re looking for a planet with life, it needs to have broadly the same mass as Earth and can’t be too close or too far from its star (otherwise known as the Goldilocks effect) so that water is in liquid form.

“Then we can look for ‘biomarkers’ – signs of life, like amino acids,” he adds. “Generally we look for long, complex molecules like carbon and nitrogen, things that forced the soup out of which life emerged.”

How do we search for such molecules when they are many light years away?

“Molecules emit light at a particular wavelength and the wavelength tells you what their make-up is,” says Gallagher.

“Water has a particular spectrum as do amino acids and organic molecules. That’s how we worked out what the sun was made of.

“For example, if there’s lots of hydrogen present there will be lots of red light: oxygen will show green light, and so on.”