Stellar atmospheres for ultrahot Jupiter exoplanets

Simulated views of WASP-121b (Image NASA/JPL-Caltech/Aix-Marseille University (AMU))
Simulated views of WASP-121b (Image NASA/JPL-Caltech/Aix-Marseille University (AMU))

An article published in the journal “Astronomy & Astrophysics” describes a research on gas giant planets very close to their star. A team of researchers used observations made with Hubble and Spitzer space telescopes and computer simulations to study in particular the characteristics of the atmosphere of the exoplanet WASP-121b but also of other similar ones, called ultrahot Jupiters. They’re so close to their stars that their dayside has very high temperatures and are in some ways more similar to a star than a planet.

Hot Jupiters are a class of exoplanets now well known since it was the first type of planets found in abundance: they’re planets very close to their star that are consequently heated so much that the dayside can reach very high temperatures. Some of them are really extreme cases: for example WASP-121b can reach 2,500° Celsius (4,600° Fahrenheit) and according to a research described in an article published in August 2017 in the journal “Nature” has a stratosphere.

Now a team led by astrophysicist Vivien Parmentier of Aix Marseille University in France tried to study what were called ultrahot Jupiters, gas giant planets so close to their star that their year can last even less than an Earth’s day with temperatures on the dayside that exceed 2,000° Celsius (3,600° Fahrenheit). Some authors of this research had already published studies on some of these extreme cases: WASP-103b, WASP-18b and HAT-P-7b, this time others of that type were included.

Space telescopes help to observe the dayside of hot Jupiters in general because the atmosphere reaches such temperatures as to emit infrared in abundance but observing their nightside, much less hot and much darker, is a problem. The image shows some simulated views of the exoplanet WASP-121b from different points of view and different degrees of illumination by its star.

These planets always show the same face to their star, like the Moon to the Earth, in what is called tidal lock, with the consequence that the dayside is much hotter than the nightside. This made it difficult to understand some characteristics such as the lack of water in ultrahot Jupiters, which is normally present in gas giants and even in hot Jupiters.

One hypothesis to explain the water scarcity in ultrahot Jupiters was that they had high levels of carbon instead of oxygen. However, traces of water have been found on the border between the dayside and nightside of some of these exoplanets, at the limits of the area that the telescopes can study. Perhaps there’s water in the dark side of these exoplanets.

To try and create a model that might explain that situation, Vivien Parmentier’s team drew inspiration from physical models of the atmosphere of stars and the failed stars called brown dwarfs as some properties overlap with those of hot and ultrahot Jupiters. Mark Marley, a researcher at NASA’s Ames Research Center and one of the authors of the research, had developed a model for brown dwarfs that was adapted to ultrahot Jupiters.

According to the new models, in ultrahot Jupiters’ atmospheres there’s a cycle of destruction and formation of water with the consequence that its traces are limited or non-existent. What happens is that the dayside is irradiated by the star so violently that it breaks water’s molecular bonds. Hydrogen and oxygen are pushed by the heat towards the nightside, where they can recombine to form clouds that can end up again in the dayside and restart the cycle.

This model explains only some ultrahot Jupiters but in other cases there are still observations to be explained. The interesting thing is that they don’t need an exotic chemical composition but it’s possible that the current models are still far from being complete. The extreme conditions on those exoplanets may require more precise detections and this means including them among the many targets for the James Webb space telescope.

Leave a Reply

Your email address will not be published. Required fields are marked *