Researchers using the James Webb Space Telescope have detected atmospheric differences on the exoplanet WASP-39 b, revealing temperature variations and distinct cloud cover in its tidally locked hemispheres. The planet, similar in size to Jupiter but closer in mass to Saturn, displays a hotter evening side compared to its morning side, due to powerful atmospheric circulations. Courtesy: NASA, ESA, CSA, Joseph Olmsted (STScI)
Near-infrared spectral analysis of the terminator confirms differences in the morning and evening atmospheres.
From the first exoplanet was discovered in 1992, thousands of planets orbiting stars outside our solar system have been confirmed by a myriad of different methods, including direct imaging, gravitational microlensing, transit measurements, and astrometry. Over the years, techniques for studying these exoplanets have evolved, with astronomers learning details about the atmospheric composition of these distant worlds.
NASA‘c James Webb Space Telescope continues to develop this area of research and deepen our understanding of the diversity of exoplanets and their atmospheres.
The last one? Webb allowed astronomers to analyze atmospheric differences between morning and evening on a tidally locked exoplanet—an amazing feat for a distant world 700 light-years from Earth like WASP-39 b.
This artist’s concept shows what the exoplanet WASP-39 b might look like based on indirect transit observations from NASA’s James Webb Space Telescope, as well as other space and ground-based telescopes. Courtesy: 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 1.3 times the diameter of Jupiterbut similar to the table Saturn orbits a star about 700 light-years from Earth, tidally locked to its parent star. This means that it has a permanent day side and a permanent 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 WASP-39 b’s eternal morning and eternal evening, with the evening appearing hotter by approximately 300 Fahrenheit degrees (about 200 Celsius degrees). They also found evidence of varying cloud cover, with the morning part of the planet forever likely cloudier than the evening.
This animation describes how Webb used transmission spectroscopy to study the atmospheres of distant exoplanets. Courtesy: NASA, ESA, CSA, Leah Hustak
Advances in Exoplanet Atmospheric Research
The astronomers analyzed the 2- to 5-micron transmission spectrum of WASP-39 b, a technique that studies the exoplanet’s terminator, the boundary that separates the planet’s day and night sides. The transmission spectrum is made by comparing 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. When they make this comparison, researchers can get information about the temperature, composition, and other properties of the planet’s atmosphere.
“WASP-39 b has become something of a benchmark planet in the study of exoplanet atmospheres with Webb,” said Nestor Espinoza, an exoplanet researcher at Space Telescope Science Institute and lead author of the study. “It has an inflated, puffy atmosphere, so the signal coming from starlight filtered through the planet’s atmosphere is quite strong.”
A light curve from the NIRSpec (Near Infrared Spectrograph) on NASA’s James Webb Space Telescope shows the change in brightness from the star system WASP-39 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 propagate light from a single bright object (such as the host star of WASP-39 b) and measures the brightness of each wavelength of light at certain time intervals. Courtesy: NASA, ESA, CSA, Ralf Crawford (STScI)
Insights into temperature and atmospheric composition
Previously published Webb 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 of the terminator region, essentially splitting the day/night boundary into two semicircles, one from the evening and the other from the morning. The data reveals that the evening is significantly hotter, a scorching 1,450 degrees Fahrenheit (800 degrees Celsius), and the morning is relatively cooler at 1,150 degrees Fahrenheit (600 degrees Celsius).
This transmission spectrum, captured using Webb PRISM’s NIRSpec (near-infrared spectrograph) in 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 change in temperature between morning and evening on the exoplanet.
A new analysis of the transmission spectrum of WASP-39 b constructs two distinct spectra of the exoplanet’s stationary day/night boundary, essentially dividing this terminator region into two semicircles, one from the evening and the other from the morning. The data reveals that the evening is significantly hotter, a scorching 1,450 degrees Fahrenheit (800 degrees Celsius), and the morning is relatively cooler at 1,150 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 inferred characteristics of the atmosphere.
Courtesy: NASA, ESA, CSA, Ralf Crawford (STScI)
Consequences of temperature variations
“It is truly stunning that we are able to analyze this small difference, and this is only possible because of Webb’s sensitivity in the near-infrared wavelengths and its extremely stable photometric sensors,” Espinosa said. “Any small movement in the instrument or with the observatory while we’re collecting data would severely limit our ability to make this detection.” It has to be extremely precise, and the Web is exactly that.
Extensive modeling of the resulting data also allowed researchers to study the structure of WASP-39 b’s atmosphere, cloud cover, and why the evening is hotter. While the team’s future work will investigate how cloud cover can affect temperature and vice versa, the astronomers confirmed the circulation of gas around the planet as the main culprit for WASP-39 b’s temperature difference.
Understanding planetary wind patterns and temperature dynamics
On a highly irradiated exoplanet like WASP-39 b that orbits relatively close to its star, researchers typically expect the gas to move as the planet orbits its star: Hotter dayside gas should move toward the nightside in the evening by a powerful equatorial jet stream. Because the temperature difference is so great, 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, the researchers determined that on WASP-39 b, the prevailing winds likely move from the night side through the morning terminator, around the day, through 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 from air that has been cooled by the night side, while the evening is buffeted by winds from air that has been warmed by the day side. Research shows that WASP-39 b’s wind speed can reach thousands of miles per hour!
Future research directions and Webb’s early scientific contributions
“This analysis is also particularly interesting because you get 3D information about the planet that you didn’t get before,” Espinosa added. “Because we can tell the evening edge is hotter, that means it’s a bit puffier. So, theoretically, there is a small bulge of the terminator approaching 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 differences of other tidally locked hot Jupiters as part of the Webb Cycle 2 3969 Common Observers Program.
WASP-39 b was among the first targets analyzed by Webb as it begins 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 the instruments of telescope and realize its full scientific potential.
Reference: “Inhomogeneous terminators of the exoplanet WASP-39 b” by Nestor Espinoza, Maria E. Steinruck, James Kirk, Ryan J. McDonald, Arjun B. Savel, Kenneth Arnold, Eliza M.-R. Kempton, Matthew M. Murphy, Lyudmila Carone, Maria Zamyatina, David A. Lewis, Dominic Samra, Sven Kiefer, Emily Rauscher, Duncan Christie, Nathan Mayne, Christian Helling, Zafar Rustamkulov, Vivienne Parmentier, Erin M. May, Arin L. Carter, Si Zhang, Mercedes Lopez-Morales, Natalie Allen, Jasmina Blecic, Leanne Dessin, Luigi Mancini, Karan Molaverdihani, Benjamin V. Rackham, Enrique Paley, Shang-Min Tsai, Eva-Maria Arrer, Jacob L. Bean, Ian J. . Crossfield, David Hegele, Eric Hebrard, Laura Kreidberg, Diana Powell, Aaron D. Schneider, Louis Welbanks, Peter Whitley, Raphael Brahm, and Nicola Kruse, 15 Jul 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 of 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 after 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 Lagrange point (L2), JWST will investigate a wide range of scientific questions, helping to reveal new insights into the structure and origin of the universe.
