Extreme heating and tidal locking result in fierce cyclones and powerful storms in the atmosphere of WASP-121 b
The Hubble Space Telescope has seen cyclones raging in the dynamic atmosphere of a hot Jupiter located 880 light years away thanks to observations and computer modelling that could one day be applied to characterise weather on smaller rocky exoplanets. The planet, dubbed WASP-121 b, has a mass about 1.16 times greater than our Solar System’s Jupiter and orbits its star at a distance of just 3.88 million kilometres (2.41 million miles). That’s just 2.6 per cent the distance between Earth and the Sun. Furthermore, WASP-121 b speeds around its star once in only 1.27 days – in other words, its year is just 1.27 Earth days long.
Being so close to its star, WASP-121 b is also tidally locked, meaning it rotates on its axis in time with its orbit such that it always shows the same hemisphere to its star. Its permanent dayside reaches temperatures around 2,329 degrees Celsius (4,224 degrees Fahrenheit), causing WASP-121 b’s atmosphere to become bloated, expanding the radius of the planet to a size 75 per cent larger than our beloved Jupiter. In this insufferably hot maelstrom, it’s suspected that evaporated iron, barium and oxides of titanium and vanadium lace the world’s atmosphere and form showers of molten metal on weather fronts between the day and night sides.
The Hubble Space Telescope observed the WASP-121 system on four separate occasions: in June and November 2016, in March 2018 and February 2019. On each occasion, the planet was in a different position in its orbit, including transiting in front of and disappearing behind its star. This allowed a team of astronomers, led by Jack Skinner of the California Institute of Technology, to determine how WASP- 121b changes brightness when seen from different angles and in different phases, like our Moon. By applying computer modelling, Skinner and his colleagues could determine what the data could reveal about the planet’s atmosphere. “The remarkable details of our exoplanet atmosphere simulations allow us to accurately model the weather on ultrahot planets like WASP-121 b,” said Skinner.
Hubble’s observations show that WASP-121 b’s atmosphere is dynamic, as the temperature gradient between the permanent dayside and everlasting nightside generates huge storms and cyclones; weather fronts also wash over the planet and regions of hot and cooler air form, grow and mix. These weather features are quasi-periodic, varying over timescales of about five of the planet’s days. “The information that we extracted from those observations was used to infer the chemistry, temperature and clouds of the atmosphere of WASP-121 b at different times,” said Quentin Changea
