Research confirms that distant world’s eternal sunrise and sunset are not alike

Pioneering new research has revealed that places where the eternal sunrise and sunset occur on a distant world have different atmospheric properties.

An international team of researchers, using NASA’s James Webb Space Telescope (JWST), has shown that there is a temperature difference between the west and east side of WASP-39b — an exoplanet located 700 light-years away from Earth. 

WASP-39b is a gas giant planet with a radius 1.3 times greater than Jupiter and a mass similar to Saturn. WASP-39b is tidally-locked to its host star, which means that it rotates about its axis in exactly the same time as it takes to orbit its host star. This causes WASP-39b to have a perpetual morning on its west side and a perpetual evening on its east side.

Using JWST’s NIRSpec (Near-Infrared Spectrograph), astronomers confirmed that the temperatures on WASP-39b’s evening side are hotter by about 200 degrees Celsius (roughly 300 degrees Fahrenheit) than on its morning side. They also found that the forever morning side of the planet is likely cloudier than the evening side. 

Astronomers observed WASP-39b’s atmosphere at 2 to 5 microns using transmission spectroscopy, a technique that studies the exoplanet’s terminator, the boundary that separates the planet’s dayside and nightside. A resulting transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves in front of the star, to the unfiltered starlight detected when the planet is beside the star. When making that comparison, researchers can get information about the temperature, composition, and other properties of the planet’s atmosphere. 

The team’s findings have been published in Nature. 

Maria Zamyatina, from the University of Exeter and a co-author of the research said: “Such observations of the evening-morning differences in exoplanet atmospheres are absolutely critical for testing our theories about the general atmospheric circulation on these planets. They also help constrain 3-dimensional numerical models used to predict the dynamical, thermal and chemical structure of these atmospheres, including models developed at Exeter”.

Previously published JWST spectra of WASP-39b’s atmosphere revealed the presence of sodium, potassium, water vapour, carbon monoxide, carbon dioxide and sulphur dioxide at the planet’s entire day/night boundary – however, there was no detailed attempt to differentiate between morning and evening. 

Now, this new study produced two different spectra from the terminator region, essentially splitting the day/night boundary into two semicircles, one from the evening, and the other from the morning. Data reveals that the evening is significantly hotter, a searing 800 degrees Celsius (1450 degrees Fahrenheit) compared to the morning’s relatively cooler 600 degrees Celsius (1150 degrees Fahrenheit). 

“WASP-39b has become a sort of benchmark planet in studying the atmosphere of exoplanets with Webb,” said Néstor Espinoza, an exoplanet researcher from the Space Telescope Science Institute and the lead author on the study. “It has an inflated, puffy atmosphere, so the signal coming from starlight filtered through the planet’s atmosphere is quite strong.” 

Extensive modelling of the data obtained also allows researchers to investigate the structure of WASP-39b’s atmosphere, the cloud cover, and why the evening is hotter. While future work by the team will study how the cloud cover may affect temperature, and vice versa, astronomers confirmed that gas circulation around the planet as the main culprit of the temperature difference on WASP-39b. 

On a highly irradiated exoplanet like WASP-39b that orbits relatively close to its star, researchers generally expect the gas to be moving as the planet rotates around its star. Hotter gas from the dayside should move through the evening to the nightside via a powerful equatorial jet stream. Since the temperature difference is so extreme, the air pressure difference would also be significant, which in turn would cause high wind speeds. 

Using General Circulation Models, 3-dimensional models similar to the ones used to predict weather patterns on Earth, researchers found that on WASP-39b the prevailing winds are likely moving from the night side across the morning terminator, around the dayside, across the evening terminator and then around the nightside. As a result, the morning side of the terminator is cooler than the evening side. In other words, the morning side gets slammed with air that has been cooled on the nightside, while the evening is hit by air heated on the dayside. Research suggests the wind speeds on WASP-39b can reach thousands of miles an hour! 

The researchers will now look to use the same method of analysis to study atmospheric differences of other tidally locked hot Jupiters, as part of the JWST Cycle 2 General Observers Program 3969. 

WASP-39b was among the first targets observed by JWST as it began regular science operations in 2022. The data in this study was collected under the Early Release Science program 1366, designed to help scientists quickly learn how to use the telescope’s instruments and realise its full science potential. 

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).