The Search for Extrasolar Planets


So far SETI, the Search for Extraterrestrial Intelligence, has not come up with any definitive signs of intelligent life outside our own planet. Piggybacking their data collection on the huge radio telescope in Aricebo they search certain sections of the sky for telltale transmissions. This is not the only way to look for life 'out there' though. One way to narrow down the search is to look for another Earth in a distant solar system like our own.

Dr Serena Viti travelled from her base at University College London to tell us about the search for extrasolar planets and some of the latest news in the field. The first thing to do, its seems, is to define your target. A planet:- doesn't emit any light of its own (only reflected rays), it has a relatively small mass somewhere between Pluto and a star, and it orbits a star. Just to confuse matters an object known as a brown dwarf can also fit the bill but its method of formation rules it out. Brown dwarfs are not a new character out of "Lord of the Rings" but are thought of as failed stars. They do not have enough inherent mass to kickstart the nuclear reactions that turn hydrogen into helium releasing photons of light in the process.

So, now you know what to look for, you have to think how you are going to find them. There are three main ways that scientists are looking for planets. The first is to closely watch the star that you think a planet may be orbiting. If you can detect a tiny wobble in the star's position then it could be being pulled off course by a planet. The breaking news on this method of detection is the discovery of two Neptune-sized planets. Both orbit their parent star in a matter of days and are very close to it.

The second method of finding planets has the rather grand title of "gravitational microlensing". It works due the fact that as light rays pass a massive object they are bent towards that object. It's as if the object is acting like a lens and focusing the light rays towards us making the star suddenly appear fractionally brighter. This method relies on a planet passing between Earth and a star and highly sophisticated detectors that can pick up the tiny differences in brightness in the star. It has succeeded in finding a planet about one and a half times the mass of Jupiter.

The final way of hunting down a planet is again linked to a star's brightness and is known as the Transit Method. As a planet crosses a nearby star the star's light will dim slightly as the planet transits across its disc. This happened back in June in our own Solar System when Venus crossed the Sun and appeared as a small black disc. The Hubble Telescope has made a major breakthrough using this technique as it has been used to analyse the atmosphere of a planet as it crossed its parent star. Initial observations show that the planet's atmosphere contains Sodium.

To date about 130 extrasolar planets have been discovered. Most of them are Jupiter-sized and trace out an elliptical orbit very close to their parent star. In the near future ESA, the European Space Agency, is proposing a mission called Darwin. It will consist of six spacecraft that fly in formation to simulate a large mirror. If this ground-breaking project comes to fruition it will allow astronomers to directly detect Earth-like planets and begin to analyse their atmospheres for signs of life.