A new study led by Université de Montréal Ph.D. student Louis-Philippe Coulombe about this exoplanet, an ultra-hot gas giant 10 times more massive than Jupiter, based on new data from the Canadian NIRISS instrument on the James Webb Space Telescope (JWST) holds many surprises.

Mapping an exoplanet

An international team of astronomers have identified water vapor in the atmosphere of the exoplanet WASP-18 b and made a temperature map of the planet as it slipped behind, and reappeared from, its star. This event is known as a secondary eclipse. Scientists can read the combined light from the star and planet, then refine the measurements from just the star as the planet moves behind it.

The same side, known as the dayside, of WASP-18 b always faces its star, just as the same side of the Moon always faces Earth. This is called tidal locking. The temperature, or brightness, map of the exoplanet shows a huge change in temperature—up to 1,000 degrees—from the hottest point facing the star to the terminator, where day and night sides of the tidally-locked planet meet in permanent twilight.

"JWST is giving us the sensitivity to make much more detailed maps of hot giant planets like WASP-18 b 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, such as a sharp drop in temperature away from the point on the planet directly facing the star, is actually seen in the data," said Megan Mansfield, a Sagan Fellow at the University of Arizona, and one of the authors of the paper describing the results.