Researchers using the James Webb Space Telescope have discovered atmospheric differences on the exoplanet WASP-39 b, revealing temperature variations and a distinct cloud cover over the tidally locked hemispheres. The planet, which is similar in size to Jupiter but closer in mass to Saturn, shows a warmer evening side compared to the morning side, which is attributed to strong atmospheric circulations. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)
Spectral analysis of the terminator in the near-infrared confirms differences in the morning and evening atmospheres.
Since the first exoplanet 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, transit measurements, and astrometry. Over the years, techniques have been developed to study these exoplanets, with astronomers learning details about the atmospheric composition of these distant worlds.
NASA‘S James Webb Space Telescope continues to develop this field of research and deepen our understanding of the diversity of exoplanets and their atmospheres.
The latest news? Webb has allowed astronomers to analyze the atmospheric differences between morning and evening on a tidally locked exoplanet — an incredible feat for a distant world 700 light-years away, 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 and other space and ground-based telescopes. Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
Webb Space Telescope Explores Eternal Sunrises and Sunsets on Distant World
Researchers using NASA’s James Webb Space Telescope have finally confirmed what models previously predicted: an exoplanet has differences between its eternal morning and eternal evening atmospheres. WASP-39 b, a giant planet 1.3 times larger in diameter than Jupiterbut a similar mass as Saturn orbiting a star about 700 light-years away from Earth, is tidally locked to its parent star. This means that 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 eternal morning and eternal evening on WASP-39 b, with evening appearing about 300 degrees warmer. degrees Celsius degrees (about 200 Celsius degrees). They also found evidence of a different cloud cover, with the part of the planet that is always morning likely being cloudier than in 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’s terminator, the boundary that separates the planet’s day and night sides. A transmission spectrum is created by comparing starlight filtered through a planet’s atmosphere as it passes in front of its star with the unfiltered starlight detected when the planet is next to its star. By making this comparison, researchers can glean information about the temperature, composition and other properties of the planet’s atmosphere.
“WASP-39 b has become a kind of reference planet when studying the atmospheres of exoplanets with Webb,” said Néstor Espinoza, an exoplanet researcher at the Space Telescope Scientific Institute and lead author of the study. “It has a puffy, bulging atmosphere, so the signal of starlight filtered 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 in front of the star. This observation was made using NIRSpec’s bright object time-series mode, which uses a grating to spread out light from a single bright object (such as WASP-39 b’s host star) and measure the brightness of each wavelength of light at set time intervals. Credit: 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 full day/night boundary – no detailed attempt has been made to distinguish between one side and the other.
Now, the new analysis builds two different spectra of the terminator region, essentially splitting the day/night boundary into two semicircles, one for the evening and the other for the morning. The data shows the evening to be significantly hotter, a scorching 1,450 degrees Fahrenheit (800 degrees Celsius), and the morning to be a relatively cooler 1,150 degrees Fahrenheit (600 degrees Celsius).
This transmission spectrum, captured using 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, and a morning-to-evening temperature variation on the exoplanet.
New analysis of WASP-39 b’s transmission spectrum builds two distinct spectra of the exoplanet’s stationary day/night boundary, effectively splitting this terminator region into two semicircles, one for the evening and the other for the morning. Data show the evening to be significantly hotter, a scorching 1,450 degrees Fahrenheit (800 degrees Celsius), and the morning to be a comparatively cooler 1,150 degrees Fahrenheit (600 degrees Celsius).
The blue and yellow lines are a best-effort model that takes into account the data, the known properties of WASP-39 b and its star (e.g. size, mass, temperature), and assumed atmospheric features.
Source: NASA, ESA, CSA, Ralf Crawford (STScI)
Implications of temperature variations
“It’s really amazing that we can resolve this tiny difference, and that’s only possible because of Webb’s sensitivity to near-infrared wavelengths and its extremely stable photometric sensors,” Espinoza said. “Any slight movement in the instrument or with the observatory during data collection would have severely limited our ability to make this detection. It has to be extraordinarily precise, and Webb is just that.”
Extensive modeling of the acquired data also allows researchers to investigate the structure of WASP-39 b’s atmosphere, its cloud cover, and why the evening is warmer. While future work by the team will examine how cloud cover can affect temperature and vice versa, astronomers confirmed that gas circulation around the planet is the main culprit behind the temperature difference on WASP-39 b.
Understanding planetary wind patterns and temperature dynamics
On 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: hotter gas from the day side should move through the evening to the night side via a strong equatorial jet stream. Because the temperature difference is so extreme, the air pressure difference would also be significant, which in turn would create high winds.
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 night side, over the morning terminator, around the day side, over 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 hit by winds from air that has cooled on the night side, while the evening side is hit by winds from air that has warmed on the day side. Research suggests that wind speeds on WASP-39 b could reach thousands of kilometers 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 weren’t getting before,” Espinoza added. “Because we can see that the evening edge is hotter, that means it’s a little bit more convex. So theoretically there’s a little swell at the terminator as it approaches the night side of the planet.”
The team’s results have been published in the journal Nature.
The researchers will now use the same analysis method to study atmospheric differences in other tidally locked hot Jupiters, as part of the Webb Cycle 2 General Observers Program 3969.
WASP-39 b was one of the first targets Webb analyzed when it began regular science operations in 2022. The data in this study were collected under the Early Release Science Program 1366, 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, July 15, 2024, Nature.
DOI file: 10.1038/s41586-024-07768-4
The James Webb Space Telescope (JWST) is a large space-based observatory that was launched on December 25, 2021. It is a collaborative project involving NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA). As a scientific successor to the Hubble Space TelescopeJWST 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 glow after the Big Bangto 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 uncover new insights into the structure and origins of the universe.