Researchers using the James Webb Space Telescope have discovered atmospheric differences on the exoplanet WASP-39 b, noting temperature variations and varying cloud cover in its tidally locked hemispheres. The planet, which is similar in size to Jupiter but closer in mass to Saturn, has a hotter evening side compared to its morning side, due to strong atmospheric circulations. Image credit: NASA, ESA, CSA, Joseph Olmsted (STScI)
A near-infrared spectral analysis of the terminator line confirms differences in the morning and evening atmosphere.
Since the first Exoplanet was discovered in 1992, thousands of planets orbiting stars outside our solar system have been confirmed by a variety of different methods, including direct imaging, gravitational microlensing, measuring transits, and astrometry. Over the years, techniques for studying these exoplanets have evolved, and astronomers have learned details about the atmospheric composition of these far-flung worlds.
NASA‘S James Webb Space Telescope advances this field of research and deepens our understanding of the diversity of exoplanets and their atmospheres.
And the latest? Webb has enabled astronomers to analyze the atmospheric differences between morning and evening on a tidally locked exoplanet—an incredible feat for a world 700 light-years from Earth like WASP-39 b.
This artist’s impression shows what the exoplanet WASP-39 b might look like, based on indirect transit observations from NASA’s James Webb Space Telescope and other space and ground-based telescopes. Image credit: NASA, ESA, CSA, Ralf Crawford (STScI)
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 have already predicted: an exoplanet has differences between its atmosphere of eternal morning and that of eternal evening. WASP-39 b, a giant planet with a diameter 1.3 times the size of Jupiterbut similar mass as Saturn orbits a star about 700 light-years from Earth and is tidally locked to its parent star, meaning 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 a temperature difference between the eternal morning and the eternal evening on WASP-39 b, with the evening appearing about 300 °C hotter. Fahrenheit degrees (approx. 200 Celsius degrees). They also found evidence of varying cloud cover, with the morning part of the planet likely cloudier than the evening part.
This animation describes how Webb uses transmission spectroscopy to study the atmospheres of distant exoplanets. Image credit: NASA, ESA, CSA, Leah Hustak
Progress in research into the atmospheres of exoplanets
Astronomers analyzed the 2- to 5-micron transmission spectrum of WASP-39 b, a technique that examines the exoplanet’s terminator, the boundary that separates the day and night sides of the planet. A transmission spectrum is created by comparing the starlight filtered by a planet’s atmosphere as it passes in front of the star with the unfiltered starlight seen when the planet is next to the star. By making this comparison, researchers can obtain information about the temperature, composition and other properties of the planet’s atmosphere.
“WASP-39 b, along with Webb, has become a kind of reference planet in the study of the atmospheres of exoplanets,” said Néstor Espinoza, an exoplanet researcher at Institute for Space Telescope Science and lead author of the study. “It has a puffy, puffy atmosphere, so the signal from starlight filtering through the planet’s atmosphere is quite strong.”
A light curve from NASA’s James Webb Space Telescope’s NIRSpec (Near-Infrared Spectrograph) shows the change in brightness of the WASP-39 star system over time as the planet passed the star. This observation was made using NIRSpec’s bright object time series mode, which uses a grid to spread out the light from a single bright object (such as WASP-39 b’s parent star) and measure the brightness of each wavelength of light at set time intervals. Image credit: NASA, ESA, CSA, Ralf Crawford (STScI)
Insights into temperature and atmospheric composition
Previously published Webb spectra of WASP-39b’s atmosphere, which showed 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 creates two different spectra from the terminator region, essentially splitting the day/night boundary into two semicircles, one for the evening and one for the morning. The data shows that it was significantly hotter in the evening, a sweltering 800 degrees Celsius, and a comparatively cooler 600 degrees Celsius in the morning.
This transmission spectrum, taken with Webb’s NIRSpec (Near-Infrared Spectrograph) PRISM bright object time series mode, shows the amounts of near-infrared starlight blocked by the atmosphere of the hot gas giant exoplanet WASP-39 b. The spectrum shows clear evidence of water and carbon dioxide, as well as a temperature difference between morning and evening on the exoplanet.
A new analysis of WASP-39 b’s transmission spectrum creates two different spectra from the stationary day/night boundary on the exoplanet, essentially splitting this terminator region into two semicircles, one for the evening and one for the morning. The data show that it was significantly hotter in the evening, a sweltering 800 degrees Celsius, and a comparatively cooler 600 degrees Celsius in the morning.
The blue and yellow lines represent an optimal model that takes into account the data, the known properties of WASP-39 b and its star (e.g. size, mass, temperature), and assumed properties of the atmosphere.
Photo credits: NASA, ESA, CSA, Ralf Crawford (STScI)
Effects of temperature fluctuations
“It’s really amazing that we were able to evaluate this small difference. This is only possible thanks to Webb’s sensitivity in the near-infrared wavelength range and its extremely stable photometric sensors,” said Espinoza. “Any movement of the instrument or the observatory during data collection would have severely limited our ability to make this discovery. It has to be extraordinarily precise, and that’s exactly what Webb is.”
Comprehensive modeling of the data obtained also allows researchers to study the structure of WASP-39 b’s atmosphere, cloud cover, and the reasons for the higher temperatures in the evening. While the team will investigate how cloud cover affects temperature and vice versa in future work, astronomers confirmed that gas circulation around the planet is the main cause of the temperature difference on WASP-39 b.
Understanding planetary wind patterns and temperature dynamics
On a highly irradiated exoplanet like WASP-39 b, which orbits relatively close to its star, researchers generally assume that the gas moves as the planet rotates around its star: hot gas from the day side should reach the night side over the course of the evening 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, three-dimensional models similar to those used to predict weather patterns on Earth, researchers found that on WASP-39 b, the prevailing winds likely move from the nightside, across the morning terminator line, around the dayside, across the evening terminator line, and then around the nightside. As a result, the morning side of the terminator line is cooler than the evening side. In other words, the morning side is hit by winds that were cooled on the nightside, while the evening side is hit by winds that were warmed on the dayside. Research suggests that wind speeds on WASP-39 b can reach thousands of miles per hour!
Future research directions and Webb’s early scientific contributions
“This analysis is also particularly interesting because it gives us 3D information about the planet that we didn’t have before,” Espinoza added. “We can see that the evening edge is hotter, that is, it’s a little more bloated. So, in theory, there’s a little swell on the terminator line that’s approaching the night side of the planet.”
The team’s results were published in the journal Nature.
The researchers will now attempt to use the same analysis method to study atmospheric differences on other tidally locked hot Jupiters as part of the General Observers Program 3969 of Webb Cycle 2.
WASP-39 b was among the first targets analyzed by Webb when it began regular science operations in 2022. Data from this study were collected as part of the Early Release Science 1366 program, which is designed to help scientists quickly learn how 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 J.M. 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 joint project of NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA). As the scientific successor to the Hubble Space TelescopeThe JWST is designed to provide unprecedented resolution and sensitivity in the infrared part of the electromagnetic spectrum. This capability will allow astronomers to study every phase of cosmic history – from the first glow after the Big Bangfrom the formation of solar systems capable of supporting life on planets like Earth to the evolution of our own solar system. Located at the second Lagrange point (L2), JWST will investigate a wide range of scientific questions and help gain new insights into the structure and origin of the universe.