I'll bet you thought that just because Earth is ludicriously overstuffed with life means that our planet is in a pretty ideal spot for living things to flourish. But, it's not. The habitable zone — which is the distance from a star where it's not too hot or too cold for life as we know it to survive — has just been redefined, and Earth almost didn't make it.
For a star like our sun, the habitable zone is somewhere between 0.99 AU and 1.7 AU. One AU is defined as one Earth-sun distance, meaning that we're just barely on the inside edge of where life is possible at all. Lucky us, right?
The habitable zone definition is looking at two things: how close can a planet be to a star without having all of its water boiled off, and how far can a planet be from a star before it loses too much heat, even if you include the strongest possible greenhouse effect. The actual numbers for these distances depend on how much energy water absorbs and how much energy carbon dioxide can hang on to, and the original habitable zone was based on numbers from twenty years ago. Updated measurements (from 2010) have tweaked the numbers a bit, moving the zone outward by about 0.04 AU.
The image above shows various cloud-free habitable zone boundaries for stars of different temperatures. The boundaries of the green-shaded region are determined by greenhouse values, while the two dashed vertical lines show the habitable zone boundaries irrespective of the stellar type. Some currently known exoplanets are shown, while the ‘?’ for Gl 581 and the Tau Ceti system of planets refers to the fact that we're not yet sure whether they actually exist or not.
This is kind of a big deal for several reasons. First, the fact that the Earth is (on this scale) close to being NOT habitable suggests that life may be way better at being life than we give it credit for. And second, it means that we can take another look at all of those exoplanets that we've found and reevaluate their potential for habitable-by-somethingability. For example, Gliese 581d (which we got maybe a little bit overexcited about back in 2011) has now been promoted from habitable zone outer edge to habitable zone sweet spot, making prospects for life there "much better," according to astronomers.
There are still lots of things that the current habitable zone model doesn't take into account, like clouds, which can have a significant effect on both heat retention and solar energy reflection. It's a safe bet that future models will shift the habitable zone once again, and we just have to hope that it Earth doesn't end up outside the lines completely, which would of course instantly kill us all.