Worlds Around Other Suns

October 6^th 1995 is not a day etched firmly in the minds of many but perhaps it should be. It was on this day that a Swiss team of astronomers announced to the world the first definitive detection of a planet orbiting another sun.

People had wondered about the existence of extra-solar planets for centuries. But finding them was to be no easy task. Unlike the stars, planets do not generate any visible light of their own but merely "shine" by reflecting their own sun's light. Imagine looking at our sun from the nearest star, a distance of about 40 trillion kilometres. The distance between the Sun and Jupiter would be appear equivalent to the distance between a car's headlamps, 50 kilometres away! Combine this with the problem that the visible light from Jupiter is 250 million times fainter than that from the Sun. Trying to find extra-solar planets is like trying to see a firefly sitting on a searchlight from many kilometres away! This is why extra-solar planets have not been seen directly - they are known to exist because of the gravitational effects they have on their parent suns.

How do you find an extra-solar planet?

The first technique tried was Astrometry. This involved precise measurement of the stars' positions in the sky and looking for the telltale 'wobbles' in their orbits caused by the gravitational tug of an orbiting planet.

Lynette Cook Detecting the 'wobble' these planets cause in their sun's motion requires very precise measurements.

For the last 50 years astronomers have looked for these wobbles and despite a handful of false claims have not found any. The blurring effects of the earth’s unsteady atmosphere makes accurate measurement of so small a motion difficult.

There is however another way to look for these 'wobbles'. Instead of measuring motion in the plane of the sky you can measure the motion along the line of sight. This is known as the Radial Velocity method and is how all the extra-solar planets have been found to date.

It works by utilising the Doppler effect. This effect can be heard on sirens. When a police car is moving towards you the pitch of its siren sounds slightly higher, and then lower as the car passes you and moves away. The same effect happens with light waves. When a star is approaching the Earth the light it emits appears to shift towards shorter wavelengths, and equivalently to longer ones if it is moving away. This can be measured by splitting the starlight into its constituent wavelengths and measuring these shifts. The effects are tiny, for example the effect of Jupiter's pull on our sun would appear as a change in the wavelengths of Sunlight of less than 1 part in 10 million when viewed from a distant star.