ESA Science & Technology08-Jul-2005 11:26:33

Extrasolar Planets

Radial Velocity

The vast majority of planetary detections so far have been achieved using the radial velocity technique from ground-based telescopes. The method requires the light from a star to be split into a spectrum, rather like water droplets in the atmosphere splitting sunlight into a rainbow.

When the spectrum is magnified, straight black lines can be seen superimposed upon the colours. These spectral lines correspond to the wavelengths of light that have been absorbed by chemicals on the surface of the star from which the light originated.

Studying these lines can show which stars have large planets around them because, as the planet orbits the star, it pulls on it with its gravitational field, forcing the star into a small orbit, or wobble. In fact, it makes it look as if the star is pirouetting around a point in space. This means that the star will sometimes be spinning towards Earth and at other times spinning away. When the star moves towards Earth, it squashes the wavelengths of the spectral lines in the light it emits. When the star travels away from Earth, the opposite happens, and the wavelengths are stretched. So, astronomers look for wobbling stars, as these must be the ones with planets in orbit around them.

This technique is limited, however, because it will never be able to detect Earth-sized worlds. With the best spectroscopes, astronomers can confidently detect motions of about 15 metres per second. However, Earth only forces the Sun to move at 0.1 metres per second. Even if a spectroscope could be made to detect this, the boiling of the star's gaseous surface would mask the effect of the planet.

Radial velocity measurements for the star 51 Pegasi taken at different times. Astronomers interpret the regularity of these measurements as indications of the presence of a planet-sized object orbiting the star.

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