ESA Science & Technology08-Jul-2005 11:27:22

Extrasolar Planets


A more promising method for detecting small worlds is to look for the drop in brightness they cause when they pass in front of their parent star. Such a celestial alignment is known as a transit. From Earth, both Mercury and Venus occasionally pass across the front of the Sun. When they do, they look like tiny black dots passing across the bright surface.

Obviously, such transits block a tiny fraction of the light. If a distant star were transited by the equivalent of Jupiter, it would cause 1% of the starlight to be lost from view. One gas giant planet, found by the radial velocity method, has also been detected using the transit method from a ground-based telescope. Star HD 209458 was discovered to possess a 51 Pegasi b-like planet (a large planet orbiting its parent star in a tight circular orbit, also known as 'hot Jupiters') and subsequently seen to dim at precisely the time that the planet was predicted to pass in front of the star.

Not every planetary system will be so aligned and a large number of stars need to be observed in order to stand a chance of catching one with the right orientation. ESA's Eddington mission is designed to survey about half a million stars, in the hopes of detecting thousands of planets, including Earth-sized worlds and smaller.

An Earth-sized planet will typically block less than one hundredth of a percent of the starlight but Eddington's sophisticated modern detectors will register the change.

The effect on the light curve when a planet passes in front of the parent star (Based on image by Hans Deeg, from 'Transits of Extrasolar Planets').

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