Ken Tapping, February 03, 2010

In the 1960's, American astronomer Frank Drake came up with a formula now known as the Drake Equation. When you put in the values for the number of sun-like stars in our Galaxy, the number of Earthlike planets and so on, you get an estimate of the number of planets out there with civilizations on them. One can chop off a bit of the equation and use it to estimate how many planets in our Galaxy might bear life of any kind. Of course, back in the 1960's the numbers put into the equation were not much more than wild guesses. However, even in what scientists believed to be the worst case scenario, the equation predicted that we cannot be alone. In the best case, the equation suggests our Galaxy and all the other galaxies are positively teeming with different forms of life.

For decades after Drake put together his famous equation, it didn't look as though we'd ever be able to get real numbers to put into it. The only planets we knew of were in our Solar System. Looking for planets orbiting other stars seemed impossible. However, since the 1960's technical developments undreamed of back then are now commonplace in astronomy. Moreover, these developments are not only available to professional astronomers; some are within reach of amateur astronomers too. To date we now know of around 400 planets orbiting other stars, some of which were discovered by amateurs, using backyard telescopes. Now we are starting to do something even more amazing; we can search at least some of those distant planets for signs of life.

Chemical life like ours is a pretty unstable thing. It is largely based upon chemicals that have to be continually manufactured in our bodies, and when released into our environment, are rapidly destroyed. If we can detect these "biochemicals" in a planet's atmosphere or oceans, chemically based life must be present. That does not tell us what it looks like or where it lives, but it is there. We are using that method to search for life on Mars, by looking for biochemicals in the Martian atmosphere.

Stars are far too hot for biochemicals to exist. If we point a telescope at a star that happens to have planets, and we detect the signatures of biochemicals, those chemicals are on at least one of the planets, and indicate the presence of life.

When a planet with an atmosphere passes between its star and us, some of the light around the edge of the planet's disc passes through that planet's atmosphere. Chemicals present in the planet's atmosphere absorb particular wavelengths in the starlight. We can identify these "missing wavelengths" and search for signatures of biochemicals in that distant planet's atmosphere.

It is early days yet. However, already some promising results are turning up. At the moment we do not see how we will find out what these alien creatures are like, or how they live, but on the basis of how much progress we've made since the 1960's, one should never say never!