• Survey says….

    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.

    • Robert Sevigny

      I think the Kepler results cry out for us to redouble our space based astronomy investment/efforts. My personal view is that we are going to find that these estimates for earth-sized planets in the habitable zone are going to turn out to be on the low side — maybe the very low side. The only way to be sure is to put up missions like the terrestrial planet finder and several other projects that were delayed or cancelled previously due to budget constraints. It should be obvious that we are on the brink of discoveries that will totally change our view of the universe and give a new and irresistible momentum to more ambitious manned and unmanned missions to the planets and eventually to the stars. This will also revolutionize SETI, by giving them real targets to pursue, not just listening to vast swaths of space. Time to get rolling

      • Shawn Domagal-Goldman

        You may be right about the “low side” thing. Personally, I think it depends on how you’re counting. I think the number for “Sun-like” stars will end up around 0.1 (10%). I think for all stars it may approach 0.5 (50%), which is astounding. We’ll get a little more info when TESS and WFIRST fly – TESS will attack planets inside the inner edge of the habitable zone quite well… and WFIRST will do the same outside the outer edge.

        I also think your other points right on the mark.

        Ad astra… and ad planetae? 😉

        • Robert Sevigny

          Thanks for the support. I am excited for the TESS and WFIRST missions, but I still wish we could revive the Terrestrial Planet Finder and the Space Interferrometry Mission (Lite). The tools we are going forward with are too limited to give us the answers we really need. Its time to step-up our game.

          • Shawn Domagal-Goldman

            I hear ya! The good news is even in a case where we don’t get everything we want, we still might get what we need. Or, at least that’s my hope. 🙂

    • Ivor Clark

      As per another thread the numbers extrapolated from the Kepler data are highly debatable. Focussed study of 42000 stars found only 10 ELP candidates, so lets not jump the gun here. Yes this was only fraction of detectable ELP’s because of the plane of observation, but even so I think millions rather than billions is more realistic. However even putting optimistic numbers into the Drake equation reveals we still could be the only techo civilisation around in this epoch in our galaxy. Any who were exposed to the red giant phase of their own suns will be long gone, if they had not already annihilated themselves. As for the proximity of an ELP in a local group of stars again if its only millions of ELPs the numbers have to be rounded down to say one star in a thousand to ten thousand, which basically means possibly one star, excluding our own Sun, on a very clear dark night will harbour an ELP. Food for thought and maybe why we are alone, effectively….

      • Torbjörn Larsson

        There is no connection between the full Drake equation and the habitability vs inhabited issue. Mainly because of biology making many or most individual traits (such as language capable intelligence) unpredictable. There is also the problem of unobservable or near unobservable pathways, LCIs that doesn’t “speak” to others most of the time.

        Meaning that we can pursue astrobiology for non-discriminatory reasons, as one biosphere looking for others and learning more about life.

        • Shawn Domagal-Goldman

          Right! For most astrobiological endeavors, we generally do not discriminate. We’d be happy with any form of life, “intelligent” or otherwise.

      • Shawn Domagal-Goldman

        I think the number of eta-Earth is certainly still debatable, but I think it should be higher than 1%, even if you’re fairly conservative in where you draw your boundaries. I think the range in uncertainty is more like 5% on the low side and 50% on the high side. All those are in the billions to tens of billions of potentially habitable worlds out there.

        As to how many have life? Or intelligent civilizations? Or, for that matter, to measure and confirm habitability and the presence of oceans? That’s what we need the next telescope for. 🙂