This is a great first for the James Webb telescope: the beginning of mapping the temperature of an exoplanet, in this case that of the superhot Jupiter WASP-18 b. As a reward, he discovered in its atmosphere traces of water molecules that were previously difficult to detect for this star.
As we know, we expect a lot from the James Webb Space Telescope in the field of exoplanets and more specifically in the study of their atmospheres. By viewing nature as a vast natural laboratory that runs millions of different experiments for us by changing the parameters that describe these exoplanets, we also hope to understand those of the solar system and how rare it and our blue planet are – or not – in the Milky Way.
One hundred astronomers today published an article in nature (An open access version can be found at arXiv) about studying with JWST for WASP-18b. It can be called superheated Jupiter because this exoplanet is a gas giant that orbits an F-type star in less than a day (23 hours) with an atmosphere that can heat up to nearly 2,700 degrees Celsius. With a mass about 10 times that of Jupiter, it was discovered in 2009 as part of a campaign to detect exoplanets by a transit method called Wide angle to search for planets (WASP), which includes robotic telescopes.
WASP-18 b is so close to its star that tidal forces cause it to rotate synchronously, like the Moon around Earth, thus always presenting the same face to its star. The laws of celestial mechanics show that it will end due to the same force as it, by colliding with WASP-18. Since it is located about 400 light-years from the solar system, it is a good target for the JWST.
Spectrum detector for the origin of an exoplanet
Astrophysicists observed WASP-18b for about six hours using one of JWST’s instruments, the Near infrared imaging and non-slit spectroscopy (NIRISS), courtesy of the Canadian Space Agency. This allowed them to detect the presence of water molecules in the atmosphere, which is remarkable because with a temperature of nearly 3,000 degrees K, these molecules are largely separated, demonstrating just how sensitive JWST’s instruments are, which bodes well for the near future.
In a NASA statement about the discovery Louis Philippe Colomba graduate student at the University of Montreal and lead author of the article on WASP-18 b in nature He explains:
” It was a great feeling to look at the JWST spectrum of WASP-18b for the first time and see the subtle but well-defined signature of water. With these measurements, we will be able to detect such particles for a wide range of planets in the coming years! By analyzing the spectrum of WASP-18b, we learn not only about the different particles that can be found in its atmosphere but also about how it appears. We find from our observations that the composition of WASP-18 b is very similar to that of its star, which means that it most likely formed from residual gas that was present just after the star was born. These results are very valuable for getting a clear picture of how alien planets like WASP-18 b, which have no analogues in our solar system, might look like. “.
Animation showing the transit of an exoplanet. For a more or less accurate French translation, click on the white rectangle at the bottom right. The English translation should then appear. Then click the nut to the right of the rectangle, then click Subtitles and finally Subtitle Automatically. Choose “French”. © NASA Solar System
Wind directed by a magnetic field?
But scientists have done a better job and this is a great precedent with the James Webb Telescope. They were able to see temperature differences between the day and night faces of WASP-18 b, thus mapping the temperature of the exoplanet’s atmosphere.
For this, they relied on the fact that in addition to the classic transit, when the exoplanet passes between its star and the JWST, there is a secondary transit that also results in a decrease in luminosity, a decrease in the light curve of the system, and this is only when WASP-18b passes behind its sun.
As the NASA press release explains, we can conclude that the temperature map shows an abrupt and significant change in temperature when we cross the separator, the boundary between shadow and light, between the two hemispheres of an exoplanet: 1,000 degrees!
” JWST gives us the sensitivity to create more detailed maps of hot giant planets like WASP-18b than ever before. This is the first time a planet has been mapped using the JWST, and it’s really exciting to see that some of what our models predicted, such as a sharp drop in temperature away from the point of the planet directly facing the star, can actually be seen in the data! says Megan Mansfield of the University of Arizona, one of the authors of the article. nature.
However, the temperature jump between the two faces is so great that the mechanism must hinder the heat distribution process. via Winds between the two hemispheres. It is possible that this is the presence of a strong magnetic field caused by an exoplanet. Winds will be forced to blow from the planet’s equator to the North Pole and South Pole, rather than from east to west, as you might expect.
