Using the James Webb Space Telescope, researchers have detected atmospheric differences on the exoplanet WASP-39b, revealing temperature variations and distinct cloudiness in its tidal hemispheres. The planet, similar in size to Jupiter but closer in mass to Saturn, exhibits a warmer evening side compared to its morning side, attributed to strong atmospheric circulations. Acknowledgments: NASA, ESA, CSA, Joseph Olmsted (STScI)
Near-infrared spectral analysis of the terminator confirms differences in the morning and evening atmosphere.
From the first exoplanet Discovered in 1992, thousands of planets orbiting stars outside our solar system have been confirmed using a myriad of different methods, including direct imaging, gravitational microlensing, transit measurements, and astrometry. Over the years, techniques have been developed to study these exoplanets, and astronomers have learned details about the composition of the atmospheres of these distant worlds.
NASA‘with The James Webb Space Telescope continues to advance the field and deepen our understanding of the diversity of exoplanets and their atmospheres.
Last? Webb allowed astronomers to analyze atmospheric differences between morning and evening on a tidally locked exoplanet—an incredible feat for a distant world 700 light-years from Earth like WASP-39 b.
This artist’s concept shows what exoplanet WASP-39b might look like, based on indirect transit observations from NASA’s James Webb Space Telescope and other space and ground-based telescopes. Acknowledgments: NASA, ESA, CSA, Ralf Crawford (STScI)
The Webb Space Telescope studies eternal sunrises and sunsets on a distant world
Researchers using NASA’s James Webb Space Telescope have finally confirmed what models had 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 similar weight to Saturn it orbits a star about 700 light-years from Earth and is tidally connected to its parent star. This means it has a constant day side and a constant night side – one side of the planet is always exposed to its star, while the other is always shrouded in darkness.
Using Webb’s NIRSpec (Near-Infrared Spectrograph), astronomers confirmed the temperature difference between eternal morning and eternal evening on WASP-39b, with the evening appearing warmer by about 300 Fahrenheit degree (about 200 Celsius degrees). They also found evidence for varying cloud cover, with the perpetually morning part of the planet likely cloudier than the evening.
This animation describes how Webb uses transmission spectroscopy to study the atmospheres of distant exoplanets. Acknowledgments: NASA, ESA, CSA, Leah Hustak
Advances in the Study of Exoplanet Atmospheres
Astronomers analyzed the 2- to 5-micron transmission spectrum of WASP-39b, a technique that studies an exoplanet’s terminator, the boundary that separates the planet’s day and night sides. The transmission spectrum is created by comparing the starlight filtered through the planet’s atmosphere as it moves in front of the star with the unfiltered starlight detected when the planet is next to the star. In this comparison, researchers can gain information about the temperature, composition, and other properties of the planet’s atmosphere.
“WASP-39b has become a sort of benchmark planet for studying exoplanet atmospheres with Webb,” said exoplanet researcher Néstor Espinoza Space Telescope Science Institute and lead author of the study. “It has a puffy, puffy atmosphere, so the signal coming from the starlight filtered by the planet’s atmosphere is quite strong.”
A light curve from the NIRSpec (Near-Infrared Spectrograph) of NASA’s James Webb Space Telescope shows the change in brightness from the WASP-39 star system over time as the planet transits the star. This observation was made using NIRSpec’s bright object time-series mode, which uses a grating to spread light from a single bright object (such as the host star WASP-39 b) and measure the brightness of each wavelength of light at set time intervals. Acknowledgments: NASA, ESA, CSA, Ralf Crawford (STScI)
Temperature and atmospheric composition statistics
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.
The new analysis now constructs two different spectra from the terminator region, essentially dividing the day-night boundary into two semicircles, one from the evening and the other from the morning. The data shows a significantly warmer evening at a scorching 1,450 degrees Fahrenheit (800 degrees Celsius) and a relatively cooler 1,150 degrees Fahrenheit (600 degrees Celsius) in the morning.
This transmission spectrum, captured by Webb’s NIRSpec (Near-Infrared Spectrograph) PRISM, shows the amount of near-infrared starlight blocked by the atmosphere of the hot gas giant exoplanet WASP-39b. The spectrum shows clear evidence of water and carbon dioxide and temperature fluctuations between morning and evening on the exoplanet.
A new analysis of the transmission spectrum of WASP-39b produces two different spectra from the stationary day/night boundary on the exoplanet, essentially dividing this terminator region into two semicircles, one from the evening and the other from the morning. The data shows a significantly warmer evening at a scorching 1,450 degrees Fahrenheit (800 degrees Celsius) and a relatively cooler 1,150 degrees Fahrenheit (600 degrees Celsius) in the morning.
The blue and yellow lines are the best-fit model that takes into account the data, the known properties of WASP-39b and its star (eg, size, mass, temperature), and the predicted characteristics of the atmosphere.
Acknowledgments: NASA, ESA, CSA, Ralf Crawford (STScI)
Consequences of temperature changes
“It’s really amazing that we’re able to analyze this small difference, 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 with the observatory while collecting data would severely limit our ability to make this detection. It has to be extremely precise, and Webb is exactly that.’
Extensive modeling of the acquired data also allows researchers to investigate the structure of WASP-39 b’s atmosphere, cloud cover and why it is warmer in the evening. While the team’s future work will study how cloud cover can affect temperature and vice versa, astronomers confirmed the circulation of gas around the planet as the main culprit for the temperature difference on WASP-39b.
Understanding planetary wind patterns and temperature dynamics
On a highly irradiated exoplanet like WASP-39b that orbits relatively close to its star, scientists generally expect gas to move as the planet rotates around its star: Warmer dayside gas should move to the nightside during the evening via 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 those used to predict weather on Earth, the researchers found that on WASP-39b, the prevailing winds likely move from the night side through the morning terminator, around the day, through the evening terminator, and then around night. As a result, the morning side of the terminator is cooler than the evening side. In other words, the morning side is hit by winds of air that have been cooled on the night side, while the evening side is hit by winds of air that has been warmed on the day side. Research suggests that wind speeds on WASP-39b may 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 tell the evening edge is warmer, that means it’s a little puffier.” So theoretically there is a small wave on the terminator approaching the night side of the planet.”
The team’s results were published in the journal Nature.
The researchers will now try to use the same analysis method to study the atmospheric differences of other tidally locked hot Jupiters as part of the Webb Cycle 2 General Observers Program 3969.
WASP-39 b was among the first targets analyzed by Webb when it began regular science operations in 2022. The data in this study were collected as part of the Early Release Science 1366 program, which is designed to help scientists quickly learn to use the telescope’s instruments and realize its full scientific potential.
Reference: “Inhomogeneous Terminators on 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 observatory launched on December 25, 2021. It is a joint project involving NASA, European Space Agency (ESA) and the Canadian Space Agency (CSA). As a scientific successor Hubble Space Telescope, JWST is designed to provide unprecedented resolution and sensitivity in the infrared range of the electromagnetic spectrum. This capability allows astronomers to study every phase of cosmic history—from the first glows onward Big Bangto the formation of solar systems capable of supporting life on planets like Earth, to the development of our own solar system. Located at the second Lagrange point (L2), JWST will investigate a wide range of scientific questions and help reveal new insights into the structure and origin of the universe.