[MUSIC] Welcome to the last week of the course on exoplanets. In the previous weeks we have learned a lot of things about exoplanets. We learned how to measure the orbits, we learned how to measure their masses, their radii. We have learned about the global properties of the exoplanet population and now we are going to, to move to move a step further and try to characterize better the planets themselves, their, their, their internal properties and in particular, their surface properties and atmospheric properties. Of course, with the masses and radii, that we can measure with the transit and radial velocity techniques. We are able to constrain already the internal structure of these objects because we, from, from these measurements we get mean density of these objects and is a very precious number from which you can derive the main composition. The main, component of these, of the interior of these objects. But, of course, mean density alone is often not enough to lift the generacies, in bulk composition that, we may have. And, of course it also provides no information whatsoever on the surface conditions. And, atmospheric properties. So in order to illustrate better this point I, I am showing here, a mass density plot. This mass density plot, is restricted to low mass planets, Neptunes and super Earth. So you can see on this plot, some objects that have been measure with sufficiently high precision. so, for which we have, good masses and good radii, which means relatively precise densities. You can see that there is a wide diversity in this, in the composition of these objects. In the plot you can also see some [UNKNOWN] curves that will give you theoretical models of internal structure for a given bulk of positions. So, for example, you will see the red curve that shows you the density of the function of mass for a planet that would be made out of pure rock. Hundred percent rocks. You also have blue curve that shows you the density of a planet. If it was made only of water. And so on top of these curves you has several objects that has been discovered in recent years and at a given mass for example if you will, if you look at masses we can find on ten earth masses. You can see that there is huge diversity in compositions. We have objects that are most likely rocky, for example Kepler 10b that has a high density for its mass, so most likely rocky composition. But at almost the same mass you find objects such as Gliese 1214b. Here, at the bottom of the plot. Which has a very low mean density that indicates significant hydrogen helium envelope to explain its low density, and you also have an intermediate cases, like 55 can crate E. In this plot. And, so, this is an object that could be made of rocks and, a significant fraction of water, but it could also contain some hydrogen, we just don't know at this point. And that's where we hit the limits of having only the mean density of these objects. And we can't find any help, unfortunately. By observing the solar system planets that we know very well. And that's because such planets simply don't exist in the solar system. There's nothing between Earth and Uranus, which has a mass of 14 Earth masses. And already has a significant Hydrogen, Helium envelope. So these objects are very exotic. And to understand them, we really need, need to gather more information. So the obvious way forward here is to have spectra of these objects. And that will allow us to measure the chemical composition of the atmosphere. Here, and a also pressure and temperature profile and also physical parameters and the goal would be to understand the atmospheric frequency conditions on these objects. And also, to understand how they were formed and evolved with time. So, before, moving to exoplanets I, we just stop within our solar system just to illustrate just to illustrate what we can get from a spectrum. On the right here, you will see the Earth reflecting spectrum. On the left, at the bottom in red, you will see the spectrum of Jupiter, two very different planets. In each case, you would see in a, in the spectra, very clear absorption lines from chemical species that are major components of of the atmospheres of, of these planets. And so you can immediately if you can, if you're able to gather, to get such a spectrum, you can immediately see that water vapour and oxygen, for example, are present in Earth's atmosphere. And on Jupiter you would see methane and ammonia features so you. It just tells you, how much information can be retrieved for, from a good quality exoplanet spectrum. Now unfortunately we have to remember that, we are not yet at that point for sol- For exoplanets. In the solar system, the planets have been studied with a luxury of details. And including with NC2 probes. Unfortunately this would remain out of reach for exoplanets in the foreseeable future. So on the observation side then. We have to obtain high quality spectra for exoplanets, with specifically designed instruments if possible. And on the theoretical side, we have to develop robust retrieval methods. To derive properties from the available information that we'll discuss, we will have only a partial view of exoplanet atmospheric physics. And we have to be able to infer properties from a limited amount of data. So that's it for this little introduction, in the next session, we are going to discuss how to interpret exoplanet spectra. Thank you. [MUSIC]
