Now, that pesky Kepler space telescope has informed us that Kepler 10c is a planet 560 light years from here that ought to be a gas giant but instead is a rocky "mega-earth," with 17 times our mass. This is one big, heavy chunk of real estate revolving around a sun-like star, but one twice as old as ours. A little more than twice as big around as us, with that weight, this is a rocky planet.
This messes with planetary expectations. A ball this size should have pulled in a whole lot of gas and shrouded itself in clouds like Jupiter, Saturn, Neptune or Uranus ... but no. For now, 10c is considered an outlier among the planets ... but that might not be true. Ol' 10c here could be common as dirt in the grand scheme of things. This increases the potential for habitable planets quite a bit ... and the age of its star suggests life could have quite a long run on such a rock. That gives a lot more time for sentient beings to rise up and develop on a massive heap like 10c.
Of course, this could also be why we don't hear from them. If creates have grown up and gained intelligence long ago on worlds like 10c, it is possible that they are now a civilization of oldsters who just want the young whippersnappers of the galaxy to leave them along and stop playing in their lawn, dag nab it. Perhaps time will tell.
For more on life in the galaxy, see: http://jsbrookspresents.blogspot.com/2014/05/100-million-planets-with-complex-life.html