While the debate continues to rage about how many planets there are in our own solar system, it is worth remembering that as of August 2006 there are well over 200 planets known to exist around other stars. When I did a talk at CAS in May 2006 that number was nearer 160! Each month more extrasolar planets, or exoplanets, are added to the list.
One of the most recently confirmed planets orbits the star beta gemini, that's the naked eye star Pollux to you and I. One of the Gemini 'twins', Castor and Pollux, the star is well known to many amateurs and is well placed in the winter sky. Next time you look at the constellation you can contemplate the fact that there are planets up there !
The planet has a mass of 2.5 times that of Jupiter and orbits at a distance of 1.64 AU from Pollux. Of course that may not be the only planet to orbit Pollux. The techniques currently used for detecting extrasolar planets are best at detecting larger planets, in close orbit around their stars. Hence the majority of planets we know about are so called 'Hot Jupiters'. As techniques improve smaller and smaller planets will be discovered, with the ultimate goal being to find Earth sized planets around other stars.The closest star known to have a planet is Epsilon Eridani. It is the third closest naked eye star to our sun, at a distance of 10.5 light years. In 1998 a dust disk was discovered around the star. Within about 35 AU of the star there was no dust, indicating the possible presence of a planetary system. In 2000 a planet Epsilon Eridani B was announced. This is a 0.8 Jupiter mass object orbiting about 3 AU from the star. A further planet Epsilon Eridani C has also been proposed, but as yet remains unconfirmed. Because of its close proximity Epsilon Eridani will be one of the first stars tobe investigated by the next generation of extrasolar planet missions, including NASA's SIM PlanetQuest , which should be able to detect any Earth mass planets orbiting the star.
The Extrasolar Planet Encyclopedia is the place to go to keep tabs on all the known extrasolar planets. However it is not the most graphical of sites, if you want a more visually appealing site, with images and artistic impressions of some of these planets, check out Extrasolar Visions. Extrasolar visions even provides a view of the night sky from the planet in question. There is also plenty of information on the New Worlds Atlas , part of NASA's PlanetQuest site. PlanetQuest also has details of some of the upcoming missions to improve our knowledge of other planetary systems as well as a round up of the surprisingly large number of current exoplanet studies.There's also an excellent summary of the various methods of detection, including the pros and cons, at the Planetary Society website. For information on the nearest stars to Sol and potential planetary systems check out Solstation .
Many thanks to the Eddington Astronomical Society for inviting me to talk at their meeting last night. I really enjoyed the evening. Hopefully this post will clear up some of the questions from the meeting and provide some follow up links.