🌧️ It rains rubies and sapphires on this planet

A place where winds blow at 11,000 mph (18,000 km/h) and it rains liquid metal, rubies and sapphires: this description worthy of a science-fiction novel actually corresponds to the reality of an exoplanet named WASP-121b, an "ultra-hot Jupiter" that defies imagination.

This gas giant orbits so close to its star that a year there lasts only 30.5 hours. Its proximity to the star is such that tidal forces have deformed it into an egg shape, and any closer approach would tear it apart. On its dayside, temperatures are high enough to vaporize metals, while on the nightside, iron or even crystals could condense and form, then fall as rain.


Thanks to the James Webb Space Telescope, astronomers have detected temperature differences between dawn and dusk on this planet. By observing how starlight is absorbed as WASP-121b passes in front of its star, they found that the evening terminator is hotter than the morning one. This difference stems from powerful winds that transport heat from the dayside to the nightside.

The measurements also revealed variations in the signals of water vapor and carbon monoxide. The hotter evening side might break down water molecules in the upper atmosphere. The cooler morning side could be partially obscured by clouds of silicates, though more advanced models are needed to confirm this.

These new data add to previous observations. The Hubble Telescope had already detected magnesium and iron escaping from the atmosphere, likely due to intense ultraviolet radiation from the star. The Very Large Telescope in Chile had revealed intricate winds and jet streams spanning half the planet.

The technique used by Cyril Gapp's team from the Max Planck Institute for Astronomy could be applied to other ultra-hot planets. It will allow comparisons of the atmospheric conditions of these distant worlds, adding an essential piece to our understanding of exoplanets. The study was published in Nature Astronomy.

Exoplanet Spectroscopy

To study the atmosphere of an exoplanet like WASP-121b, astronomers use spectroscopy. When the planet passes in front of its star (a transit), a small fraction of starlight passes through its atmosphere. The molecules present absorb certain specific wavelengths, creating a kind of fingerprint.

By analyzing this light with sensitive instruments like those on James Webb, one can identify the chemical composition of the atmosphere, its temperature, and even wind movements. Each molecule leaves a unique signature: water, carbon dioxide, methane, etc.

This technique makes it possible to probe the atmosphere at different altitudes and longitudes, offering a three-dimensional map of extraterrestrial weather conditions.