Hubble observes a hot Jupiter with a changing atmosphere
A team of astronomers have found evidence for massive cyclones along with other dynamic weather activity occurring on a hot Jupiter 880 light years away, providing important information about weather patterns on distant planets.
Over the past few decades, detailed space and ground-based observations of our planetary neighbours in the solar system have revealed that their atmospheres are constantly changing, much like weather on Earth. However, although a similar atmospheric variability should be true for exoplanets, studying this is much more challenging. Due to the vast distances these planets are from Earth, along with the fact that most orbit much larger and brighter stars, observations need to be combined to increase the strength of the exoplanets signal. This gives astronomers a picture of the exoplanet’s average atmosphere, however, it does not provide any information on if there is variability. To study this, higher quality data taken over longer periods of time is needed.
This was the aim for a team of astronomers who gathered Hubble observations of a hot Jupiter named WASP-121 b spanning several years. Four sets of archival observations from Hubble were used, including exoplanet transits in June 2016, its secondary eclipse in November 2016, and the brightness of the planet as a function of its phase angle to the star in March 2018 and February 2019.
"The assembled data set represents a significant amount of observing time for a single planet and is currently the only consistent set of such repeated observations," said Quentin Changeat, one of the principal investigators of the team and an ESA Research Fellow at the Space Telescope Science Institute. "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. This provided us with an exquisite picture of the planet changing over time."
WASP-121 b is extremely close to its parent star with a rapid orbital period of 1.27 days. This close-in orbit means the planet is tidally locked, so the same hemisphere always faces the star. As a result, the day-time temperatures reach a scalding 2150K. Additionally, the planet is blasted with UV light from the host star, heating the planets upper atmosphere causing magnesium and iron gas to escape into space. The strong gravitational tidal forces from the star have also pulled the planet into a more oblate shape.
The Hubble observations uncovered WASP-121 b’s dynamic atmosphere, varying over time. Dramatic differences are seen between observations, where huge weather fronts, storms and cyclones are repeatedly created and destroyed due to the large temperature difference between the exoplanet’s day and night side. The data displayed a shift in the hot spot along with variability in its atmospheric chemical composition. Sophisticated computational models were used by the team to explain the changes, suggesting quasi-periodic weather patterns.
"The remarkable details of our exoplanet atmosphere simulations allows us to accurately model the weather on ultra-hot planets like WASP-121 b," explained Jack Skinner, a postdoctoral fellow at the California Institute of Technology in Pasadena, California, and co-leader of this study. "Here we make a significant step forward by combining observational constraints with atmosphere simulations to understand the time-varying weather on these planets.”
Hubble has just begun its cycle 31 observations on the 1st of December, with two-thirds of its time dedicated to imaging studies, whilst the remainder is for spectroscopic studies such as those used for WASP-121 b.
Cover image: NASA, ESA, Q. Changeat et al., M. Zamani (ESA/Hubble)
Journal source: Quentin Changeat et al, Is the atmosphere of the ultra-hot Jupiter WASP-121b variable?, arXiv (2024). DOI: 10.48550/arxiv.2401.01465