Astronomers find traces of water in the atmosphere of an ultra-hot gas giant planet

Known as WASP-18 b, this planet is located approximately 400 light-years away from Earth. (Photo: Nasa)

The international group of scientists found traces of vapor in the star called WASP-18b, located 400 light years from Earth.

An international team, made up of scientists from the Institute of Astrophysics of the Spanish Atlantic Archipelago of the Canary Islands (IAC), identified water vapor in the atmosphere of WASP-18 b, an ultra-hot gas giant extrasolar planet, the IAC reported Wednesday. WASP-18 b, which is about 400 light-years from Earth, is ten times larger than Jupiter and has an orbital period of less than one day, said in a statement adding that the finding is published in Nature magazine.

The extreme proximity to which it is located from its star, its relative proximity to Earth and its great mass, have made it, since its discovery in 2009, a coveted research object, explains the IAC.

Now, observations made with the James Webb Space Telescope (JWST) have produced the most detailed map to date of a gas giant planet. “It was a great feeling to look at the JWST spectrum of WASP-18 b for the first time and see the subtle but precisely measured signature of the water,” says Louis-Philippe Coulombe, a graduate student at the University of Montreal and lead author of the paper. article. With this type of measurement, “we will be able to detect this type of molecule on a wide range of planets in the coming years,” he says. For the study of WASP-18 b, the team mapped its temperature or luminosity by detecting the secondary eclipse, that is, the moment when the planet slips behind its star and reappears, and studied the thermal differences along its daytime hemisphere, the side that always faces the star.

The mapping obtained by the JWST shows a “huge” temperature change, of up to 1,000 degrees Celsius, from the hottest point on the side facing the star to the so-called terminator, or line of transition between the illuminated part and the part of the star. in the planet’s shadow, they are in a permanent twilight. Since the planet is much colder in the terminator, the scientific team suspects the existence of a strong magnetic field that would be preventing the winds from effectively distributing the heat towards the night side. In this way, the winds would be blowing from the planet’s equator towards the north and south poles, instead of from east to west as might be expected.

The IAC says that the study, which has also recorded thermal changes at different levels of the planet’s atmosphere, has found in its spectrum slight traces of water vapor at different heights, despite the fact that temperatures are almost 2,700 °C and such extreme heat would break most of the water molecules. The ability to detect such a small number of these molecules demonstrates the extraordinary sensitivity of the JWST. In the words of Enric Pallé, a researcher at the IAC and co-author of the study: “By analyzing the spectrum of WASP-18b, we not only learn about the various molecules that can be found in its atmosphere, but also about the way in which it was formed.” He adds that, according to his observations, WASP-18b’s composition is very similar to that of its star, which means that it probably formed from the remnants of gas left behind just after its birth.