24.07.2024

The James Webb Space Telescope has for the first time detected the difference between the morning and evening of a tidally locked gas giant planet.

This artist’s concept shows what the exoplanet WASP-39 b could look like.
NASA / ESA / CSA / R. Crawford (STScI)

In an unprecedented step, astronomers have used the James Webb Space Telescope to measure differences between an exoplanet’s atmosphere at morning and evening. The team puts these differences down to fierce 10,000 kilometer per hour (6,000 mph) winds that whip around the planet. Fully understanding such factors is a vital step in accurately modelling the climates of exoplanets.

The planet in question is WASP-39b, a Saturn-mass giant that orbits a Sun-like star in a little more than four days. Such proximity means that WASP-39b is tidally locked — it always shows the same face to its star, just as the Moon does to Earth. One side is bathed in perpetual, scorching daylight, 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 light from the star filtered through the planet's puffy atmosphere, the atmosphere left tell-tale imprints in the star's spectrum that aren't present when the planet isn't there. This transmission spectroscopy technique gives astronomers important details about the temperature and chemical composition of the planet's upper atmosphere.

While WASP-39b has been studied in this way before, Webb’s exquisite new measurements home in on differences across the planet's terminator, the line that divides night from day. They reveal an evening 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 a separate measurement of a direct evening and morning spectrum of an exoplanet has been possible,” says team member Maria Steinrück (University of Chicago).

These are incredibly detailed and therefore difficult measurements to make. “Any tiny movement in the instrument or with the observatory while collecting data would have severely limited our ability to make this detection,” says study lead Néstor Espinoza (Space Telescope Science Institute). “It must be extraordinarily precise, and Webb is just that.”

The team went on to model the climate of WASP-39b using techniques similar to those used by weather forecasters here on Earth. They found that the temperature difference leads to changes in air pressure, which generates strong winds. “With speeds reaching 10,000 kilometers 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 hint at a cloudier morning terminator, compared to a relatively cloud-free evening counterpart.

Carole Haswell (Open University, UK), who was not involved in the research, says that the observed difference between morning and evening has important consequences for studying other hot and gaseous exoplanets. “Most [atmospheric] models just lump the whole atmosphere together with a single set of parameters for the clouds,” she says. “These findings suggest that one might need to be careful about assuming the whole atmosphere behaves in the same way.”

“What’s most exciting to me is the potential this reveals for further studies,” Haswell says. It is certainly a ringing endorsement of the kind of intricate work Webb is capable of. She adds: “There are astronomers all over the world thinking about exactly what observations to propose with [Webb] to answer their questions about exoplanet atmospheres.”

Quelle: Sky&Telescope