Researchers using the James Webb Space Telescope have detected atmospheric changes on the exoplanet WASP-39 b, revealing temperature changes and distinct cloud cover in its tidally locked hemispheres. The planet, similar in size to Jupiter but closer in mass to Saturn, exhibits a hotter evening side compared to its morning side, attributed to strong atmospheric circulations. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)
Near-infrared spectral analysis of the terminator confirms changes in the morning and evening atmosphere.
From the beginning exoplanets discovered in 1992, thousands of planets orbiting stars outside our solar system have been confirmed through a myriad of different methods, including direct imaging, gravitational microlensing, transit measurement, and astrometry. Over the years, techniques have evolved to study these exoplanets, with astronomers learning details about the atmospheric compositions of these distant worlds.
NASAS ‘ The James Webb Space Telescope is continuing to advance this field of study and deepen our understanding of the diversity of exoplanets and their atmospheres.
The last? Webb has allowed astronomers to analyze atmospheric differences between morning and evening on a tidally locked exoplanet – a remarkable achievement for a distant world 700 light-years away from Earth like WASP-39 b.
This artist’s concept shows what exoplanet WASP-39 b might look like based on indirect transit observations from NASA’s James Webb Space Telescope, as well as other space-based and ground-based telescopes. Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
The Webb Space Telescope probes eternal sunrises, sunsets on distant worlds
Researchers using NASA’s James Webb Space Telescope have finally confirmed what models have previously predicted: An exoplanet has differences between its eternal morning and eternal evening atmospheres. WASP-39 b, a giant planet with a diameter 1.3 times larger than Jupiterbut measure similar to Saturn orbiting a star about 700 light-years from Earth, it is tidally bound to its parent star. This means that it has a continuous day and a continuous night – one side of the planet is always exposed to its star, while the other is always covered by darkness.
Using Webb’s NIRSpec (Near Infrared Spectrograph), astronomers confirmed a temperature difference between eternal morning and eternal evening in WASP-39 b, with the evening appearing hotter by approximately 300 Fahrenheit degrees (about 200 centigrade degrees). They also found evidence for different cloud cover, with the permanent morning part of the planet likely to be cloudier than the evening.
This animation describes how Webb uses transmission spectroscopy to study the atmospheres of distant exoplanets. Credit: NASA, ESA, CSA, Leah Hustak
Advances in Exoplanet Atmospheric Studies
Astronomers analyzed the 2- to 5-micron transmission spectrum of WASP-39 b, a technique that studies the exoplanet terminator, the boundary that separates the planet’s day and night sides. A transmission spectrum is created by comparing the starlight filtered through a planet’s atmosphere as it moves in front of the star, with the unfiltered starlight detected when the planet is near the star. When making this comparison, researchers can obtain information about the temperature, composition, and other properties of the planet’s atmosphere.
“WASP-39 b has become a standard planet type in the study of exoplanet atmospheres with Webb,” said Néstor Espinoza, an exoplanet researcher at Space Telescope Science Institute and lead author on the study. “It has a puffy, puffy atmosphere, so the signal from starlight filtered through the planet’s atmosphere is quite strong.”
A light curve from NASA’s James Webb Space NIRSpec (Near-Infrared Spectrograph) telescope shows the change in brightness from the star system WASP-39 over time as the planet transited the star. This observation was made using NIRSpec’s Bright Object Time Series mode, which uses a grating to scatter light from a single bright object (such as WASP-39 b’s host star) and measure the brightness of each wavelengths of light at certain time intervals. Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
Insights into temperature and atmospheric composition
Webb’s previously published spectra of WASP-39b’s atmosphere, which revealed the presence of carbon dioxide, sulfur dioxide, water vapor, and sodium, represent the entire day/night boundary—there was no detailed attempt to distinguish between one side and the other.
Now, the new analysis constructs 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. The data shows that the evening is significantly hotter, 1450 degrees Fahrenheit (800 degrees Celsius) and the morning is relatively cooler 1150 degrees Fahrenheit (600 degrees Celsius).
This transmission spectrum, captured using Webb’s NIRSpec (Near-Infrared Spectrograph) object-time-series bright mode, shows the amount of near-infrared starlight blocked by the gas giant exoplanet’s atmosphere hot WASP-39 b. The spectrum shows clear evidence for water and carbon dioxide, and a temperature difference between morning and evening on the exoplanet.
