The second main method used is emission spectroscopy. This involves measuring the variation in light at the time of occultation, when the planet passes behind its star. Shortly before and shortly after the occultation, it is possible to observe the light emitted by the day side of the planet. During the occultation, the planet is hidden by the star and its dayside emission is thus occulted. By observing occultations at different wavelengths in the infrared, it is possible to determine the thermal structure of the atmosphere. This is because, depending on the temperature, the spectrum of the light emitted by the planet takes on different shapes. “By observing occultations, we can measure the light emitted by the planet from its day side. Depending on the presence of atmospheric compounds which block different wavelengths, we can observe more or less deeply in the atmosphere. When we observe at a wavelength which is not absorbed by the atmospheric compounds, we can probe the deep layers of the atmosphere. The thermal emission is proportional to the temperature of the atmosphere at the depths probed. By comparing measurements at different wavelengths, we are able to map the vertical distribution of the atmospheric temperature and determine, for example, whether it increases or decreases with altitude”. Up to recently, it was widely thought that temperature inversions were common in the atmospheres of hot Jupiters, that there was a layer in their atmosphere where the temperature increased with altitude (like the stratosphere of the Earth). However, recent observations seem to show the opposite. With the exception of a very few candidates, the atmospheric temperature appears to be constant, or in certain cases, decreases with altitude. Thermal inversion would therefore be a marginal phenomenon in the atmospheres of hot Jupiters. 

Choosing the best targets…

Among the co-discovered planets, Laetitia Delrez was tasked with publishing the data gathered for a number of them. While their names are not very exotic (WASP-68 b, WASP-73 b, WASP-88 b, WASP-121 b…), these discoveries have thrilled the team. “We calculated the mass and radius of these planets, which allowed us to deduce their density and to have some idea of their composition. We also determined their basic orbital parameters and their distance from Earth… Among these new planets, I then made a list of the most favorable ones for atmospheric study”. Those planets with the most promising profiles were either extremely hot with very short orbital periods, or they had a relatively low density. This was the case with WASP-49b, WASP-80b and WASP-103 b. “The higher the density of the planet, the more compact it is and the less extended the atmosphere is. Conversely, a planet with a low density has a more extended atmosphere, which is the case for WASP-49 b and WASP-80 b. The more extended the atmosphere is, the more the chemical signals expected using transmission spectroscopy will be strong and easily observable. WASP-103 b interested us due to another characteristic. It orbits its star in only 22 hours, which means that it is an extremely hot planet, with a surface temperature of more than 2000 K on its day side. Its thermal emission is therefore relatively strong, which makes it easier to measure its occultation”. 

… Choosing the right instruments

Once the first characteristics of the planets had been determined and the best profiles selected, the astrophysicist could request observing time on larger telescopes. “In particular, we obtained time on one of the VLT telescopes in Chile, whose primary mirror is eight meters in diameter. This telescope is equipped with a spectrograph, FORS2, which makes it possible to separate the light from the star into different wavelengths in the visible spectrum. We were thus able to obtain precise transit light curves at different wavelengths simultaneously. For WASP-49 b, we did not observe any variation in the transit depth with wavelength. We obtained a flat transmission spectrum, which means that we did not detect any signature of atmospheric compounds which might seem frustrating. In fact, this is a rather frequent result for hot Jupiters. We think that the atmospheres of these planets harbor clouds at high-altitude that hide the spectral signatures of the atmospheric components. But for some other planets, we observed clear atmospheres showing rather strong atmospheric signatures. Water vapor, for example, has been detected in several atmospheres of hot Jupiters. All these observations create exciting dynamics. For example, it would be interesting to understand why we observe cloud formations on some planets but not on others. What is the composition of these clouds? What role does temperature play in their formation? To understand the atmospheric physics of these planets, we must continue to collect this type of data and compare them with models to obtain the strongest constraints on their atmospheric properties”.

The next generation of instruments such as the JWST and recently started programs such as SPECULOOS offer a future full of promise. The methodologies developed to study the atmospheres of hot Jupiters will be soon applicable to terrestrial planets and will lead to great advances in the search for habitable environments. Atmospheric studies of the TRAPPIST-1 planetary system have already begun. As for JWST, it will, from space, be able to observe more planetary systems in the galaxy with greater precision.