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  • Writer's pictureNikki Latham

Hubble Detects Exotic Water Cycle & Metal Clouds In A Hot Jupiter Exoplanet Atmosphere

Scientists have explored the nightside hemisphere of an enormous planet, far outside our solar system. They discovered that the planet has metal clouds and rain made of liquid gems.

A team of researchers, including Professor Nathan Mayne from the University of Exeter, has made a new discovery about the fascinating atmosphere of the exoplanet WASP-121b with NASA’s Hubble Space Telescope.

WASP-121b, located approximately 900 light years from Earth, is a gas giant exoplanet commonly referred to as a ‘hot Jupiter’. It's bigger than our own Jupiter and has a mass just over 3 times that of the Earth, meaning it's much puffier! This exoplanet orbits its host star every 1.3 days and is around the closest distance it could be before the star's gravity would start ripping it apart.

A close proximity of the top of the atmosphere to the Earth's surface means that it is heated to a boiling point of 2,500 degrees Celsius—the temperature at which iron exists in gas form rather than solid.

The researchers led by Thomas Mikal-Evans from the Max Planck Institute for Astronomy made the first detailed measurements of an exoplanet's nightside atmospheric conditions.

They used previous measurements from the dayside hemisphere to determine how water changes physical states when moving between the hemispheres.

The study revealed a fascinating insight into the conditions on the nightside hemisphere. While airborne metals and minerals evaporate on the hot dayside, liquid gems rain from the night side and clouds of metal fill the atmosphere.

The researchers believe this new discovery will provide a giant leap in understanding the cycles of matter and energy in exoplanets' atmospheres.

The study was published last year in Nature Astronomy.

Professor Nathan Mayne, co-author of the study said: “Studies like Tom has led here show the real power of combing cutting-edge observations with state-of-the-art modelling. Facilities like the HST and now the JWST, combined with the knowledge and tools often developed to understand our own planet’s atmosphere are combining to teach us about these fascinating worlds. “It is incredibly exciting to think what we will learn in the near future about the range of planets our galaxy is home to, and how this can feed back into a better understanding of our own changing world.” On Earth, water constantly changes form. Solid ice melts into liquid water, which evaporates and then condenses into droplets to form clouds. The cycle closes when those droplets grow to raindrops, which eventually fall down to fill rivers and oceans. However, new data from the Hubble Space Telescope reveals a water cycle on WASP-121 b that behaves completely differently. On the side of the planet facing the central star, the upper atmosphere becomes as hot as about 3000 degrees Celsius. At such temperatures, water molecules begin to glow and even break down into their atomic components. The Hubble data also reveal that the temperature drops by approximately 1500 degrees Celsius on the nightside hemisphere. This extreme temperature difference between the two hemispheres can cause strong winds to sweep around the entire planet from west to east, dragging water molecules along with them. The higher temperatures on the dayside allow hydrogen and oxygen atoms to recombine, forming water vapour before being blown back around to the dayside and the cycle repeats. Temperatures never drop low enough for water clouds to form throughout the cycle, let alone rain. The clouds on WASP-121 b are mainly made of metals such as iron, magnesium, chromium and vanadium. Previous observations have revealed these gases as part of the hot dayside clouds. New Hubble data indicate that temperatures drop low enough for water vapor to condense into clouds on the nightside. The winds blowing eastward would transport this metal vapor back toward the dayside, where it evaporates again. Although aluminium and titanium are both metals, they were not detected by the gas analysis on WASP-121 b. This is likely because these metals have condensed and rained down into deeper layers of the atmosphere, making it inaccessible to observations. Astronomers believe this rain to be more unusual than any in the Solar System. For instance, aluminium condenses with oxygen to form the compound corundum. With impurities of chromium, iron, titanium or vanadium, we know it as ruby or sapphire. Liquid gems could therefore be raining on the nightside hemisphere of WASP-121 b. An animation illustrating the measurements obtained with the Hubble Space Telescope to characterise the stratosphere of WASP-121 b can be viewed here: Credit: T. Mikal-Evans (MPIA) / T. Müller (MPIA/HdA)

Picture Credit: Patricia Klein and MPIA

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