On October 16, 2012, the discovery of Earth's nearest exoplanet was announced. Alpha Centauri Bb, so named because it orbits the 'B' (second) star in the Alpha Centauri system, and is the first planet discovered orbiting this star (planets are ordered alphabetically from the date they were discovered, i.e. 'b', 'c', 'd', etc. skipping 'a'). This planet appears to have a mass similar to our own, but orbits much closer to its parent star and is unlikely to be able to support life. Still, the discovery is important for a number of reasons. First, the fact that we have found planets as close as our neighbouring star provides further support that the number of exoplanets in the galaxy could be quite large (currently estimated to be about 1.6 planets per star). Second, at 4.3 light years away, there is even the possibility of sending a probe to investigate this planet. However, even travelling at one-tenth the speed of light (which is well beyond our current capabilities) it would take 40 years to reach the star (not including acceleration and decceleration) and another four years for data transmission. However, this trip is much more reasonable than probing the previously known closest exoplanet.
The number of exoplanet candidates discovered by Kepler alone has grown to over 2300 (as of 2012). A few of the more interesting discoveries are mentioned here, with links provided for further study.
Alpha Centauri Bb, our Closest Neighbour
The sixth planet found to be orbiting the star HD40307 is likely at a suitable distance from it's parent star to support liquid water, one of the conditions many believe is necessary for life. At roughly 7.1 Earth-masses, this planet classifies as a Super Earth. The other planets in the system are believed to be too close to their sun to support life, which means that this solar system is organized quite differently than our own. The discovery is significant because it is the first planet discovered relatively close to Earth-size that can support liquid water, and potentially life.
55 Cancri e, Diamond Planet
This exoplanet, orbiting a star approximately 40 light years away, is roughly eight times as massive as the Earth and orbits with a period of only 18 hours, very close to its parent star. What makes this planet most remarkable however, is that an estimated one third of the planet by mass is composed of graphite and diamond, which is about two or three earth masses total. The planet is expected to have a thin layer of graphite mixed with diamond, followed by a very thick layer of diamond much purer than any found on Earth. Below this may even be liquid diamond. Scientists are excited to study the chemistry on this planet, which is quite different than what we see on Earth.
Sample of academic sources for other exoplanet discoveries:
Other recommended links: