The mission of the Kepler spacecraft, as you might remember, was to look at a patch of the sky, find transiting planets, and determine how many Earth-like planets are in the habitable zones of sun-like stars. Sadly, the Kepler spacecraft was disabled before it had a chance to look for long enough to be able to see these things that are far enough away from their stars. And while the Kepler spacecraft has been revived and will be continuing to make interesting discoveries it will not answer that question that it was designed to do. How many stars that are like the sun have Earth-like planets or at least terrestrial planets in the habitual zone. You can mourn for Kepler and say it was a big failure. Except that you would be crazy. Kepler has been tremendously successful. The discoveries have been astounding and if nothing else, I would say what Kepler has done is shown us that we knew, of course that hot of Jupiter's existed close to their stars. But now we know that there are hot terrestrial planets and by hot I mean terrestrial planets inside the orbits of Mercury, that these are incredibly common in the galaxy. Now, as you remember the way that the Kepler spacecraft works, and it sees a transit. It's a lot easier to see a transit of something that's close like this then far away, so there was a tendency to find these things, but that doesn't matter. We didn't know that they were there before at all. They're now there in abundance. If we wanted to study planets that were abundant, this would be the sorts of things to do. Now of course, these are all too close to their stars, they are fried and they're not going to be habitable. But Kepler also found that small planets, by small I mean rocky, terrestial-ish sized planets are common around not just sun-like stars, but also around these cooler stars that we talked about a little earlier. M Dwarfs, M Dwarfs are proportionally, maybe this big compared to the sun. They are cool, they're red. They are the most common type of star in the galaxy, and they have very close planets just like sun-like stars. There two really exciting things about that. One, is that their close planets because the red dwarf, the M dwarf, is so cool, their close are going to be proportionally cooler too. In fact, some of them could well be inside the habitable zone, and that's great. But there's actually another really exciting thing about M dwarfs. And I'm going to show you by showing you the big picture of the sun right here. Here's proportionally about the size of the M dwarf. Hear is an Earth-sized planet that was transiting across the sun. Making a very small signature that would be very difficult to detect. Take that same planet, move it across the surface of the M dwarf. It is proportionately bigger, because the planet itself is proportionately bigger at that M dwarf. That means many different things. One, it's easy to find close planets. So you finding close planets around M dwarfs is a relatively easy thing to do compared to more distant planets around sun-like stars. They're common, they make a big signature if you look at their transmission as they make a transit and of course these close ones could be. Be the temperature that would actually be in the habitable zone. In all of Kepler's discoveries, there is one such planet that best fits this whole description. And I'll show you a representation of it now. This is Kepler-186. Kepler-186 is crazy. Here's for scale, the solar system, Mercury's orbit is right here. These are all to scale. Mercury's orbit here. Venus, green is supposed to be the habitable zone. So in the Earth, the habitable zone extends out through here. And here's the Kepler-186 system. All of these planets, b, c, d, e, four planets packed way inside of Mercury's orbit and inside of the habitable zone here. But f Kepler-186f, you should remember that name. It's the same size as the Earth and it's in the habitable zone, sort of. You know, so again, habitable zone shmabitable zone we don't really know what a habitable zone is. This is on the outer edge of the habitable zone, by this calculation, it's kind of the equivalent to the location of Mars in our solar system. So you can try to guess what the planet would be like and you would come up with something that's cold that maybe is Mars like. Now, Kepler-186, itself is actually the star, the M dwarf right here is so far away that it's particularly faint. It's not going to be possible to get a spectrum abit in transit. But there is this great project of which I'm a big fan being carried out by David Sharvelorn, a professor at Harvard University. And what they're doing is going with a series of small telescopes both in the northern hemisphere and the southern hemisphere, and systematically looking at all the close, nearby bright M dwarfs because they're nearby. And looking for those to see if they have signs of planets. How do you do it? Well, you just look, you set there and stare at it for long periods of time and look for a dip in the light, just like Kepler did. It's harder to do from the ground, but you're looking at bright things and so these small telescopes can do the task. The great thing about using small telescopes, bright stars, is that when they find planets, the host stars, these M Dwarfs, are close enough, they're bright enough that you