Let us see what we can learn about those planets or planetary systems. Now, all the pictures that I'll show you here are artist's impressions, right. We haven't gotten pictures like this of extrasolar planets and will not for a long, long time. There are many interesting individual cases. The one that was in news recently was planet was found around the nearest star to us, Alpha Centauri, which is actually a binary star. So, it's the beacon point that has, at least, one planet going around it. So, if and when someday we have interstellar travel, this will likely be one of the first destinations to get. More recently, there was this five planet system that they've seen eclipses with Kepler satellite that correspond to not one, but five planets in a row. And some of them actually are in the habitable zone. There was a recent, very recent, case of first Earth-like planet by mass at least in habitable zone around a red dwarf star. So, what is it we can do about studying it? From radial velocities, you can measure velocities obviously, you can measure period, cuz that's the variation. Therefore, you know what the size of the orbit is, for you've got velocity, you got period. And then from that, you can infer mass using Kepler's Laws, especially if you know what kind of star it is, and we have some understanding of stellar masses. But also depending on the shape of the radial velocity curve, is it an ellipse and how tilted it is to you, you can also infer something about the shape of the orbit. So measure eccentricities of orbits. In transits, you obviously can measure planetary radius because it's the square of the radius that matters, how much light is obscured just from the depth of the eclipse. And now, if you already had mass from radial velocity measurements, you can infer density. If you know the density, you can start making statements about possible chemical composition like we do in our solar system. The rocky planets might have densities of the order of five grams per cubic centimeter gaseous giants, mult two, and so on. And so, you can start making some statements in combination with likely temperature since you know the radius and distance from the star, and what could be physical state on it. If you know how far the thing is from its parent star, you can infer its temperature, we can module albedo, and there is a very clever trick of measuring composition of the atoms. Here, I'll show you in a moment. But first, these basic quantities, the sizes and the masses. And so, this is the distribution of the known exoplanets in short period orbits, periods that are like Mercury. Because planets on larger orbits takes longer time to discover them, and so there's an obvious selection effect and distribution of their masses. We can see that planet sizes tend to pile up at the low end. This is not a surprising thing because you expect there to be more smaller planets than big ones. But now, we know for sure since the selection effect is against finding smaller planets, the bigger planet, easier to detect, that majority of planets in our galaxy are smaller. And the same thing applies about the masses. What's shown there for comparison are Jupiter and Neptune. Now, what about planetary axes and shapes? Well, here there is a very strong bias to find planets on smaller orbits, because this is where you see higher radial velocities, this is where you see more often the eclipses. And so, most planets that we now know are these small orbits, like within Earth's orbit radius, but we do see some further out. And so, there's probably many, many more larger distances. One surprise that came out of this is the, many of the extrasolar planets have very eccentric orbits. Orbits of planets in our solar system are almost circles. They're very slightly eccentric eclipses, and that result to be actually something of an exception. And there's been a lot of interesting dynamical studies why this could be, and how gravitation interaction planets and their planet start in migrate planets closer in and so on. But if there has been one important upshot of these studies, it is this. And this is the plot of distribution of sizes. And now, because you know the geometry, and you can assume that on average, orientations of orbits of planets would be isotropic. So you can make statistical statement, how many there has to be in order for us to be able to see them from our direction? That's pretty simple geometry. So you can boost up the numbers using that argument. And when doing that, the estimate is that there are more planets than there are stars in the Milky Way by at least a factor of two. And this is lower limit because it's based on the other stuff that we know. In other words, planets and planetary systems are very, very common. So there are at least 400 or 500 billion planets in the Milky Way, probably many more. Now I mentioned to you that you can measure atmospheres of extrasolar planets, and that's pretty cool. Heather Knudsen, here at GPS division, is one of the people working on that. And the idea there is that as the planet is transiting the disc, the planet star. Some of the star light will go through the edge on atmosphere, and the absorption spectrum will change ever so slightly. So, if you take a really high signal noise spectrum of the star, before or after the eclipse and during the eclipse, and subtract it to with proper scaling to account for the dip in the transit. Then you have differential absorption spectrum is due to planets' atmosphere alone. Now you think this is going to be really hard, and you're right. This has never been possible before until we had super high precision photometry with Kepler and large telescopes, like Keck, to measure the spectrum. So then, of course, everybody wants to know how many of these hundreds of billions of planets can support life? And so, there is a big variety of those. They tend to be a little big, because that's the selection effect. But as time goes on, we keep finding more and more planets that are in their stars' habitable zones. And so in principle, they can have liquid water on their surface. What they do with that is another story. And one last thing, a relatively new discovery, and this one is gravitational microlensing really played an important role, is that there are planets with no stars. They're just loose planets floating between stars in interstellar space. And how do we know that? Well, because they can cause gravitational microlensing events, but their masses are much, much smaller than even brown dwarfs. And so, if you can observe them in infrared and so you can infer the physical properties, and now, there is maybe few of these now. And so, there are planets that's just outside any planetary system. Now, how did they get there? The likely possibility is that they got ejected from their solar systems through dynamical interactions. Just like say, Jupiter, in our own solar system perturbs orbits of comets and asteroids. Occasionally, some of them get enough kinetic energy to get kicked out. Well, in the early days of planetary systems, such things could have been more common. Or, if you have a planetary system forming around a binary star, similar things can happen, and it can clear up some of the planets, push them into interstellar space. Or, they may have even formed independently during the star formation process. They weren't even massive enough to become brown dwarfs, but they become interstellar Neptunes or something like that.
