Webb telescope captures sunrise and sunset on a hot Saturn – Sky & Telescope

In an unprecedented step, astronomers have used the James Webb Space Telescope to measure differences between an exoplanet's atmosphere in the morning and in the afternoon. The team attributes these differences to the powerful 10,000-kilometre-per-hour (6,000 mph) winds that whip around the planet. Fully understanding these factors is a vital step toward accurately modelling exoplanet climates.

The planet in question is WASP-39b, a Saturn-mass giant that orbits a Sun-like star in just over four days. Such proximity means that WASP-39b is Tidally locked — always shows the same face to its star, like the Moon to the Earth. One side is bathed in perpetual, scorching light, while the other side experiences eternal night.

An international team of astronomers used Webb's near-infrared spectrograph to examine the planet as it passed in front of its star. As the star's light filtered through the planet's puffy atmosphere, the atmosphere left telltale fingerprints in the star's spectrum that aren't present when the planet isn't there. transmission spectroscopy The technique provides astronomers with important details about the temperature and chemical composition of the planet's upper atmosphere.

While WASP-39b has been studied this way before, Webb's exquisite new measurements focus on differences across the planet. terminatorthe line that divides night from day. They reveal a nighttime temperature of around 800 °C (1,472 °F), compared to a morning temperature of around 600 °C. The results are published in Nature.

“This is the first time that it has been possible to measure the evening and morning direct spectra of an exoplanet separately,” says team member Maria Steinrück (University of Chicago).

These are incredibly detailed measurements, and therefore difficult to make. “Any small movement in the instrument or observatory during data collection would have severely limited our ability to make this detection,” says study leader Nestor Espinoza (Space Telescope Science Institute). “It needs to be extraordinarily precise, and Webb is.”

The team went on to model WASP-39b’s weather using techniques similar to those used by meteorologists here on Earth. They found that the temperature difference causes changes in air pressure, leading to strong winds. “With speeds reaching 10,000 kilometres per hour or more, these winds are four times faster than those on Neptune, the windiest planet in our solar system,” says team member Laura Kreidberg (Max Planck Institute for Astronomy, Germany). The team’s findings also point to a cloudier late morning, compared to a relatively cloud-free late afternoon.

Carole Haswell (Open University, UK), who was not involved in the research, says the observed difference between morning and afternoon has important implications for the study of other hot, gassy exoplanets. “Most[atmospheric]models simply lump the entire atmosphere together with a single set of cloud parameters,” she says. “These findings suggest that perhaps we need to be cautious about assuming that the entire atmosphere behaves the same way.”

“What I’m most excited about is the potential this holds for future studies,” Haswell says. It’s certainly a strong endorsement of the kind of complex work Webb is capable of. He adds: “There are astronomers around the world thinking about exactly what observations to propose with[Webb]to answer their questions about exoplanet atmospheres.”