can now go to things like Hubble Space Telescope, use that to measure the transmission spectrum. And in fact that's been done for a star called GJ 1214, just a name of a star. It's only about 40 light years away. And it, not surprisingly, for this, one of these early discoveries, it's a little bit close to its host star. Remember, they're easier to discover when they're close. So those are the ones you're going to find first. And it turns out to be a little bit toasty. So it's not going to be a place that is actually in the habitable zone of this star. But a very nice project was carried out looking to see what its atmosphere looked like. And it's one very similar to the one that you saw from Professor Knudson where you see a very featureless looking spectrum indicative of high clouds that are sort of preventing you from being able to see what's really going on on that planet. I think, though, that's just a proof of concept. This is the easiest current way to go explore potentially Earth-like planets. Now we say potentially Earth-like planets in the sense that it's about the size of the Earth, it's presumably a terrestrial planet, but its environment is pretty weird. The temperature might be the right temperature to support Liquid water, but the star is very red, very cool, very close by. And one other strange thing about this, is an interesting one to look at back on the habitable zone plot, you remember from a few lectures ago, here is where the habitable zone is and we're now talking about M dwarfs down here. And the habital zone is indeed quite close inside the orbit of Mercury. But another thing happens when you get a planet quite close. If a planet is closer to its star than a certain distance, then that planet will be tightly locked to that star, tightly locked means the same face, faces it all the time, just like the same face of the moon faces us all the time, and just like Mercury is not quite tidily locked. It's right at that radius right here. But Mercury is tidily forced by the sun to rotate three times for every two times it goes around the sun so it's almost in a state of being tidily locked. The closer you are to the star the more likely you are to be locked. Things like the Gillian satellites are all tidily locked in cells tightened are all tidily locked. And these distances are similar to those so and interesting things happen in this Emmett Dwarf that we're talking about. And these M Dwarf planets to be in the habital zone. In fact even if you're more like just a K Star just a little bit cooler than the Earth. To be in the habital zone you have to be tightly locked to your host star. Now, that's a strange thing. If we're going to talk about Earth-like planets, this would be quite strange. There would be a spot where you could stand on that planet. And you look straight up and the sun is there. And it never moves. And there would be spots where it would be perpetually sunset. And then of course, half of the planet would be perpetually dark. What would a planet like that be like? Well people thought pretty hard about this, because again, the idea that looking for things around these M dwarfs is quite compelling. And have come to the conclusion if you have a planet that is totally hot on one side, all the sunlight comes in from here and no sunlight on this side. As long as there is abundant enough atmosphere or even an ocean, that that heat could redistribute fairly effectively, and you could indeed have habitable conditions throughout. People have even speculated for these even closer planets around M dwarfs. Things that are too close to be in the habitable zone that perhaps you have sunlight here. And it's really super hot on this side. And they're winds that are pulling the heat around to the backside. And maybe just in these little spots here, you can have just the right conditions to maintain life at the poles at the ring around where you're in perpetual sunset. To those things, I say. Yeah, maybe I think what were really look for is the best possible cases where you would have planetary wide biosphere and those the ones that really don't like to look at in detail. The first ones that we're going to find I suspect will be this M Dwarfs, would be some habitable zone and dwarf planets, that are the size of the Earth. And it would not surprise me if these are discovered and explored soon. If they're not discovered by these projects that are taking place right now, from small telescopes, they will certainly be discovered by the newest mission to find planets that NASA is launching, the TESS is the transiting Exoplanet Satellite Survey or maybe survey satellite. Seems a perfectly good acronym and here it is looking at all these little things, looking for exoplanets. What has is these four different telescope is basically what it's going to do is go around the sky, the whole sky looking at only the brightest stars, and looking for transits in front of those brighter stars, including M dwarfs. The nice thing about this compared to Kepler, Kepler was able to study more stars because it looked at one patch in the sky. But the Kepler stars were moderately faint, so harder to study. This will be the mission that defines the next set of planets that we will be studying to try to understand the habitability of other worlds in our galaxy.
