Giant, massive, and orbiting very close to their star. Of all the exoplanets discovered to date, hot Jupiters are the easiest to detect and observe. Though life is impossible on these planets, they are nonetheless fascinating because of their extreme environments and their unknown and equally intriguing origins. As an increasing number of studies are being conducted, these enigmatic objects reveal more and more secrets. Although they do share some common properties, they are also showing today an increasingly wide range of profiles. At the crossroads of various international campaigns for the detection and characterization of exoplanets and atmospheric study programs (WASP, TRAPPIST), Laetitia Delrez, a researcher at the Origins in Cosmology and Astrophysics Laboratory of the University of Liege, currently in postdoc at Cambridge, has painted a very detailed picture of these planets based on information gathered over a four-year period. She defended her thesis(1) recently and was awarded the prestigious Gruber Foundation Fellowship which is awarded every year by the International Astronomical Union (IAU).    

Over the last twenty years, the field of Exoplanetology has made remarkable progress. From a single exoplanet discovered in 1995, telescopes all over the world have since detected thousands of them. These discoveries resulted from intense research programs targeting the whole sky, even its most remote parts. These encouraging observations and subsequent statistical studies led astrophysicists to say that almost every star harbors a planetary system, resulting in around one hundred billion planets in our galaxy alone. While knowledge about these planets is increasing rapidly, a great number of questions and uncertainties remain. It is true that the tens or, more often, hundreds of light years that separate us from them, make it difficult to observe them. Of course, developments in this area all stem from the same motivation, to identify planets with suitable conditions for life to exist and to probe their atmospheres in the search for possible biomarkers (water, carbon dioxide, ozone etc.). The best candidates are terrestrial planets orbiting in the habitable zone of their star. But apart from a few potential candidates that might yield up their intimate secrets more easily (See: A trio of Earths 40 light years away?), determining the characteristics of these small planets with precision is not currently feasible. 

Artist's view of an hot Jupiter, agiant gas planetorbiting near its star.
Credit: NASA.

Getting close to the extreme conditions of hot Jupiters 

Apart from these privileged but somewhat secretive targets, there are the gas giants, more particularly, those which orbit close to their star (nearer than Mercury’s orbit around the Sun). Massive, large and orbiting their star in just a few days, these planets generically classed as hot Jupiters reveal more about themselves. They are indeed easier to observe, whether by using the radial velocity or transit method. These planets make it possible not only to learn a lot about the physics of the planetary systems of our galaxy, but also to calibrate and improve our current methods and instruments. And that is not all. “Hot Jupiters are fascinating objects in themselves because of their extreme conditions”, comments Laetitia Delrez, an astrophysicist at the Origins in Cosmology and Astrophysics Laboratory of the University of Liege, currently in postdoc at Cambridge. “These planets orbit their star in a few days, sometimes less (see on this subject An exoplanetary drama: a planet collapses on its star). “This proximity generates considerable tidal effects, which generally distort the planets and in most cases bring them closer to their star until, after several tens or hundreds of millions of years, they reach the Roche limit where they are disrupted. They are also subject to strong irradiation, up to several thousand times that of the Earth. The surface temperature can reach over two thousand degrees Kelvin. They are tidally locked and thus have a permanent day side and a permanent night side. This phenomenon leads to significant day-night temperature differences, which may result in supersonic winds. These winds play a part in the atmospheric circulation and in the heat distribution between the two hemispheres”. 

While there are similarities between them, hot Jupiters also show very varied profiles, for example in terms of mass and radius, and thus density. Certain planets have a high density, that is to say a small radius in view of their high mass and irradiation. These planets might contain a significant amount of heavy elements (rocks, ices), up to several tens of times the mass of the Earth. In contrast, others have a very low density, up to ten times smaller than that of our Jupiter. These planets are also generally the most highly irradiated and therefore the hottest. Some hot Jupiters which have a lower mass, have a similar profile to Saturn, while others have a mass up to 13 times that of Jupiter, etc. Ultimately, the name “hot Jupiter” refers to a group of planets with quite different characteristics but which should be presented in a more orderly way. It is to this very task that Laetitia Delrez devoted herself for four years. The young astrophysicist has just defended a thesis under the direction of Michaël Gillon, in which she presents an impressive analysis of the question. Her contributions to Exoplanetology are varied and range from the co-discovery of new planets and their characterization to the study of some of their atmospheres. These involved a considerable amount of observations that were performed in the context of several research programmes such as WASP and TRAPPIST.

(1) Delrez L., Detection and characterization of transiting hot Jupiters, Sept. 2016.