George, of Sawston Village School, asks :-

How many solar systems are there?

Gerry Gilmore, an astronomer at Cambridge University's Institute of Astronomy, responded.

This is an extremely interesting question, which many astronomers are working to answer in many different ways. The simple answer is `a very large number'!

The simplest way to know there are many other solar systems is that we see them forming. The Hubble telescope has produced beautiful pictures of what are (strangely) called PROPYLDs, which are young stars, just forming now, which have baby solar systems growing around them. These often look rather like hamburgers, when we look at them side-on, with the newly-formed star surrounded by a dark ring of dust, gas, rocks and comets, which are slowly forming new planets and moons. These seem to be very common, so it is probable that most new stars form with solar systems. That would mean there are many tens of millions of young solar systems out there.

What about older stars? Here it is harder, since the faint light from a planet is lost in the bright glare of its Sun when we try to see them. Huge new telescopes are being designed now to do this direct imaging, but they will not be ready for 10-15 years yet.

In the meantime, we have to make do with another method, which uses gravity. A sun and its planets all orbit about the centre of all the mass, for our Sun this is roughly on the surface of the Sun. That is, the Sun is itself orbiting about a point near its surface, and so is made to move by the planets around it.

We can use that motion to look for planets around other stars, by accurately measuring the speed of other stars. This process is very successful, and has so far found well over 100 Solar Systems in addition to our own.

Two excellent web pages are http://www.obspm.fr/encycl/encycl.html and http://antwrp.gsfc.nasa.gov/apod

From all this we can try to answer you question: how many solar systems are there? The answer seems to be that approximately one star in 20 has a solar system of some form. That means there are in total several thousand million solar systems in total in the Milky way Galaxy alone.