From the Greek words ‘Exo’, meaning outside and ‘Planette’ meaning wanderer, exoplanets or extrasolar planets are simply planets that exist outside our solar system. As of April 2010, four hundred and fifty three exoplanets were known to us. By June 2011, that number had climbed to five hundred and sixty three. Counting the number of exoplanets would be similar to counting the stars in the sky, there are trillions upon trillions of them, counting them isn’t really an option. Currently there are 716 known exoplanets.
For many years, up until 1992 philosophers guessed that exoplanets existed, but weren’t entirely sure, there wasn’t a way to know how common they were or how similar to planets in our solar system they were. Several terrestrial-mass planets orbiting the pulsar PSR B1257+12 were discovered, this was the first proven discovery of an exoplanet. The first confirmed detection of an exoplanet orbiting a main-sequence star wasn’t made until 1995 with a giant planet in a four-day orbit around the nearby star 51 Pegasi.
Most of our known exoplanets are pretty large and most closely resemble Neptune or Jupiter. This is biased of course because larger planets are much easier to observe, the smaller would logically be less visible and less known. There are ‘free floating’ masses in space or those that orbit brown dwarfs but these can’t, by definition, be called planets.
With the discovery of exoplanets, the idea that there could be life on other planets isn’t as farfetched as it once was. Several exoplanets orbit what is known as their star’s ‘habitable zone’, meaning in theory they could support life. Planetary habitability is the measure of how habitable a planet is. Earth and its surroundings is the only system known to support life and appears to be favorable to life’s flourishing. It’s much more likely for life to exist on some exoplanet than another one in the Solar system.
The current big story is of Keplar 22b, the first known exoplanet to orbit in its sun-like star’s habitable zone. It’s 600 light years away from Earth and orbits around Kepler-22. Its radius is about 2.4 times that of earth, or 60% of Neptune. Not much is known about its mass and surface composition but it’s guessed it’s most likely similar to Neptune with a mass of around 35 earth masses. There’s also a possibility that it’s an ‘ocean like’ world with only about 10 earth masses. Depending on its actual mass, it could be rocky, liquid, or gaseous.
There’s an encyclopedia of extrasolar planets at http://exoplanet.eu/ which contains information on all known and potential exoplanets.