WASP-18 b Illustration

An illustration shows a yellow-orange exoplanet in a three-quarter view against the star-smattered black of space. The planet's gaseous atmosphere fades from a very bright dayside to a much dimmer nightside and there are subtle bands going north-south. It is at its brightest to the right of center.
May 27, 2023
CreditNASA/JPL-Caltech (K. Miller/IPAC)
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WASP-18 b, seen in an artist concept, is a gas giant exoplanet 10 times more massive than Jupiter that orbits its star in just 23 hours. Researchers used NASA’s James Webb Space Telescope to study the planet as it moved behind its star. Temperatures there reach 5,000 degrees Fahrenheit (2,700 C).

Scientists identified water vapor in the atmosphere of 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 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 the star, just as the same side of the Moon always faces Earth. The temperature, or brightness, map 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.