The new analysis of the transmission spectrum of WASP-39 b constructs two different spectra from the stationary day/night boundary in exoplanets, essentially dividing this terminator region into two semicircles, one from the evening and the other from the morning. The data shows that the evening is significantly hotter, 1450 degrees Fahrenheit (800 degrees Celsius) and the morning is relatively cooler 1150 degrees Fahrenheit (600 degrees Celsius).
The blue and yellow lines are the best-fit model that takes into account the data, the known properties of WASP-39 b and its star (eg size, mass, temperature), and the assumed characteristics of the atmosphere.
Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
Implications of temperature variations
“It’s truly amazing that we’re able to analyze this small change, and it’s only possible because of Webb’s sensitivity at near-infrared wavelengths and its extremely stable photometric sensors,” Espinoza said. “Any slight movement in the instrument or observatory during data collection would have severely limited our ability to make this discovery. It has to be extremely accurate, and the Webb is just that.”
Extensive modeling of the obtained data also allows researchers to investigate the structure of WASP-39 b’s atmosphere, cloud cover and why the evening is hotter. While future work by the team will study how cloud cover can affect temperature and vice versa, astronomers confirmed the circulation of gas around the planet as the main culprit of the temperature change in WASP-39 b.
Understanding planetary wind patterns and temperature dynamics
In a highly irradiated exoplanet like WASP-39 b that orbits relatively close to its star, researchers generally expect the gas to move as the planet orbits its star: The hottest gas from the day should move during the evening to the nightside through a powerful gas. 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 those used to predict weather patterns on Earth, the researchers found that in WASP-39 b the prevailing winds are likely to move from the night side along the morning terminator , around the day, across the evening terminator and then around the night side. As a result, the morning side of the terminator is cooler than the evening side. In other words, the morning side is buffeted by winds of air that have cooled on the night side, while the evening is buffeted by winds of air that has warmed during the day. Research suggests that wind speeds on WASP-39 b could reach thousands of miles per hour!
Future Research Directions and Webb’s Early Scientific Contributions
“This analysis is also particularly interesting because you’re getting 3D information about the planet that you didn’t get before,” Espinoza added. “Because we can say that the evening edge is hotter, that means it’s a bit puffier. So, theoretically, there is a small ripple in the terminator that approaches the night side of the planet.”
The team’s results are published in the journal Nature.
The researchers will now seek to use the same analysis method to study the atmospheric changes of other hot, tidally locked Jupiters as part of the Webb Cycle 2 General Observers Program 3969.
WASP-39 b was among the first targets analyzed by Webb after it began regular science operations in 2022. The data in this study was collected under the Early Release Science 1366 program, designed to help scientists quickly learn how to use telescope instruments and realize full scientific potential.
Reference: “Inhomogeneous Terminators in the Exoplanet WASP-39 b” by Néstor Espinoza, Maria E. Steinrueck, James Kirk, Ryan J. MacDonald, Arjun B. Savel, Kenneth Arnold, Eliza M.-R. Kempton, Matthew M. Murphy, Ludmila Carone, Maria Zamyatina, David A. Lewis, Dominic Samra, Sven Kiefer, Emily Rauscher, Duncan Christie, Nathan Mayne, Christiane Helling, Zafar Rustamkulov, Vivien Parmentier, Erin M. May, Aarynn L. Carter, Xi Zhang, Mercedes López-Morales, Natalie Allen, Jasmina Blecic, Leen Decin, Luigi Mancini, Karan Molaverdikhani, Benjamin V. Rackham, Enric Palle, Shang-Min Tsai, Eva-Maria Ahrer, Jacob L. Bean, Ian JM Crossfield, David Haegele, Eric Hébrard, Laura Kreidberg, Diana Powell, Aaron D. Schneider, Luis Welbanks, Peter Wheatley, Rafael Brahm, and Nicolas Crouzet, 15 July 2024, Nature.
DOI: 10.1038/s41586-024-07768-4
The James Webb Space Telescope (JWST) is a large, space-based observatory launched on December 25, 2021. It is a collaborative project involving NASA, European Space Agency (ESA), and the Canadian Space Agency (CSA). As a scientific successor of Hubble Space Telescope, JWST is designed to provide unprecedented resolutions and sensitivities in the infrared range of the electromagnetic spectrum. This capability allows astronomers to study every phase of cosmic history—from the first glows to the last Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System. Positioned at the second Lagrangian point (L2), JWST will explore a wide range of scientific questions, helping to uncover new insights into the structure and origins of the universe.