Using the Hubble Space Telescope, an international team have been examining the unusual atmospheric conditions of exoplanet hot Jupiter WASP-121 b, and established how water changes physical states when moving between the hemispheres.
Like all hot Jupiters, WASP-121 b’s rotation is tidally locked to its orbit around its parent star resulting in a cold hemisphere and a hot hemisphere. By merging the data from the dayside and nightside hemispheres, the team’s analysis captures how an exoplanet atmosphere functions as a global system, observing the complete water cycle of an exoplanet for the first time. This includes the discovery of evaporation of airborne metals and minerals on the hot dayside, and metal clouds and liquid ruby and sapphire rain on the cooler night side.
The study, published in Nature Astronomy, is a step towards deciphering the global cycles of matter and energy in the atmospheres of exoplanets.
Dr Hannah Wakeford of the University of Bristol’s School of Physics said: “The water cycle on earth is trapped close to the ground constantly changing from liquid to gas to solid. However, the new Hubble data reveals a water cycle on WASP-121b high in the atmosphere that is completely alien to us on Earth.”
On the side of the planet facing the central star, the upper atmosphere can heat to 3000 degrees Celsius causing the water to begin to glow, and many of the molecules to break down into their atomic components. The Hubble data shows that the temperature drops by approximately 1500 degrees Celsius on the nightside hemisphere. This extreme temperature difference between the two hemispheres gives rise to strong winds that sweep around the entire planet from west to east, dragging the disrupted water molecules along. Eventually, they reach the nightside where the lower temperatures allow the hydrogen and oxygen atoms to recombine, forming water vapour again before being blown back around to the dayside where the cycle repeats. Temperatures never drop low enough for water clouds to form throughout the cycle, let alone rain.
Instead, clouds on WASP-121 b mainly consist of metals such as iron, magnesium, chromium and vanadium. The new Hubble data indicates that temperatures drop low enough for the metals to condense into clouds on the nightside which are blown around to the dayside, where they evaporate.
Aluminium and titanium were not among the gases detected in the atmosphere of WASP-121 b. They are thought to have condensed and rained down into deeper layers of the atmosphere, not accessible to observations. This rain would be unlike any known in the Solar System. Aluminium condenses with oxygen to form the compound corundum, one of the most durable gemstones and recognised as ruby or sapphire.
“We have seen indications of these types of clouds forming in other planetary atmospheres but these data show that there may be processes deeper in the planets trapping clouds that are beyond our reach,” said Dr Wakeford. “This will have implications for the total energy of the planet and how it transports material around”
Lead author Thomas Mikal-Evans from the Max Planck Institute for Astronomy (MPIA) in Heidelberg said: “Despite the discovery of thousands of exoplanets, we’ve only been able to study the atmospheres of a small fraction due to the challenging nature of the observations. So far, most of these measurements have provided limited information, such as basic details on the chemical composition or average temperature in specific subregions of the atmosphere.”
Co-author David Sing from the Johns Hopkins University in Baltimore, USA, explained: “To probe the entire surface of WASP-121 b, we took spectra with Hubble during two complete planet revolutions.”
Dr Wakeford added: ““The Hubble Space Telescope is in low Earth orbit going around the Earth once every 90 minutes. For this program, we designed it so that we would point at the same target for 26 Hubble orbits in a row, that is a very challenging set-up but it has given us amazing insight into WASP-121b’s atmosphere.”
WASP-121 b was discovered in 2015 in the constellation Puppis. Its mass is about 20% greater than that of Jupiter and has a diameter that is nearly twice as large.
The first discovery of an exoplanet orbiting a Sun-like star more than 25 years ago introduced a new and exotic class of planets, a hot Jupiter. Hot Jupiters are Jupiter-like giant gas planets on close orbits around their parent stars, separated by only a few stellar diameters. Due to their proximity, the irradiation from the star heats the planet to several hundred to a few thousand degrees Celsius. Of the almost 5000 known exoplanets, more than 300 are such hot Jupiters.
The team plan to use the James Webb Telescope within the first year of its operation to discover more about the planet. By covering wavelengths beyond Hubble’s range, these observations will help them to determine the amount of carbon in the atmosphere, which could hold clues about how and where WASP-121 b formed in the protoplanetary disk. The measurements will even be precise enough to learn about the wind speeds at different altitudes inside the atmosphere.
Paper:
‘Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b’ in by Thomas Mikal-Evans et al in Nature Astronomy.