You may have heard the news, announced earlier this week, about the presence of tens of billions of habitable planets in our galaxy. This is big news for two reasons: one is related to the simplest astronomy one can do; the other is related to astronomy so difficult and complicated we don’t even know how to do it yet. (But we’re working on it.)
But before we get to that, let’s set the stage with an analogy. You can think of the Kepler mission as a poll of the stars in the sky. In a poll, you sample a subset of the population, and ask everyone you talk to the same set of questions. Who are you supporting for President? What political party are you affiliated with? What is your opinion on this hot-button issue? And so on… Then, based on their answers, and some statistical corrections and extrapolations, you come up with an estimate for what the broader population thinks about a topic.
Well, the Kepler mission is doing something similar. It has been staring at a small patch of sky – about the size the of the palm of your hand when outstretched – and asking the stars in that sky a consistent set of questions. How many planets do you have? How big are those planets? What is the orbiting distance (the distance between the planet and the star) of those planets? And so on… Well, based on the number of planets the mission has found, their properties, and some statistical extrapolations from those data (the press release was on a paper that took a nice statistical approach to the Kepler data set), we have an estimate for the number of planets in our galaxy that could harbor life.
That answer appears to be in the tens of billions. Now, here’s the point where the scientist in me wants to be precise and complete, and tell you that there’s some debate over how many billion… but the amazing thing about the era we live in is that even pessimists will give you an answer in the billions. Optimists may put the number closer to 100 billion. Either way, that’s a LOT of planet, and it’s tremendously good news for astronomy both simple and complex.
The simple astronomy is to go out into the sky and count 5 stars. According to the study, at least one of them probably has a habitable planet around it. More pessimistic estimates might say you have to count 10 or 20 stars before you find a habitable planet. More optimistic estimates say you have to count only 2 or 3. But the crazy thing is… you only need the digits on your hands (and maybe your feet) to count enough stars to find a potentially habitable planet. Go out into the night and do this some time…. even in a city with lots of light pollution you should be able to count enough stars to cover a few potentially habitable planets.
So do these planets have life? Sadly, we don’t know yet. All we know about the population of planets is what we asked in the survey – specifically, their sizes and their distance from their host star (their Sun). We don’t know if these planets have oceans, or atmospheres, or the chemicals needed for life, or many of the other factors that have led to the presence of life on Earth. And we definitely don’t know if they actually have life. In order to test that, we need to study individual planets in something that is less like survey and more like a focus group.
Getting to that focus group is going to require two advancements. First, we have to find planets that are closer to us than the ones Kepler has found so far. Most of those planets are too far away for us to conduct a follow-up interview. In our analogy of the focus group, imagine we could only conduct the focus group in our neighborhood. We don’t know where most of the planets in our stellar neighborhood are, but NASA is currently planning a search for them with the TESS (Transiting Exoplanet Survey Satellite) Mission. In the poll analogy, TESS is doing the equivalent of a census of our neighborhood.
Meanwhile, we have to figure out how to conduct our focus group… which means we have to find a way to take a picture of individual planets in our neighborhood. That’s where the difficult astronomy comes in. Eventually, our goal is to get an image of some nearby, potentially habitable worlds, and analyze that image for evidence of habitable environments and of life. That is extremely difficult to do, because the star is so much brighter than the planet. Think about trying to track a ball or a plane as it passes by the Sun in the sky – the Sun blinds you and you struggle to see the object you’re tracking. The same thing happens with stars and planets – the stars are so bright, they make it impossible to see the planets orbiting it. We need to find a way to build a telescope with the equivalent of a nice pair of sunglasses. I’m not convinced we know how to do it quite yet, but every single day some incredibly brilliant work is moving us closer to being able to build such a mission.
This new study is very important to that mission… because the distance to the nearest habitable planet is a major driver of how big a telescope has to be in order to see our nearest potential Earth-like neighbor… and the size of the telescope is one of the big drivers of the cost of the telescope. So, putting the logic chain together, if potentially habitable worlds are more common, there’s a good chance there’s one relatively close to us, and if there’s relatively one close to us, then we may be able to image it with a relatively inexpensive telescope, and if we can do it relatively inexpensively, we may be able to start to search for life beyond our solar system a little bit sooner.
So that’s two pretty good reasons that this is a big deal. The first is related to astronomy you can do with your eyes and your fingers (and, if you’re a pessimist, your toes). The second is related to astronomy that can only be done with a mission that at this point is nothing more than a concept. And we’re working on that mission constantly, to bring us closer to the day when we can sit the planets in our neighborhood down to an interview, and find out if they host any life.
In the mean time, head out into the night and count to ten. Then smile, because one of those planets has some fantastic possibilities.