An exoplanet that rains rubies and sapphires

Discovered in 2015, 850 light-years from Earth in the constellation La Naupe, exoplanet WASP-121b is a doomed world. Too close to its star, it goes around it in just 30 hours, and will eventually be dislodged by tidal forces… in a few tens of millions of years. Signs of this announced destruction are already visible as the star is no longer round but has taken on the shape of a rugby ball. Observed several times by the Hubble telescope, it offers astronomers the opportunity to study its complex weather system, which hides amazing features.

Locked “Hot Jupiter”

WASP-121b is a “super-hot Jupiter” type planet: its mass is about 1.2 times that of the solar system giant, but it is much larger, about twice that of the latter. Its proximity to its star, which is hotter and brighter than the sun, means the planet is “locked in” by the play of gravity: the two objects are in synchronous rotation. Like the Moon around the Earth, WASP-121b always represents the same hemisphere facing the star. One can reach temperatures as high as 3,000°C, while the other, much cooler one, remains submerged in the night and exposed to the cold of space. A situation faced by all exoplanets found in this configuration. The large temperature difference between the two hemispheres causes strong winds, the speed of which can reach 18,000 km/h. They convey a heap of elements evaporated by the heat reigning on the illuminated face. Previous studies have already revealed the presence of heavy metals such as iron, chromium, magnesium and aluminum, as well as water molecules, in the atmosphere.

New Hubble studies by Thomas Mikal-Evans of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, have revealed more about the atmospheric circulation around this exoplanet. To do this, the Mikala-Evans team tracked two complete orbits of WASP-121b (in 2018 and 2019) using the space telescope’s spectroscopic camera. The instrument observes light and breaks it down into its component wavelengths, the intensity of which gives astronomers clues about the temperature and composition of the atmosphere. The latter focused in particular on the spectral line of water and studied its changes during two orbits.

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