The long migration theory  

Hot Jupiters are astonishing and their proximity to their star is even more so. It is practically inconceivable that, during their formation, there was enough matter so close to the star to form a gas giant. In all likelihood, they must have formed at the periphery of their system, beyond the snow line. “For some reason, once they were formed, these giants must have been driven to migrate towards the center of the system”, says Laetitia Delrez. “This unknown cause has been the subject of much debate. The main theory is that there was a gravitational interaction between the planet and the accretion disc which orbits the star at the time of its formation. The planet might also have interacted with other planets formed at the same time, or might have migrated under the influence of another star…” What is interesting in these migration theories is that, in our solar system, four terrestrial planets, including our own, orbit between the Sun and Jupiter. If the systems harboring a hot Jupiter included a planet similar to our own, it is very likely that, during migration, the gas giant would have ejected it or would at least have strongly perturbed its orbit. “This is one of the reasons why we are trying to understand how these planets have evolved and why our Jupiter has fortunately not migrated toward the Sun after its formation”.

Diagram illustrating the two main methods used to study the atmospheres of transiting exoplanets.

WASP and TRAPPIST, a fruitful collaboration

Since 2010, TRAPPIST, ULg’s telescope based in La Silla in Chile has carried out various programs for detecting and studying exoplanets, comets and asteroids in the Solar System (See Astrophysicists from Liege are in seventh heaven). Notably, it has been used in the context of a close collaboration with WASP, an English exoplanet detection program searching for transiting hot Jupiters. Laetitia Delrez has been ensuring this collaboration during her thesis. The WASP mission is to examine, from Earth, almost the entire sky in the search for transiting planets. Their instruments cast a very wide net but are limited in terms of resolution. They can even sometimes mix several stars into one single light signal. If they observe a transit, they often do not have the necessary precision to determine which of the observed stars has been eclipsed. This is where TRAPPIST comes in. “Once WASP has collected a list of candidates”, explains the young researcher, “we get their coordinates and we can then observe them with greater precision. Depending on the light curves that we obtain during transits, the shape of the signal and its depth, we can then determine whether the signal is indeed that of a planet or not. A large part of my thesis consisted in observing these candidates and beginning to constrain the system parameters as soon as we found a planet”. Since the beginning of this fruitful collaboration, 105 planets out of 550 candidates were identified. At the source of the detections, Laetitia Delrez was perfectly placed to observe newly-discovered planets. She could then select the most interesting ones and focus on a second more challenging task, studying the properties of their atmospheres. 

The technical feat of studying atmospheres 

Of all the major characteristics to be identified concerning an exoplanet, the properties of its atmosphere (its chemical composition or the temperature distribution as a function of altitude) are the most challenging to determine. Where small telescopes can, in certain conditions, identify a terrestrial planet several tens of light years away and make it possible to determine its radius, for example, only the most precise instruments can be used for studying the atmosphere of these distant worlds. Once again, hot Jupiters, possessing a more extended and hotter atmosphere than temperate terrestrial planets are more suitable for this kind of study.  

Two methods are mainly used. The first is transmission spectroscopy. “During a transit, some of the starlight passes through the atmosphere of the planet. The atmosphere therefore acts as a filter. Each gaseous component absorbs certain wavelengths of the light. This is a chemical signature. Depending on the composition of the atmosphere, the light will be absorbed to a greater or lesser extent at different wavelengths. When we observe transits at these different wavelengths, we can therefore search for variations in the depth of the transit and recreate what is called the transmission spectrum of the planet, which reveals its atmospheric chemical composition”.