Exoplanets are worlds orbiting distant stars. Astronomer Néstor Espinoza of Space Telescope Science Institute spoke with EarthSky’s Deborah Byrd on Monday, May 20, 2024, about this diversity of worlds beyond our sun and planets. He talked about the nearby, fascinating TRAPPIST-1 system, located some 40 light-years away. He talked about Proxima Centauri b, the closest exoplanet at 4 light-years. And he touched on some current and future exoplanet missions.
- Since the first exoplanet was discovered in 1992, thousands of planets orbiting stars outside of our solar system have been confirmed through a myriad of different methods, including direct imaging, gravitational microlensing, measuring transits, and astrometry. Over the years, techniques have evolved to study these exoplanets, with astronomers learning details about the atmospheric compositions of these far-off worlds.
- NASA’s James Webb Space Telescope is continuing to advance this field of study and deepen our understanding about the diversity of exoplanets and their atmospheres.
- The latest? Webb has allowed astronomers to parse out the atmospheric differences between the morning and evening on a tidally locked exoplanet – an incredible achievement for a distant world 700 light-years away from Earth like WASP-39b.
Webb Space Telescope released this story on July 15, 2024.
Webb finds differences between eternal sunrises and sunsets
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 atmosphere. WASP-39b – a giant planet also known officially as Bocaprins, with a diameter 1.3 times greater than Jupiter, but similar mass to Saturn, orbiting a star about 700 light-years away from Earth – is tidally locked to its parent star. This means it has a constant dayside and a constant nightside – 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 eternal evening on WASP-39 b, with the evening appearing hotter by roughly 300 Fahrenheit degrees (about 200 Celsius degrees). They also found evidence for different cloud cover, with the forever morning portion of the planet being likely cloudier than the evening.
Probing WASP-39b’s ‘puffy’ atmosphere with starlight
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 dayside and nightside. A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves in front of the star, to the unfiltered starlight detected when the planet is beside the star. When making that comparison, researchers can get information about the temperature, composition, and other properties of the planet’s atmosphere.
Néstor Espinoza, an exoplanet researcher at the Space Telescope Science Institute (STScI) and lead author on the study, explained why this exoplanet is of particular interest:
WASP-39b has become a sort of benchmark planet in studying the atmosphere of exoplanets with Webb. It has an inflated, puffy atmosphere, so the signal coming from starlight filtered through the planet’s atmosphere is quite strong.
Previous observations ID’d gasses but lacked details of eternal sunrises, sunsets
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 differentiate between one side and the other.
Now, the new analysis builds 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. Data reveals the evening as significantly hotter, a searing 1,450 degrees Fahrenheit (800 degrees Celsius), and the morning a relatively cooler 1,150 degrees Fahrenheit (600 degrees Celsius).
Espinoza said the fine quality of Webb and its instruments made the observations possible:
It’s really stunning that we are able to parse this small difference out, and it’s only possible due Webb’s sensitivity across near-infrared wavelengths and its extremely stable photometric sensors. Any tiny movement in the instrument or with the observatory while collecting data would have severely limited our ability to make this detection. It must be extraordinarily precise, and Webb is just that.
3D model lets astronomers study windy exoplanetary atmosphere’s structure
Extensive modeling of the data obtained also allows researchers to investigate the structure of WASP-39b’s atmosphere, the cloud cover, and why the evening is hotter. While future work by the team will study how the cloud cover may affect temperature, and vice versa, astronomers confirmed gas circulation around the planet as the main culprit of the temperature difference on WASP-39b.
On a highly irradiated exoplanet like WASP-39b that orbits relatively close to its star, researchers generally expect the gas to be moving as the planet rotates around its star: Hotter gas from the dayside should move through the evening to the nightside 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 the ones used to predict weather patterns on Earth, researchers found that on WASP-39b the prevailing winds are likely moving from the night side across the morning terminator, around the dayside, across the evening terminator and then around the nightside. As a result, the morning side of the terminator is cooler than the evening side. In other words, the morning side gets slammed with winds of air that have been cooled on the nightside, while the evening is hit by winds of air heated on the dayside. Research suggests the wind speeds on WASP-39b can reach thousands of miles an hour!
This analysis is also particularly interesting because you’re getting 3D information on the planet that you weren’t getting before. Because we can tell that the evening edge is hotter, that means it’s a little puffier. So, theoretically, there is a small swell at the terminator approaching the nightside of the planet.
Study eternal sunrises, sunsets leads to investigation of other ‘hot Jupiters’
The team’s results have been published in Nature.
The researchers will now look to use the same method of analysis to study atmospheric differences of other tidally locked hot Jupiters, as part of Webb Cycle 2 General Observers Program 3969.
WASP-39b was among the first targets analyzed by Webb as it began regular science operations in 2022. The data in this study was collected under Early Release Science program 1366, designed to help scientists quickly learn how to use the telescope’s instruments and realize its full science potential.
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
Bottom line: The Webb Space Telescope probed eternal sunrises and sunsets on exoplanet WASP-39b. They found differences in the atmospheres.
Read more: What are Hubble and Webb looking at right now?
Via Webb Space Telescope