Much is expected from determining the composition of the atmospheres of various exoplanets around stars in the Milky Way close to the Sun with the James Webb Space Telescope JWST. A number of articles have just saluted the work in this direction that has been carried out with the atmosphere of the exoplanet WASP-39 b, a known hot Jupiter. No biosignature yet, but indications of the formation process.
A few months ago, a research team led by Natalie Batalha of the University of California Santa Cruz proudly announced an important and promising result for the program analyzing the atmospheres of exoplanets, made possible by the James Webb Space Telescope. Astrophysicists had just provided definitive evidence for the presence of carbon dioxide in the atmosphere of an exoplanet, in this case a gas giant planet orbiting a sun-like star just 700 years away -Light of the Solar System via an announced publication in the famed journal Nature , but a version is freely available on arXiv.
The exoplanet in question is a hot Jupiter called WASP-39 b, which has a mass of about a quarter of that of Jupiter and a diameter 1.3 times larger because it is heated to about 900 °C. It was first detected on Earth in 2011 using the transit method. Its fixed orbital period is only about four days and the radius of its orbit is barely one-eighth the distance between the Sun and Mercury.
So there was no point in hoping to find life there, but it was a good example of the possibilities now open with the JWST, with the hope of eventually determining perhaps compelling biosignatures of the existence of nearby life forms on Exoterres the sun in the Milky Way. A non-obvious problem, as astrophysicist Franck Selsis has explained on numerous occasions.
The study of exoplanets has revealed an incredible variety of planetary system architectures, as well as planetary types in terms of mass, radius, temperature and composition. Observation methods make it possible today to examine the structure and composition of their atmosphere and thus open up a considerable field of research in comparative planetology. A conference by Franck Selsis was held here in 2014, organized by the Bureau des Longitudes (Academy of Sciences) and the Earth Sciences Department of the ENS. © Ecole Normale Supérieure – PSL
Remember that the transits make it possible to implement the observation capacities of the JWST, made possible in particular by its near infrared spectrograph: NIRSpec (there are in fact three instruments of this type that equip the JWST). When an exoplanet passes in front of its star, the light transmitted through its atmosphere shows absorption lines, which can be highlighted by scattering with the spectrograph, and which are a kind of barcode revealing the presence, identity, and abundance of atoms and molecules in that atmosphere .
In the case of WASP-39 b this is particularly easy as its atmosphere is greatly expanded by the thermal radiation of its host star, its close proximity to the Sun and its frequent transits, allowing it to collect statistics, as we say in the jargon of natural sciences, which help increase the accuracy of measurements over time by reducing the noise that interferes with the signals you are looking for.
Five articles on molecules in the atmosphere of WASP-39b
We see evidence of this today with the detection of new molecules in the atmosphere of WASP-39 b by the JWST, which has been mobilized again, and by the Early Release Science (ERS) Transiting Exoplanets team, which is a group of Canadian astronomers from the university belonged to Montreal under the direction of Professor Björn Benneke. As explained in a press release from the Institute for Research on Exoplanets (iREx), this group had already contributed to the detection of CO2 in the atmosphere of WASP-39 b and therefore continued their analyzes with the Canadian instrument Near-Infrared Imager and Slitless Spectrograph ( NIRIS).
Among the molecules newly discovered with three of the JWST instruments today we find the first demonstration in an exoplanet’s atmosphere of molecules of sulfur dioxide (SO2), known on Earth to be produced by chemical reactions involving ultraviolet light light can be generated by photochemistry.
“This is the first time we’ve seen concrete evidence of photochemistry – chemical reactions triggered by energetic starlight – on exoplanets. I see this as a really promising perspective to advance our understanding of exoplanet atmospheres,” says Shang-Min Tsai, a researcher at the University of Oxford in the UK and ESA lead author, in an ESA statement, one of five articles published today those about the formation of sulfur dioxide in the atmosphere of WASP-39 b.
This is just the tip of the iceberg, modeling the effect of a star’s radiation on chemical reactions in an atmosphere. In the case of Earth, of course, we know how to do it, but there are models for the other planets in the solar system as well. The discovery of SO2 in the atmosphere of WASP-39 b is therefore a prelude to expanding and testing models of photochemistry in exoplanet atmospheres. Models that will help us in the coming decades to better define what might be compelling biosignatures.
In this context, Hannah Wakeford, an astrophysicist at the University of Bristol in the UK who studies the atmospheres of exoplanets, adds in the ESA statement on the results obtained with the molecules discovered today: “We predicted what we JWST would show, but it was more accurate, diverse and beautiful than I originally thought.”
The cosmogony of the exoplanets
Determining the atmospheric composition of exoplanets with the James-Webb not only enables exobiologists to search for biosignatures. In fact, cosmochemists can also determine abundance ratios between the elements that make up the detected molecules.
As in the solar system, the carbon-oxygen or potassium-oxygen ratios provide information about the conditions in the protoplanetary disks and about the mechanisms of planet formation.
In the case of WASP-39b, the relationships already found point to processes already discovered in the Solar System case, explained Kazumasa Ohno, a UC Santa Cruz exoplanet scientist who worked on the Webb data.
“The abundance of sulfur compared to hydrogen suggests that the planet has likely experienced a significant accumulation of planetesimals that can supply these constituents to the atmosphere. The data also show that oxygen is much more abundant in the atmosphere than carbon. This suggests that WASP-39 b originally formed far from the host star. By accurately revealing the details of an exoplanet’s atmosphere, the Webb telescope’s instruments have exceeded scientists’ expectations and promise a new phase in the study of the galaxy’s vast diversity of exoplanets. »
Detailed explanations by Franck Selsis on the problem of detecting and interpreting biosignatures. © Academy of Sciences
Five scientific articles about the exoplanet WASP-39 b by the scientific team of the JWST Transiting Exoplanet Community Early Release Science Program were published on arXiv on November 21, 2022. Here they are linked: