Astronomers are continually finding more planets outside our solar system, known as exoplanets. More than 2000 years ago Greek philosophers began discussing the possibility of many inhabited worlds. This philosophical question is nowadays a topic of research for modern science. But how far is our technology in determining whether these exoplanets could harbour life?
So far there have been over 300 exoplanets discovered. The vast majority of these planets are similar to our Jupiter - gaseous and massive. It is estimated that about 10 % of all stars have such planets orbiting around them. There is also the "super-earth" category, consisting of very large rocky planets. Around 30 % of Sun-like stars could have planets of this kind. But the most interesting planets for us are the smaller terrestrial planets with a comparable mass to that of the Earth.
If light from an exoplanet can be measured, it is possible to study the chemical composition of its atmosphere, which could perhaps lead to traces of life. The first molecule that was detected in the atmosphere of an exoplanet was water. However, in order to find evidence of life the search must continue.
Already the initial task of finding an exoplanet is very challenging. The extremely brilliant light from the parent star outshines the small amount of light coming from an exoplanet. Astrophysicist Jean-Philippe Beaulieu of the Paris Astrophysics Institute offers the analogy of standing on Mars and observing a nuclear explosion on the Earth. Finding an exoplanet is similar to wanting to measure the light from a fire-lighter one hundred metres away from the nuclear explosion.
After having found an exoplanet, scientists face an ever more challenging task of analysing the atmosphere. So far success has been limited to massive, hot gaseous exoplanets that have an exceptionally close orbit to their stars. Much is now known about this type of planet. Like Jupiter, they are composed of mostly hydrogen. Molecules have also been found in their atmospheres, including water-vapour, methane, carbon-dioxide and carbon monoxide.
For other exoplanets a remarkable degree of accuracy is required to learn something about their atmospheres. These planets can not be directly seen with today's telescopes, so indirect methods and modelling is used.
Computer models are used regularly as research tools. Tommi Koskinen, a Finnish astrophysicist, has created a simulator that can predict the dynamics of upper-atmospheres of giant exoplanets, depending on their orbiting distance. One simulation can show, for example, how the atmosphere under particular conditions heats up and expands to the point that it escapes into space, leading to a large rate of mass loss for the planet.
At the Kiepenheuer Institute for Solar Physics in Freiburg, Southern Germany, the first polarised light in the atmosphere of a hot, gaseous giant was detected. Analysing this light showed that molecules, atoms or possibly dust particles had been scattered in it. With better measurements the scientists of the institute could identify the type of molecules and the type of dust.
In 1995 the Geneva Observatory in Switzerland was involved in the discovery of the first exoplanet. Now its researchers are not only studying atmospheres of exoplanets, but are searching for small, rocky Earth-like planets. They use various techniques, the most efficient of which uses radial velocity measurements of the stars. A planet will have a very slight gravitational force on the parent star, causing the star to wobble as the planet orbits. This motion can be detected as variations in the velocity of a star. The effect is easier to detect for larger planets and planets with a close orbit. After improving their instruments, the observatory can find planets as small as four-times the mass of the Earth, corresponding to a planet one-and-a-half-times bigger than the Earth.
The ultimate goal will be to study the atmosphere of an Earth-like planet, to determine whether it contains the building blocks that could support life or if there are any signs of life itself.