The number of planets discovered to be orbiting around other stars in our galaxy seems to be increasing at an amazing rate. These planets, known as exoplanets (extrasolar planets) have been found to range from massive and hot gas giants, many times larger than Jupiter, to rocky and icy worlds only a few times larger than our own Earth. But, how do astronomers actually set about finding them?

Planet Hunting Method 1 – The Wobble

The technique which has proven the most successful in finding exoplanets is that of looking for a ‘wobble’ in the position of the star. How this works is that, just like in our solar system, the Sun and the planets all really orbit around a common ‘centre of gravity’, which can be thought of as the location of the average mass of the entire solar system. Because the mass of the Sun is so much larger than the combined masses of all of the other planets, the centre of gravity is actually somewhere inside the Sun, but not exactly at its centre.

As with the Sun, other stars with their own planetary systems will orbit around their own centres of gravity. By studying the properties of the light coming from a star, astronomers can tell whether the star is spinning more or less perfectly about its centre (like a spinning top), or slightly off-centre. If it is spinning slightly off-centre, then this may well indicate that the star could have planets in orbit around it.

Planet Hunting Method 2 – The Transit

Another technique which astronomers use to detect whether other stars have their own solar systems, is one where the amount of light from a star may vary as the result of a planet passing in front of it.

Much like when the planet Venus ‘transits’ across the Sun, the total amount of light coming from the star is very slightly reduced since the planet is, in effect, blocking the view of the entire star. The bigger the planet, the bigger this effect. The most powerful telescopes can also be used to detect the planet’s colour and atmospheric chemistry by this method, but finding exoplanets in this way is very difficult, as it relies on a certain amount of luck. Firstly, from our point of view, the orbits of any planets must actually go across the star and secondly, we must also need to be looking at the right time, as such transits do not last long.

Planet Hunting Method 3 – Direct Viewing

The direct viewing technique is exactly as the name suggests – directly observing a planet in orbit around another star. Different to the previous two methods, which rely on effects that the exoplanets themselves have on the stars, direct viewing relies on finding special stars of a certain type and size that would not drown out the ability to see planets through their glare.

These special stars are known as ‘brown dwarves’, which are much smaller than our Sun and sometimes only generate enough power to shine in the infrared. Through large infrared telescopes, it has been possible to see exoplanets in orbit around very faint brown dwarf stars, too dim to be seen in visible light.

About the author:

Steve West is a systems engineer, astrophysics graduate and lifelong enthusiast for all things to do with space. For more interesting stuff about astronomy, telescopes, astronomy gifts and educational gadgets, visit: http://www.squidoo.com/home-star-projector