The more we discover about exoplanets, the more we seem to be discovering that Earth-like planets (small rocky planets stably orbiting within the goldilock's zone) are not at all rare. This doesn't necessarily surprise me. I remember back in the 1990s and early 2000s it was not yet predominantly accepted that planets themselves were common. It now seems obvious that planets outnumber stars by a minimum of 2:1. I would deduce that the ratio is likely much higher. Therefore, it does not surprise me at all that 1 in 6 solar systems are likely to have an Earth-like planet. Just last year astronomers discovered that our closest star system possesses an Earth-sized planet:

http://www.theadvancedapes.com/theratchet/2012/10/19/discove...

And in December 2012 astronomers discovered an Earth-like planet for the first time. All of these discoveries happened as soon as scientists were able to detect Earth-sized planets orbiting in the goldilock's zone. They hadn't been found in the 1990s and early 2000s because the technology and detection methods to find them had yet to be developed (not because they were rarer than other types of planets). By 2020s we will have likely discovered hundreds, if not thousands, of Earth-like planets. For those not particularly well-informed scientifically, this should revolutionize their understanding of the Universe and our place within it. I would imagine that by the 2030s - when Hubble Telescope successors have been scanning our galaxy for several years - we will also likely have images of these Earth-like planets. And that will truly revolutionize more than just astronomy. It should make astrobiology and astrochemistry, real scientific fields of inquiry.

For more on exoplanet discoveries from 2012, see the link below (#9):

http://www.theadvancedapes.com/theratchet/2012/12/22/top-sci...

And if your interested in listening to a podcast on exoplanet discovery, see the link below:

http://www.theadvancedapes.com/itunes-podcast/2012/11/2/exop...