2023-06-02 09:14:13
This is a great first for the James-Webb telescope: the beginning of the mapping of the temperature of an exoplanet, in this case that of the ultra-hot Jupiter WASP-18 b. As a bonus, he discovered in its atmosphere traces of water molecules 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 atmosphere. By considering nature as a vast natural laboratory performing millions of different experiments for us by changing the parameters describing these exoplanets, we hope to also understand those of the Solar System and how rare it and our Blue Planet are – or not – in the Milky Way.
A hundred astronomers published today an article in Nature (an open-access version can be found at arXiv) about the study with the JWST of WASP-18 b. It can be called an ultra-hot Jupiter since this exoplanet is a gas giant that orbits an F-type star in less than a day (23 hours) with an atmosphere whose temperature can rise to almost 2,700 °C. With a mass about 10 times that of Jupiter, it was discovered in 2009 as part of the exoplanet detection campaign by the transit method called Wide Angle Search for Planets (WASP) and which involves automated telescopes.
WASP-18 b is so close to its star that tidal forces cause it to rotate synchronously, like the Moon around the Earth, and therefore always presenting the same face to its star. The laws of celestial mechanics show that it will end up due to its same forces by crashing into WASP-18. As it is located about 400 light-years from the Solar System, it is a good target for the JWST.
A revealing spectrum of the origin of the exoplanet
Astrophysicists observed WASP-18b for about six hours with one of JWST’s instruments, the Near-Infrared Imager and Slitless Spectrograph (NIRISS), provided by the Canadian Space Agency. This allowed them to detect the presence of water molecules in the atmosphere, which is remarkable because with its temperature of almost 3,000 kelvins, these molecules are largely dissociated, which shows how the JWST instruments are sensitive, which bodes well for the near future.
In NASA’s statement about this discovery Louis-Philippe Coulombegraduate student at the University of Montreal and main author of the article on WASP-18 b in Nature explain :
« It was a great feeling to look at the JWST spectrum of WASP-18b for the first time and see the subtle but precisely determined signature of water. With such measurements, we will be able to detect such molecules for a wide range of planets in the years to come! By analyzing the spectrum of WASP-18b, we learn not only about the different molecules that can be found in its atmosphere but also about how it came into being. 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 leftover gas that was present just after the birth of the star. ‘star. These results are very valuable for getting a clear picture of how strange planets like WASP-18 b, which have no equivalent in our solar system, came into being. ».
An animation showing the transits of the exoplanet. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © NASA Solar System
Winds channeled by a magnetic field?
But scientists have done better and this is a great first with the James-Webb telescope. They managed to see temperature differences between the daytime and nighttime faces of WASP-18 b, thus sketching out a temperature map of the atmosphere of the exoplanet.
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 which also results in a drop in luminosity, a dip in the system’s light curve, and that’s just when WASP-18b passes behind its sun.
As the NASA press release explains, we can deduce that the temperature map shows a sudden and significant change in temperature when we cross the terminator, the border between shadow and light, between the two hemispheres of the exoplanet: 1,000 degrees!
« JWST gives us the sensitivity to create much more detailed maps of hot giant planets like WASP-18b than ever before. This is the first time a planet has been mapped with JWST, and it’s really exciting to see that some of what our models predicted, like a sharp drop in temperature away from the point of the planet directly facing the star, is actually 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 large that a mechanism must hinder the process of heat distribution. via winds between the two hemispheres. It is possible that it is the presence of a strong magnetic field generated by the exoplanet. It would force the winds to blow from the planet’s equator to the north pole and to the south pole, instead of east to west, as you would otherwise expect.
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