A bit late, but hopefully still relevant.
There’s been a lot of hype about NASA’s Kepler telescope’s discovery of a potentially earth-sized planet orbiting a distant star, and there should be. Space exploration and discovery is hugely important. There’s a huge, complicated universe of fascinating stuff out there that we are only now just beginning to explore. But to say that this puts us closer than ever to finding extraterrestrial life is a massive oversimplification. The reality is, the odds of finding a planet like this at some point have always been fairly high, while the odds of finding life beyond the Earth is the same as it always has been.
The Drake Equation, which is purely theoretically but widely accepted equation for estimating the probability that we will ever be make contact with an alien civilization, explains this in painstaking detail (this focuses purely on the probability aspect of it, assuming the possibility of alien life could exist, all theological and philosophical arguments for and against such a prospect aside).
The probability that a planet will end up in the habitable zone of a star should actually be fairly high. It’s not uncommon for stars and planets to have objects orbiting them, and simple statistics would dictate that eventually these objects would fall into the habitable zone (or Goldilocks zone) where the heat from the star isn’t too intense or too weak to support life. The newly discovered planet, Kepler-186f, orbits its star once every 130 days on the far edge of the habitable zone.
The fact that the planet is roughly the same size of earth is a much bigger and more rare discovery, but also not unprecedented. After all, in our own solar system, Venus is roughly the same size as earth. And since the size and mass of the planet is based on the Kepler telescope monitoring variations in a star’s appearance, there’s definitely some margin for error, although this method is actually a lot more accurate than it sounds (you can read more about these methods here). But while the size qualification is harder to come by, it’s still one of the easiest parts of the equation. After all, there are many rocks in the world the size of the Hope Diamond, but very few rocks are actually diamonds, and very few of those are actually on par with the Hope Diamond.
The part where the Drake Equation makes the search for life much, much more difficult is the next variable, “the fraction of planets that could support life that actually develop life at some point.” It would be an understatement to say that this variable is a billion times more unlikely to occur than the previous one.
This is not to say that NASA’s discovery wasn’t important. The fact that there are other planets orbiting other systems just adds to the wonder and complexity of the universe. But to say that we’re now closer than ever to discovering extraterrestrial life seems like an exaggeration at best. Space is so huge, so complex, and so weird, that there’s no need to exaggerate.