Using Hubble, scientists observe weather on exoplanet

by Martijn Luinstra

Using data from NASA’s Hubble Space Telescope, an international team of astronomers has detected atmospheric variability on an exoplanet called WASP-121 b. The team analyzed observations taken with Hubble over several years and used computer simulations to demonstrate that the observed variations in the atmosphere could be explained by weather patterns.

WASP-121 b, also known as Tylos, is a Jupiter-sized exoplanet located about 880 light-years from Earth. Taking only 30 hours to complete one orbit around its host star, the planet is so close to the star that it is almost torn apart by gravitational forces. Additionally, the exoplanet is tidally locked to its star, meaning that the same hemisphere always faces the star and gets extremely hot from the constant exposure to the star’s ultraviolet light.

“This is a hugely exciting result as we move forward for observing weather patterns on exoplanets,” said one of the team’s principal investigators, Quentin Changeat of the Space Telescope Science Institute in Baltimore, Maryland. “Studying exoplanets’ weather is vital to understanding the complexity of exoplanet atmospheres on other worlds, especially in the search for exoplanets with habitable conditions.”

To study this extreme world, the scientists had to construct a data set from multiple observations taken over three years. They then analyzed that data and used computer simulations to reconstruct the conditions in the exoplanet’s atmosphere that could have caused the observed atmospheric variability.

“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 Changeat. “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.”

But why did the astronomers need a special data set to observe weather on WASP-121 b?

Most exoplanets like WASP-121 b cannot be observed directly, but only by carefully measuring their effect on the light emitted by their host stars. When an exoplanet passes between its star and the Earth, the star will appear slightly dimmer than normal. By measuring these so-called transits, astronomers can learn a lot about the transiting exoplanet.

However, observing an exoplanet’s atmosphere is even harder. When a star’s light dims during a transit, an even smaller fraction of the light passes through the planet’s atmosphere, which slightly alters the spectrum of light detected by the telescope. By analyzing how the spectrum changes, astronomers can determine the composition of the planet’s atmosphere.

To increase the strength and reduce the noise of these minute signals, astronomers often combine multiple observations into an average, but this makes it impossible to measure changes over time. When a single observation is enough to obtain a clear signal, the limited availability of telescopes means the observation is rarely repeated.

Fortunately, WASP-121 b has been observed four times with Hubble’s Wide Field Camera 3. Totaling about 90 hours of observation time, Hubble collected data on the exoplanet in 2016, 2018, and 2019. This allowed the team to analyze the exoplanet’s atmosphere at multiple points in time, without having to deal with incompatibilities between observations made with different telescopes.

However, the data was not all processed the same way, as the observations from 2018 and 2019 were processed with a different method for another study. The team, therefore, had to reprocess these observations before they could analyze them.

Eventually, the astronomers found that the exoplanet’s hot spot had moved around between the different observations, and the composition of the chemical atmosphere appeared slightly different. They then used computer simulations to find weather patterns that could explain the observations. These showed that the temperature differences between the exoplanet’s star-facing and dark side could cause giant storms and cyclones to appear and disappear.

“The remarkable details of our exoplanet atmosphere simulations allows us to accurately model the weather on ultra-hot planets like WASP-121 b,” said co-author Jack Skinner of the California Institute of Technology in Pasadena, California. “Here we make a significant step forward by combining observational constraints with atmosphere simulations to understand the time-varying weather on these planets.”

The recent study is one of many that have been conducted on WASP-121 b in recent years. Many telescopes have observed the planet and its star, including the Spitzer Space Telescope, the James Webb Space Telescope, and ground-based telescopes. A study published in 2019 reported that the planet’s atmosphere gets so hot that metals like iron and magnesium escape the planet’s atmosphere instead of condensing into clouds.

Changeat et al.’s study was accepted for publication in The Astrophysical Journal Supplement journal.

(Lead image: Artist’s impression of the exoplanet WASP-121 b and its parent star WASP-121. Credit: NASA, ESA, Q. Changeat et al., M. Zamani (ESA/Hubble))

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