This is incredible! Even though I was expecting a number like this, actually hearing it for real is stunning. In 15 years we've found about 500 planets orbiting other stars, but in the almost two years since Kepler launched it may have easily tripled that number! Now, to be careful: these are candidate planets, which means they have not been confirmed. But in most cases these look pretty good, and if these numbers hold up it indicates that our galaxy is lousy with planets. They're everywhere. And it gets better: of those planets found, 54 are in their stars' habitable zones. Now, many of these are massive planets unlikely to be Earth-like, but the huge news is that five are near-Earth sized, and one is actually very close to Earth's size! If this pans out, then it implies there could be a million Earth-like planets in the Milky Way galaxy. Holy Haleakala. OK, so what's the scoop here as far as science goes?
Kepler detects planets by looking for the tell-tale dip in light as they pass in front of their star, and the amount of light blocked tells us the size of the planet. Kepler is staring at one part of the sky, continuously looking at 156,000 stars for these dips. After 23 months in orbit, it now has a passel of 1235 candidate planets. Of these, 68 are roughly Earth-sized, 288 are bigger than Earth, 662 are roughly Neptune-sized, 165 are Jupiter-like, and 19 are larger than Jupiter. Just these numbers themselves are pretty amazing. If 70 are Earth-sized and 20 are bigger than Jupiter, this means that planets like Earth are actually far more common than super-Jupiters! Up until now we couldn't know this. In fact, far more super-Jupiters were found because they're bigger and easier to detect. But now it looks like there may be many more Earth-sized planets than we thought. Now, to be careful, these are not necessarily Earth-like planets: they may be very close to their stars and very hot, or may not have atmospheres, or may have poisonous atmospheres, and so on. But you have to realize that the more Earth-sized planets we find, the more likely it is that some, just by chance, are in their star's habitable zone where liquid water can exist on the surface. And to be clear, "Earth-like" doesn't mean it's a twin of our home planet, just that conditions are similar -- Mars is Earth-like in many ways, for example. But this news by Kepler means it's far more likely that we'll eventually find a planet that looks very much like our own. Mind you, Kepler is only looking at a sample of stars that is one one-millionth of all the stars in the Milky Way. So it's not totally silly to take these numbers and multiply them by a million to estimate how many planets there may be in the galaxy... 70 million Earth-size planets, and a million in the habitable zone of their stars. A frakking million. In our galaxy alone. And while I have to note that some of these candidate planets may not pan out (they may turn out to be false alarms), as time goes on Kepler is likely to find more smaller planets in its sample, since the farther out they are from their star the more slowly they orbit, so more time is needed to uncover them. I strongly suspect that that million number is not likely to shrink.
[UPDATE: the transit method employed by Kepler only finds planets that happen to have orbits edge-on as seen from Earth. That means it doesn't even see planets that have orbits tilted with respect to us, meaning that this estimate is almost certainly too low. And I was worried about overplaying this number! My thanks to Proesterchen in the comments for reminding me of this.]
Also, I don't want to dismiss the massive gas giants either. As we see with Jupiter and Saturn, big planets can have big moons. If Jupiter orbited the Sun where Earth is instead of five times as far out, its moon Europa wouldn't be a frozen world, it would be an ocean world as big as our moon! Saturn's moon Enceladus is also a frozen iceball, and would be a liquid waterworld if it were closer to the Sun. That artwork at the top, done by my friend and planetary scientist Dan Durda, might look fanciful to you, but what Kepler is showing us is that there very well may be moons with views like this. Maybe not with plants and birds and other life, but I'll be honest: this is starting to look less like science fiction and more like science. During the Kepler press conference, planetary astronomer Debra Fischer called this "an incredible, historic moment." I agree! For years we've been making progress toward finding another blue-green world around another star, and this news means we've taken a really big stride in that direction. For the first time in human history, we can look out into the night sky and actually and realistically and scientifically consider the presence of other Earths out there. Science! I love this stuff. Image credits: Dan Durda; NASA/Tim Pyle