Finding life on other worlds: the rainbow connection
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Associate Professor Jeremy Bailey |
A number of missions are being planned for the next 10 years to search the universe for planets that can sustain life. Associate Professor Jeremy Bailey believes that if we find them, they'll be somewhere over the rainbow.
"The most basic question we want to ask is 'is there life elsewhere in the universe or are we unique?'," says Bailey, Associate Director of the Australian Centre for Astrobiology at Macquarie University. "There's possible locations where we might find life in our own solar system, such as on Mars or Europa, but beyond that, if there's nothing in this solar system the next step is to look for life in other solar systems."
However, looking for life in the universe is a little like looking for a needle in a haystack because of the vast number of possible sites. Astronomers will therefore focus on Earth-like planets, where they figure the chances of life are more probable. After all, if life could thrive on Earth, it's possible it could do so on other planets where conditions are similar.
A new approach
In preparation for the upcoming space missions, astronomers have begun developing ways of detecting planets with Earth-like conditions using a technique called spectroscopy, which is able to determine the gasses that make up the planet's atmosphere. However Bailey believes scientists should be looking for something entirely different.
"As far as we can tell, water is really the only essential requirement for life," he says. "We're finding on Earth an increasing range of different environments in which you can find life - organisms called extremophiles can survive in freezing temperatures, in virtually boiling water, in acid conditions, high pressure environments and so on. But the one thing there has to be is liquid water, because that's fundamental to life as we understand it."
Somewhere over the rainbow
Spectroscopy cannot detect liquid water in an atmosphere, so Bailey believes we should be searching for another atmospheric effect we're very familiar with on Earth - rainbows.
"The process that causes rainbows is the scattering of light off droplets of liquid. Light rays go into the droplet, reflect off the inside, and come out on a different angle," he explains. "It can't happen on a planet where there is no liquid water because it depends on the droplets being spherical. Just about the only natural way that spheres are produced is by liquids because of their surface tension - they naturally form a spherical shape. Anything else that you might come across in an atmosphere - solid particles for example - will almost certainly have an irregular shape."
If a planet with clouds of liquid water is viewed from sufficiently far away, at a certain angle there should be an increase in light due to this scattering effect. While this increase is difficult to detect because the planets are so near to their stars, the fact that rainbows are also highly polarised should make it easy for astronomers to pick out a polarisation signature.
Polarisation measurements of rainbow scattering have already been used in the past to determine the nature of the clouds of Venus, and to distinguish liquid and ice clouds in the Earth's atmosphere, and Bailey sees no reason why they couldn't be used on the next NASA mission to search for habitable planets.
For more information, contact Associate Professor Jeremy Bailey at jeremy.bailey@mq.edu.au