What astronomers think
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. As of 2019 there are over 4,000 known.  More are continually discovered, Most known exoplanets 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 instruments but some could sustain life like the Earth, we don't know yet. COROT-Exo-7b is one of 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. Kepler 10b is certainly a rocky planet like Earth  but COROT-Exo-7b and Kepler 10b orbit close to their suns and must be too hot for our kind of life. Others like COROT-Exo-7b may be at the best distance for life.  Scientists plan to look for water and oceans on exoplanets as water sustains our type of life.  Kepler 22 is at a distance from its sun where liquid water is likely  as are Kepler 186f and Kepler 438b. 
- Planetary systems
- Gliese 581 and its Planets There may be life in this system.
- Carbon Planet These are similar to earth like planets but low Oxygen would be a problem for any life.
- Other types of exoplanets
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- Volunteers wanted for planet hunt help find exoplanets yourself
- COROT discovers smallest exoplanet yet, with a surface to walk on
- New method to seek life on exoplanets
- Scientists announce planet bounty
- Scientists spot nearby 'super-Earth' Despite what the article claims other Red dwarf stars are known to have planets, notablyGliese 581
- 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?