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Study unveils fascinating insights on WASP-121B, a strange “warmer than hot” exoplanet

On WASP-121B, a hot Jupiter-like exoplanet, it’s hot – hotter than hot. That is the result of an orbit very close to a bright and warm host star. But also, according to a recent study, due to the heavy metals escaping from the upper atmosphere of the planet.

WASP-121B is an exoplanet discovered in 2015, about 850 light-years away from our Solar System, somewhere in the constellation of Stern. It is a hot Jupiter planet known especially for the water that has been detected in its atmosphere. It is also an ideal target for investigations of the future James-Webb space telescope, which will scan the infrared for water and carbon dioxide.

But in the framework of Panchromatic Comparative Exoplanet Treasury (PanCET) – the first large-scale comparative study of distant worlds in the ultraviolet, visible and infrared domains launched on the Hubble Space Telescope – WASP-121a reserved another surprise to astronomers. In search of spectral signatures of magnesium and iron in the light of the stars filtering through the atmosphere of the exoplanet, they made an astonishing observation.

For the first time, a stream of gas of heavy metals was observed escaping from the planet. In principle, hot planets the size of Jupiter remain cool enough inside to condense these heavy elements into clouds. But WASP-121 b orbits so close to its star that the temperature of its upper atmosphere reaches more than 2,500 ° C.

A surprisingly clear phenomenon

So it would essentially be the ultraviolet light (UV) of the sun of this amazing exoplanet, a sun warmer and brighter than ours, which would warm its upper atmosphere to allow heavy metals to escape. “And these metals make the atmosphere more opaque to UV, which further contributes to the warming of the upper atmosphere,” says David Singer, a researcher at Johns Hopkins University in Baltimore, United States.

“We were hoping to have the chance to observe this phenomenon on this planet that we know is extreme. But we were surprised by the clarity of the data and the presence of heavy metals as far away from the planet. WASP-121B is actively stripping its atmosphere,” says Singer. This result brings some additional elements to the understanding of how the planets lose their primordial atmospheres. When they are formed, the planets surround themselves with an atmosphere made essentially of hydrogen and helium, the most abundant elements of the Universe. An atmosphere they tend to lose as they get closer to their star. “In the case of WASP 121b, hydrogen and helium gas escape, dragging with them heavy metals,” concludes David Singer.

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