On Earth, the temperature is just right for water to form clouds in the atmosphere. The water eventually condenses, rains down to the surface, and then evaporates back up to complete the water cycle. It has been predicted that if a planet were hot enough, it might instead form clouds made of silicates, the main ingredient in rocks. Under these extreme conditions, a silicate cycle featuring clouds and rain made of sand is expected. While seemingly farfetched, the hottest of the 5,000+ known exoplanets – that is, planets outside of our solar system – may be toasty enough for this process to occur.

A recent study identified one such exoplanet, called WASP-107 b. This strange giant planet is a scalding 480 ^oC (880 ^oF) at its outer layers and its year lasts only 6 days. The researchers who led the recent study used a technique called transmission spectroscopy with the James Webb Space Telescope to identify the gasses in its atmosphere, unambiguously detecting silicate clouds for the first time. In addition, the researchers were surprised to find sulfur dioxide (SO₂) but no methane (CH₄). The giant planets in our own solar system – like Jupiter and Saturn – have methane, so the scientists expected to find it in the atmosphere of WASP-107 b. Sulfur dioxide is a known photochemical product, meaning high-energy starlight can cause its production. While sulfur dioxide was not expected to exist under WASP-107 b’s atmospheric conditions, it may be explained by the planet’s exceptionally low density. Given these conditions, starlight may be able to penetrate deep enough into the atmosphere to initiate photochemistry. Why the planet has such a low density remains an open question.

WASP-107 b is the latest addition to the growing tapestry of exoplanet atmosphere studies, further uncovering the exciting complexity of planetary climates. Yet it poses new challenges for the astronomy community, perhaps leaving more questions unanswered than answered.

This study was led by Achrène Dyrek, a postdoctoral research fellow at the French Alternative Energies and Atomic Energy Commission, Michiel Min, a senior scientist at SRON Netherlands Institute for Space Research, and Leen Decin, a Professor of Astrophysics at the Institute of Astronomy of KU Leuven.

Managing Correspondent: Collin Cherubim

Press Article: Webb Telescope Peers Into Puffy Planet with Clouds of Sand (Sky & Telescope)

Original Journal Article: SO2, silicate clouds, but no CH4 detected in a warm Neptune (Nature)

Image Credit: Pixabay/placidplace