An Exoplanet or Extra Solar Planet is a planet orbiting another sun or star outside the Solar System. Astronomers are searching for exoplanets because it is possible that some, perhaps many of them of them may sustain alien life.  SETI is seriously looking for alien signals. In late 2009 over 400 exoplanets are known.  Most are Gas giants at least as big as Jupiter, Saturn, Uranus and Neptune. Some are many times heavier than Jupiter. Smaller exoplanets may be very common but are harder to find with present intsruments.
Others are smaller and could sustain life like the Earth. COROT-Exo-7b is the smallest yet discovered and has a diameter less than twice that of Earth. Scientists expected such planets before it was found and were not surprised to find it. COROT-Exo-7b orbits close to its sun and must be too hot for our kind of life but others like COROT-Exo-7b may be at the best distance for life. Scientists plan to look for water and oceans on exoplanets as life possible on such planets. 
Recently in 2017, NASA's Spitzer Space Telescope discovered the largest batch of Earth-sized, habitable-zone planets that surround a single star and could potentially harbor life! The system was named TRAPPIST-1 (The Transiting Planets and Planetesimals Small Telescope) after the Belgian operated telescope located in Chile. The discovery set a new record for the greatest number of habitable-zone planets found surrounding a single star outside of our solar system. The system of planets is relatively close to us as it is about 40-light years or 235 trillion miles from Earth and contains both a good atmosphere and possibly liquid water. The sun (TRAPPIST-1) is an ultra-cool dwarf, which is so cool that planets with liquid water could survive orbiting very close to it. Here's a detailed video of the newly found TRAPPIST-1 system.
- Planetary systems
- Gliese 581 and its Planets There may be life in this system.
- Carbon Planet
- Chthonian Planet
- Circumbinary Planet
- Eccentric Jupiter
- Hot Jupiter
- Ocean Planet
- Extrasolar planet Wikipedia article
- COROT discovers smallest exoplanet yet, with a surface to walk on
- New method to seek life on exoplanets
- Scientists announce planet bounty
- First Time Photos of Extrasolar Planets! Well worth watching
- First Image of an Alien Planet
- First "Earth-like" planet found (April 2007) Video about a planet where there may be flattened bugs.
- Do Other Solar Systems Exist? Explaining how other planets are discovered.
- Life in the Universe, Just on Earth, or Everywhere? Life and exoplanets.
- Can a Planet Have Two Suns?
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