Humanity's inability to find Earth-like planets may not be the result of limited technology or a lack of ambition -- we might simply be too early, and in the wrong place. Astronomers poring over Hubble and Kepler data now believe that only 8 percent of the universe's potentially habitable planets exist. Based on the amount of hydrogen and helium gas left over from the Big Bang, there's still plenty of opportunity for those planets to form. These planets are most likely to appear in either dwarf galaxies or giant galaxy clusters, where the stars haven't used up all the surrounding gas.
The problem, as you might have gathered, is that the Milky Way galaxy isn't part of that bunch. Most of its gas has already been used up. And while there's an estimated 1 billion habitable planets in the galaxy, that's a drop in the bucket when the Milky Way holds as many as 400 billion stars. Earth is not only relatively early, it's in an uncrowded cosmic neighborhood. That's disappointing if you were hoping for a Star Trek-like abundance of extraterrestrial life, but it at least puts Earth's loneliness in context.
[Image credit: NASA, ESA, and G. Bacon (STScI)]