Researchers reported a striking daily cloud cycle on the Hot Jupiter WASP-94A b using observations from the James Webb Space Telescope. The study, published in Science, shows that sand-like clouds made of magnesium silicate form on the planet's mornings and largely vanish by evening. Isolating the clouds allowed a clearer measurement of atmospheric composition and corrected earlier, cloud-averaged results from Hubble.
The team observed the planet during a transit and compared spectra from the leading edge as it began to cross the star (morning) with the trailing edge as it completed the transit (evening). At the morning edge air flows from the night side to the day side, producing cloud formation; at the trailing edge the flow returns from day to night, and skies are clear. The clear evenings therefore let the researchers study the atmosphere without cloud interference.
They offer two possible mechanisms to explain the cycle. One is strong winds that lift clouds on the cooler night side and carry them toward the hot day side, where they are pushed deep into the interior and hidden. The other is that clouds form in darkness and then vaporize when exposed to more than 1,000 degrees on the day side, similar to fog burning off but on an extreme scale.
- The new, localized JWST data reduces an earlier discrepancy: Hubble averages suggested hundreds of times more oxygen and carbon than Jupiter, while JWST shows about five times those amounts.
- The team found the same cycle on WASP-39 b and WASP-17 b and will pursue a larger JWST program to study cloud cycling across many exoplanets.
Additional coauthors come from several universities and observatories. Funding and support came from multiple foundations and agencies, including NSF, NASA and the European Union's Horizon program.
Difficult words
- striking — very noticeable or surprising in appearance or effect
- cycle — series of events that repeat in ordercycling
- composition — what something is made of or contains
- transit — when an object passes in front of another
- spectrum — measurements showing light intensity by wavelengthspectra
- vaporize — change from liquid or solid to gas quickly
- discrepancy — difference between two things that should match
Tip: hover, focus or tap highlighted words in the article to see quick definitions while you read or listen.
Discussion questions
- How do clear evening skies on WASP-94A b help scientists measure the atmosphere differently than earlier averaged data?
- Which of the two mechanisms (wind transport or vaporization by heat) do you find more convincing, and why? Use reasons from the article.
- What are possible benefits of studying cloud cycles across many exoplanets with JWST for our understanding of atmospheres?
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