[MUSIC] So welcome back to the latest news from exoplanetary atmospheres. So in these parts, we are going to discuss something called aerosols. So previously we have seen that in the tropospheres of exoplanets, we are now able to routinely detect water vapor for hot gas giants and for warm Neptunes. This is best done with transit transmission spectroscopy in the near infrared, in particular with the Hubble Space Telescope. But we've also seen that the interpretation of the comparative studies that have started to determine the formation and evolution of planetary atmospheres by comparing this signature in different planets is challenging. And today we are going to discuss the main culprit of this interpretation problem. And this culprit is actually pictured in the sketch of the atmosphere. And it's figured as the little clouds that are on top of the troposphere. These clouds, actually we're going to use a more generic term of aerosols. So aerosols are covering clouds, hazes, droplets, and dust particles. And we have to use a generic term because we actually don't know what these are made of. What we do know, however, is the impact they have on our measurements. And this impact is illustrated in the plot below, which represents the transit transmission spectrum. It's a model for hot gas giants called HD 189733b. And you have different models with different colors corresponding to different amounts of aerosols in the planetary atmosphere. And the lighter, the clearer, so the light green spectrum on the bottom has no cloud in it. And you can see the difference as we put clouds higher and higher in the planetary atmospheres. The spectrum becomes flatter and flatter. So this impact is what explains this muted water feature that we've seen previously. So if you have clouds or aerosols, you basically dump the water signature. And this is because aerosols scatter light in the optical in particular. So light scattering is a bit more complex than photo absorption by molecules. But it's a part of the spectroscopic extinction that can occur in a planetary atmosphere. So we're going to discuss that a bit, and we're going to make a simplification here to start with. We're going to assume that scattering is first performed by particles that are very small with respect to the wavelength at which you're looking at. So we are going to assume that the particles that are scattering the light spherically symmetric. So they have a size, a, and one of their main properties is their refractive index, which depends on the nature of the material, on the composition of the particle. Now, when we do this assumption, we are in what we call the Rayleigh scattering regime. And the Rayleigh scattering regime has the very interesting properties, first of all, to be analytically understandable. So this formula here is giving the scattered intensity given an intensity I0. And the Rayleigh scattering by a small particle will basically scatter the lights almost isotropically around the particle. And a very interesting feature is emphasized in the equation with the blue square. It's the fact that the scattered intensity strongly depends on the wavelength. And in this case, it depends on the inverse of the wavelength at the power 4. This makes Rayleigh scattering very efficient to scatter blue light and relatively inefficient to scatter longer wavelength, such as yellow or red. And this is basically what explains why the sky on Earth is blue. Is because the light from the Sun is scattered by molecules of nitrogen and oxygen, which are very small compared to the optical wavelength. Now, what happens if the size of the scattering particle increases is that we are leaving the Rayleigh regime as soon as the size of the particle reach about one-tenth of the wavelength. And then we are entering the Mie scattering regime. And the scattering is not as much isotrope and is not as much traumatic. And eventually when the particle size is of the same order as the wavelength, we are in the same situation as in a haze or in a cloud, where white light is scattered achromatically. Basically, all wavelength are scattered by the same amount. And the only thing you can see is white everywhere. And this explains why the cloud on Earth, the water clouds, which are made out of droplets which are much bigger than the molecules of nitrogen and oxygen. That explain why these clouds are white. Now, scattering, as we saw, can produce flat transmission spectrum, so featureless spectrum. Which for us are a bit annoying, because we like to detect spectroscopic features to better constrain our model. But these aerosols are also able to absorb the lights, but at longer wavelength and depending on their composition. And then you can see some spectral feature which depends of what these particles are made. And the thing is, is for these very irradiated exoplanets, the particles are quite exotic. Because we are talking about planets where the temperature can be above 1,000 degree. And so the material that are able to exist as dust particles or droplets, or so on, are completely different from water on Earth. We are talking about hazes or clouds made out of silicate particle, iron oxide particle. And you see here the list of all possible things that scientists are thinking about. And all these different species have also different spectroscopic signature and scattering properties. And these are gathered in this plot, so I'm not going to go over all of it. I'm just going to zoom here on one panel showing one specific type of particles, and not only the scattering property. So here you have the effective extinction cross section of the particles as a function of the wavelength. So not only did it depend on the composition, but it also depend on the size of the particle. And usually the bigger the particle, the flatter the spectrum gets. And you can see that if you look at the top curves in the optical regime, they produce a very flat spectrum. Now, there are spectroscopic signatures. And here, for instance, you have one here, another one there, and another one there. And so that's something that we hope we will be able to access with new observations. Now, the intensity of these features, as I have just said, also depends on the size and also on where the particles are located in the atmosphere. The higher the aerosols, the flatter the spectrum, simply because the hazes will mask a larger chunk of the atmosphere. And determining how and where aerosols form, how they grow, how they evolve is a very hot topic in planetary atmospheres. And it is completely link with the composition and the evolution of the planetary atmosphere. So being able to characterize these hazes will actually reveal a lot of what happens in exoplanetary atmospheres. So it's not only just to get rid of them and detect the molecular species that lies beneath. It's also because they are interesting themselves. And astronomers think that the best tool to obtain spectroscopic features for these aerosols is going to be the James Webb Space Telescope, which is going to be launch in 2021. And the James Webb Space Telescope, which actually is as big as a tennis court, it's mirror is much bigger than the mirror of the Hubble Space Telescope. This telescope not only is bigger, but will also cover wavelength that Hubble is not capable of covering. In particular, the mid-infrared, where most of the spectroscopic signature of hazes can be found. So as a summary, we've seen that aerosols scatter light in planet atmospheres. And their scattering properties depend on their composition, their altitude, and their size. Now, these aerosol all create broad-band, flat optical signature. But they also have some promising spectroscopic signature in the mid-infrared that we hope we will be able to access with the James Webb Space Telescope. Thank you for your attention. [MUSIC]
