I believe that life is common throughout the universe and that we will find another Earth-like planet within a decade.
The mathematics alone ought to be proof to most people (billions of galaxies with billions of stars in each galaxy and around most of those stars are planets). The numbers suggest that for life not to exist elsewhere in the universe is the unlikely scenario. But there is more to this idea than a good chance. We've now found more than 130 planets just looking at nearby stars in our tiny little corner of the Milky Way. The results suggest there are uncountable numbers of planets in our galaxy alone. Some of them are likely to be earthlike, or at least earth-sized, although the vast majority that we've found so far are huge gas giants like Jupiter and Saturn which are unlikely to harbor life. Furthermore, there were four news events this year that made the discovery of life elsewhere extraordinarily more likely.
First, the NASA Mars Rover called Opportunity found incontrovertible evidence that a briny--salty-sea once covered the area where it landed, called Meridiani Planum. The only question about life on Mars now is whether that sea—which was there twice in Martian history—existed long enough for life to form. The Phoenix mission in 2008 may answer that question.
Second, a team of astrophysicists reported in July that radio emissions from Sagittarius B2, a nebula near the center of the Milky Way, indicate the presence of aldehyde molecules, the prebiotic stuff of life. Aldehydes help form amino acids, the fundamental components of proteins. The same scientists previously reported clouds of other organic molecules in space, including glycolaldehyde, a simple sugar. Outer space is thus full of complex molecules—not just atoms—necessary for life. Comets in other solar systems could easily deposit such molecules on planets, as they may have done in our solar system with earth.
Third, astronomers in 2004 found much smaller planets around other stars for the first time. Barbara McArthur at the University of Texas at Austin found a planet 18 times the mass of Earth around 55 Cancri, a star with three other known planets. A team in Portugal announced finding a 14-mass planet. These smaller planets are likely to be rock, not gas. McArthur says, "We're on our way to finding an extrasolar earth."
Fourth, astronomers are not only getting good at finding new planets around other stars, they're getting the resolution of the newest telescopes so good that they can see the dim light from some newly found planets. Meanwhile, even better telescopes are being built, like the large binocular scope on Mt. Graham in Arizona that will see more planets. With light we can analyze the spectrum a new planet reflects and determine what's on that planet—like water. Water, we also discovered recently is abundant in space in large clouds between and near stars.
So everything life needs is out there. For it not to come together somewhere else as it did on earth is remarkably unlikely. In fact, although there are Goldilocks zones in galaxies where life as we know it is most likely to survive (there's too much radiation towards the center of the Milky Way, for example), there are almost countless galaxies out there where conditions could be ripe for life to evolve. This is a golden age of astrophysics and we're going to find life elsewhere.