Discovery promises better understanding of atmospheric composition in cooler planets
NASA’s James Webb Space Telescope (JWST) has observed the planet WASP-80 b, as it passed in front of and behind its host star, revealing spectra indicative of an atmosphere containing methane gas and water vapor.
While water vapor has been detected in over a dozen planets, until recently, methane — a molecule abundantly found in the atmospheres of Jupiter, Saturn, Uranus and Neptune within our solar system — has remained elusive in the atmospheres of transiting exoplanets when studied with space-based spectroscopy.
An artist’s rendering of the warm exoplanet WASP-80 b whose color may appear bluish to human eyes due to the lack of high-altitude clouds and the presence of atmospheric methane identified by NASA’s James Webb Space Telescope, similar to the planets Uranus and Neptune in our own solar system. Image credit: NASA.
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Arizona State University scientists Luis Welbanks and Michael Line, of the School of Earth and Space Exploration, along with Taylor Bell from the Bay Area Environmental Research Institute (BAERI), have been studying WASP-80 b and the significance of JWST discovering methane in exoplanet atmospheres. Their findingsContributing authors on this research are Everett Schlawin, University of Arizona; Jonathan J. Fortney, University of California Santa Cruz; Thomas P. Greene, NASA Ames Research Center; Kazumasa Ohno, University of California Santa Cruz; National Astronomical Observatory of Japan; Vivien Parmentier, Université Côte d’Azur; Emily Rauscher, University of Michigan; Thomas G. Beatty, University of Wisconsin-Madison; Sagnick Mukherjee, University of California Santa Cruz; Lindsey S. Wiser, Arizona State University; Martha L. Boyer, Space Telescope Science Institute; Marcia J. Rieke, University of Arizona; and John A. Stansberry, Space Telescope Science Institute. have been recently published in Nature.
With a temperature of about 825 kelvins (1,025 degrees Fahrenheit), WASP-80 b is what scientists call a “warm Jupiter” — or a planet that is similar in size and mass to the planet Jupiter but has a temperature that’s in between that of “hot Jupiters,” like the 1,450 kelvins (2,150 F) HD 209458b exoplanet, and “cold Jupiters,” like our own, which is about 125 kelvins (-235 F).
WASP-80 b goes around its red dwarf star once every three days and is situated 163 light-years away from us in the constellation Aquila. Because the planet is so close to its star and both are so far away from us, spotting the planet separately from its star would be like trying to see a single strand of hair from 9 miles away. Therefore, we can’t see the planet directly with even the most advanced telescopes like JWST, and instead researchers study the combined light from the star and planet using methods like the transit method, which has been used to discover most known exoplanets, or the eclipse method.
“This was the first time we had seen such an obvious methane spectral feature with our eyes in a transiting exoplanet spectrum, not too much unlike what could be seen in the spectra of the solar system giant planets a half a century ago,” said Welbanks, who is a NASA Hubble Fellow at ASU’s School of Earth and Space Exploration.
“Using the transit method, we observed the system when the planet moved in front of its star from our perspective, causing the starlight we see to dim a bit,” he said. “It’s kind of like when someone passes in front of a lamp and the light dims. During this time, a thin ring of the planet’s atmosphere around the planet’s day/night boundary is lit up by the star, and at certain colors of light where the molecules in the planet’s atmosphere absorb light, the atmosphere looks thicker and blocks more starlight, causing a deeper dimming compared (with) other wavelengths where the atmosphere appears transparent. This method helps scientists like us understand what the planet’s atmosphere is made of by seeing which colors of light are being blocked.”
The measured transit spectrum (top) and eclipse spectrum (bottom) of WASP-80 b from NIRCam’s slitless spectroscopy mode on NASA’s James Webb Space Telescope. In both spectra, there is clear evidence for absorption from water and methane whose contributions are indicated with colored contours. During a transit, the planet passes in front of the star, and in a transit spectrum, the presence of molecules makes the planet’s atmosphere block more light at certain colors, causing a deeper dimming at those wavelengths. During an eclipse, the planet passes behind the star, and in this eclipse spectrum, molecules absorb some of the planet’s emitted light at specific colors, leading to a smaller dip in brightness during the eclipse compared to a transit. Image Credit: BAERI/NASA/Taylor Bell.
Meanwhile, using the eclipse method, the research team observed the system as the planet passed behind its star from Earth’s perspective, causing another small dip in the total light received.
All objects emit some light, called thermal radiation, with the intensity and color of the emitted light depending on how hot the object is. Just before and after the eclipse, the planet’s hot dayside is pointed toward Earth, and by measuring the dip in light during the eclipse, researchers were able to measure the infrared light emitted by the planet.
For eclipse spectra, absorption by molecules in the planet’s atmosphere typically appears as a reduction in the planet’s emitted light at specific wavelengths. