For over fifteen years now, astronomical techniques have been sensitive enough to detect planets beyond the solar system by the tiny effect they have on their parent star.These are called extra-solar planets or exoplanets, and more are announced at an ever-quickening rate.

At the time of writing this a grand total of 217 new worlds have been discovered, with 20 alien solar systems known with multiple planets. An up-to-date list of all known extra-solar planets can be seen at http://planetquest1.jpl.nasa.gov/atlas/atlas_index.cfm . None of the exoplanets discovered so far are thought to be potential abodes for life – because of limited sensitivity we have mostly discovered gas giants (which have no habitable surface). However, a few do orbit nicely within their star’s habitable zone and if they possess large moons these may well be suitable for life. As our instruments improve, and especially with the launch of sophisticated space telescopes, we will be able to find small terrestrial planets. Once alien Earths have been discovered, there is even the ability to assess the make-up of their atmosphere and check for signs of life. The combination of high levels of oxygen and gases such as methane in the air, which would react together very rapidly, suggests the presence of widespread photosynthetic life keeping the levels topped-up (as on Earth).