ESA Science & Technology08-Jul-2005 11:28:19

Extrasolar Planets


Orbiting its parent star, a planet's motion will alter the wavelength of light it reflects from the star. When the planet is moving towards Earth, the wavelengths of light are squashed. On the other side of its orbit, the planet moves away from Earth and so the light is stretched to longer wavelengths. This behaviour is known as the Doppler effect.

The star is also pulled by the planet (see radial velocity technique) creating a smaller Doppler shift in its own light but always in the opposite direction to that of the planet's light. If the intensity of light coming from the star and planet are plotted on a graph, they combine into a shape called the Spectral Energy Distribution. As the planet orbits and the star wobbles, the shape of the graph changes.

In principle, computer analysis would be able to separate which parts of the signal were caused by the star's light, leaving just the planet's signal for analysis. However, 4-metre diameter telescopes do not collect enough light to make this technique work and no one has yet tried it with an 8-metre telescope.

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