[MUSIC] Welcome to this new week of the diversity of exoplanets. So this week we're going to talk about the latest news from exoplanetary atmospheres. So, and this introduction, we will go throughout the basic concepts and we are going to call back notions that you have seen in the previous weeks. In particular during week three about planet detection and especially the power of transiting planets. And during week six about the observational approach to exoplanet atmospheres. So this is aimed at providing you with the recent update of the exciting discoveries in the subfield of exoplanetary science. Atmospheres of exoplanets is arguably the most frantically moving area of the characterization of exoplanets. And it is actually likely that this course is going to be outdated in a couple of years. On the other hand, we thought that it is really worth trying to convey you the rapidity of evolution of this field and the excitement that it produces. So what are the questions we're going to address during these weeks we're going to try to see what the atmospheres of exoplanets are made of. How are they structured and how can they evolve? And for this, I will base many of the ideas on these plots that is showing you the size of known exoplanets as a function of the irradiation of the insulation they receive from their stars. The X-axis show the insulation it's expressed with the Earth as a unit. So it means one means that the planet receive the same amount of energy that the earth is receiving from the sun. And you already know that exoplanets, the one we know receive a lots more irradiation than the earth and that must have an impact on the planetary atmosphere. So this axis is also inverted, so the higher the irradiation, the higher irradiation will show up on the left of the plot, the less irradiated planet will show up on the right. There are three main families of exoplanets we are going to talk about. There are hot gas giants which are shown in the upper left corner and these are objects that can be irradiated more than 10,000 times the earth. So it's a huge amount of energy they receive. There are super Earth that we are coming super early because of basically a lack of better description. And these planets are very diverse, they can be extremely irradiated, but they can be also more templates. They can be rocky, they can be made out of ice all that can be smaller version of larger planets, Neptune-sized objects. And you can see that there is a gap in these plots that we are going to talk about towards the end of this course. And it contains a few Neptune-sized objects which are not as irradiated as the hot gas giants above them. So we call them warm Neptunes. So what I propose you to do now is to go throughout a journey across planetary atmosphere using why not a hot air balloon. And we're going to start from the bottom and go through all the layers of exoplanet atmospheres. So let us start with tropospheres which are the part of the atmosphere where climate happens where clouds also a cure and you will see this will have an impact on our observations. So, I will refer a lot to the measurements, to the observations because that is what is driving our understanding of these planetary atmospheres. And to do that, I will call back notion of spectroscopy and in particular using spectroscopy of transits and eclipses. So this is the electromagnetic spectrum from the ultraviolet to the near infrared throughout the optical. And one of the first things we are going to do is to do transmission spectroscopy. So, as a reminder when Starlight is filtered throughout the limb of the transiting planet atmosphere, the planet's atmosphere imprints its composition and structure in the Starlight. And when decomposing the Starlight with a spectrograph, it's possible to retrieve the composition and structure properties of the planetary atmosphere. So it works in the optical it works also in the near infrared and in the ultraviolet, another technique is using eclipses of the planets by the star to access the decide emission spectrum of the planet. And that works well in the near infrared because that's where the planets are shining. But that also works in the optical except that here, instead of accessing the thermal emission from the planets, what we will see is the reflected light of the star on the planet and that produces different observables. So let us start right now, going in the troposphere and using primary transit spectroscopy of transits in the new infrared. And this was used in particular to detect water vapor in atmospheres of hot gas giants. This is mainly done for several years now with the Hubble space telescope and its wide-field camera, three instruments. This is a low resolution spectrograph and by low resolution, I mean a spectral resolution of about 130. So, one example of transmission spectrum for a hot gas giant HD 29458 B. Is shown on the right. And what you see here is a bump in the transmission spectrum at around 1.3, 1.4 microns and this bump is due to extra absorption by water vapor in the planetary atmosphere. Now, this kind of measurements have become almost routine for astronomers. So, this is the most recent example and in the following, I'm only going to show you observation that I have been obtained during the past year. This is a very exciting detection because contrary to the previous one which was for hot gas giants, this is for a planet called wasp 107 B. It's a planet that has the size of gas giants, but the mass of Neptune so it means that the object is very bloated probably because of a strong stellar irradiation. So what you see here clearly is the signature of water again, at 1.38 micron. I'm always going to show plots where the data are shown as dots with error bars and the models are shown as corrode curve over plotted on the data. So in this case, you see water clearly, in this other case, you don't see what you're clearly if you compare the data, the data points with the blue model spectrum, where has the blue model spectrum predict a bump. Again, the data are rather flat, they show a flatline and we will see how we can understand this. And in the several observations that have been obtained during the past year, in some cases, you can see the water feature. In some other cases, the water feature appears damped or even muted. Here is another case where the water feature is very well seen, it has a big amplitude and it can even be seen very well by going to other wavelength and for this planet here called HAT-p-26b. It's a warm Neptune with a good spectral coverage and we see the water feature pretty clearly. And that even a load the authors of this discovery to claim, they can use this and measure the abundance of heavy elements. Mainly the oxygen in the water to a value that is five times solar value, with a bigger bar plus 21 minus 4. So this is not that precise, but this is a good start because the idea here is really to do these kind of things for several exoplanets and compare the values between each other. And this is interesting because putting in perspective this kind of value between exoplanet and solar system planets as it's done in the plot. I show here well, the abundance of heavy elements is shown as a function of the planet Mars. Well, we hope that it will produce hints about how planets have formed and how atmospheres have evolved and what we hope to find is correlation between values like this. So the abundance of heavy elements and either the Mars, the distance to the star, the level of radiation and so on. And now there is enough planets for which the water feature has been measured so, detected or not, so that it's possible to start doing these comparative studies. So, for instance, this comparative study focuses on the smallest planet in the sample and conclude that there may be a correlation between the strength of the water feature. And the temperature of the object and this could be understood as the cooler objects. They have clouds that hide the parts of the water feature and damp and even mute a part of the water feature. However, another studies that is expanding on the whole sample does not reach such a conclusion. So, as a summary for this first part, we've seen that water is now routinely detected in the troposphere is of hot gas giants and a few warm Neptunes. It's best done in transmission spectroscopy in the near infrared, in particular using Hubble. And comparative studies of large exoplanets temple are starting and well interpreting their results remains difficult because we are just at the beginning. So in the next course, we are going to address one of these difficulties. Thank you for your attention. [MUSIC]
