The hidden side of an exoplanet for the first time revealed in detail

It is one of the best known and most studied exoplanets by scientists: WASP-121bclassified as a planet of type Hot Jupiter (slightly more massive than Jupiter for nearly twice its size), is the first exoplanet on which a atmosphere partly composed of water vapor had been identified. Due to its extreme proximity to its star— it completes a orbit in just 30 hours — WASP-121b is lock-in by the effect of tide : performing an orbit at the same rate as it turns on itself, the planet has one side perpetually exposed to its star (a “day” side where temperatures can reach 3,000 K), while the opposite side is immersed in permanent darkness.

The Hubble Space Telescope as a backup to observe the hidden side

Although spectroscopic data had already been collected on the lighted side of WASP-121b, allowing water vapor detection, among other things, the hidden side remained unknown: about 10 times darker than the “day” side, the measurements of the little light reflected by the “night” side requires high-precision observation tools. The team therefore appealed to the Hubble Space Telescopeand more particularly to its spectroscope, to perform measurements on the far side during two orbits. The use of a spectroscope makes it possible here to measure the intensity of the light received as a function of the wavelength, thus providing valuable clues to the composition of the atmosphere.

And the combination of measurements on the “day” and “night” sides allows for the first time a global visualization of the system represented by the atmosphere of the exoplanet, offering scientists the possibility of making various models to understand its major physical properties and the major processes that govern it. By tracking the concentrations of water vapour depending on the altitude, the team was able to reconstruct temperature profiles for the two faces: thus, the illuminated face would have a temperature climbing up to 3,500 K for the highest atmospheric layers, and descending to the around 2,500 K for the deepest observable altitudes; on the “night” side, the temperature seems to increase with depth, with temperatures ranging from 1,500 K, for the highest layers, to 1,800 K for the deepest layers.

A very exotic atmosphere

From these temperature profiles, the scientists were able to model the chemical species most likely to be present in WASP-121b’s atmosphere, as well as certain processes that take place there, such as its water cycle, although more intense than on Earth: on the illuminated side, the atoms constituting the molecules of water are separated under temperatures above 3,000 K, then are sent to the hidden face, due to the gradient temperature between the two faces. The atoms ofhydrogen and oxygen reassemble under the milder conditions of the “night” side, then are again sent to the lighted side, all thanks to winds reaching gears 5 kilometers per second.

But water would not be the only one circulating in the tumults of this atmosphere: astronomers also discovered that the dark side was cold enough to harbor clouds composed ofmetallic elementsor even corundum — the crystal form of the’aluminum oxide which can sometimes form, on Earth, rubies when mixed with traces of other metals. Thus, the team of scientists even advances to imagine possible rains of gems liquids coming from the clouds of the atmosphere of WASP-121b, during their journey between the two faces.

To dig even deeper into their new findings, the team has already reserved several slots to observe WASP-121b’s atmosphere again, but this time with the space telescope. James Webband hope to be able to map a possible cycle of the carbon monoxide. These observations could, among other things, make it possible to better understand the conditions of formation of this type of exoplanet.