Strange water cycle and mineral clouds on exoplanet WASP-121 b

Artist’s impression of the exoplanet WASP-121 b.


Photo: Patricia Klein and MPIA

A group of astronomers, led by Thomas Michael Evans of the Max Planck Institute for Astronomy, has made the first detailed measurement of atmospheric conditions on the night side of hot Jupiter WASP-121b. By including measurements from the day side, they determined how water changes its physical state as it moves between the two hemispheres of exoplanet WASP-121 b.

While minerals and minerals evaporate into the air on the hot day side, on the cooler night side there are mineral clouds and rain made of liquid gemstones. The study, published in Nature Astronomy, is a major step in deciphering the global cycles of matter and energy in the atmospheres of exoplanets.

The first discovery of an exoplanet orbiting a Sun-like star more than 25 years ago introduced a new and exotic class of planets, the hot Jupiter. Hot planets are gas giants that are similar to Jupiter and orbit closely around their parent star, a few star diameters away. Due to its proximity, the planet is heated by several hundred to several thousand degrees of temperatures by the radiation of the star. Of the approximately 5,000 known exoplanets, more than 300 are hot Jupiter planets.

Using the Hubble Space Telescope, an international collaboration of scientists led by Thomas Michael Evans of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, he examined the atmospheric characteristics of hot Jupiter WASP-121 b. Astronomers discovered this exoplanet in 2015 in the constellation Puppis, 855 light years away. Its mass is about 20% greater than that of Jupiter, while WASP-121 b is almost twice its diameter.

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“Although thousands of exoplanets have been discovered, we have only been able to study the atmosphere for a small part because of the challenging nature of the observations,” says Michal Evans. “To date, most of these measurements have provided limited information, such as basic details about chemical composition or average temperature in certain subregions of the atmosphere.”

The most detailed exploration of the night environment of an exoplanet

With the new observations, astronomers have gained the most detailed insight yet into the conditions of an exoplanet’s nighttime hemisphere. Like all hot Jupiters, WASP-121 b’s rotation is gradually locked into its orbit around its parent star. Thus, a single 30-hour orbit around the star takes as much time as it takes the planet to rotate once on its axis. As a result, the hemisphere facing the star always suffers from reddening of the surface of the star. Likewise, the cooler night side is constantly facing cold and dark spaces. By combining data from both hemispheres, day and night, the team’s analysis leads to the first comprehensive view of how an exoplanet’s atmosphere functions as a global system.

“To examine the entire surface of WASP-121 b, we took Hubble spectra during two complete planetary revolutions,” explains co-author David Singh of Johns Hopkins University in Baltimore, USA. Using this technology and backed by data modeling, the group explored the planetary upper atmosphere of WASP-121 b, observing the full water cycle of an exoplanet for the first time.

Alien toilet on WASP-121 B

On Earth, water is constantly changing its physical state. Solid ice melts in liquid water. The water evaporates, turns into a gas, and then condenses in droplets to form clouds. The cycle ends when these droplets grow into raindrops that eventually fall to fill rivers and oceans. However, new Hubble data reveals that the water cycle on WASP-121 b looks very different.

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On the side of the planet facing the central star, the temperature of the upper atmosphere rises by 3000 degrees Celsius. At such temperatures, the water begins to glow, and many molecules decompose into their atomic components. Hubble data also shows that the temperature in the night hemisphere drops by about 1,500 degrees Celsius. This extreme difference in temperature between the two hemispheres leads to strong winds that engulf the entire planet from west to east, pulling with them disrupted water molecules. Finally, they reached the night side. The lower temperatures allow the hydrogen and oxygen atoms to combine again to form water vapor before exploding again to the side of the day and the cycle repeating. Temperatures never drop enough for water clouds to form throughout the cycle, let alone precipitation.

Metallic clouds and rain made of liquid gemstones

Instead of water, the clouds on the WASP-121 b are primarily composed of minerals such as iron, magnesium, chromium, and vanadium. Previous observations revealed the spectroscopic signals of these metals as hot side gases. New Hubble data indicates that temperatures are low enough for minerals on the night side to condense into clouds. The same east wind that carries water vapor on the night side would also blow these mineral clouds to the day side, where they evaporate again.

Curiously, aluminum and titanium were not among the gases observed in the atmosphere of WASP-121b. A possible explanation for this is that these minerals have condensed and rained down in deeper layers of the atmosphere, which are inaccessible by observations. This rain will be different from any other rain we know of in the solar system. For example, aluminum condenses with oxygen to form corundum. With inclusions of chromium, iron, titanium or vanadium, we know it as sapphire or sapphire. So it can rain a liquid gemstone on the night hemisphere of WASP-121 b.

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Overview of the James Webb Space Telescope

“It’s exciting to study planets like WASP-121 b, which are very different from those in our own solar system, because it allows us to see how their atmospheres behave under extreme conditions,” said co-author Joanna Barstow of The Open University in Milton Keynes. , United kingdom. Michael Evans adds: “To better understand this planet, we will be observing it with the James Webb Space Telescope in its first year of operation.” By covering wavelengths outside the range of the Hubble telescope, these observations will allow the team to quantify the amount of carbon in the atmosphere, which could provide clues about how and where WASP-121 b formed in the protoplanetary disk. In fact, the measurements would be accurate enough to learn more about wind speeds at different altitudes in the atmosphere.

source: Max Planck Institute for Astronomy

Winton Frazier

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