Asymmetric aerosol distribution observed on the warm Saturn planet WASP-69 b

Star system: WASP-69 | Distance from the Sun: 164 light years

The warm Saturn-sized exoplanet WASP-69 b exhibits significant atmospheric differences between its morning and evening sides. New observations reveal an asymmetric distribution of aerosols—tiny particles suspended in the atmosphere—which create distinct cloud conditions across the planet. This discovery helps explain how planets at temperatures near 1,000 Kelvin manage the cycle of cloud formation and evaporation as material travels around the globe.

Researchers analyzed the planet's transmission spectrum, which is the light filtered through the atmosphere as the planet passes in front of its star, using the JWST space telescope. They found that the evening side of the planet shows clear evidence of water vapor, while the morning side does not, indicating that high-altitude clouds are blocking these molecular signals on one side but not the other.

The difference is driven by temperature variations between the two sides, with the evening side being over 300 Kelvin hotter than the morning side. This heat causes clouds formed on the morning side to evaporate as they move toward the evening, leaving a clearer atmosphere where water vapor can be detected. This pattern was confirmed by advanced 3D climate models of the planet's circulation.

Additional analysis confirms that WASP-69 b possesses an escaping tail of metastable helium extending more than three times the planet's radius. The findings suggest that many giant planets in this temperature range share these complex, uneven cloud patterns.

Key findings
- Aerosols are distributed unevenly across the morning and evening sides of WASP-69 b.
- Water vapor is detected on the hotter evening side but masked by clouds on the morning side.
- The evening side is approximately 300 Kelvin warmer than the morning side.
- Morning clouds partially evaporate as they cycle toward the hotter evening side.
- An escaping tail of metastable helium extends to over three times the planet's radius.

Scientific study

Asymmetric Aerosol Distribution on the Terminators of the Warm Saturn WASP-69 b Revealed by JWST NIRISS/SOSS (external link)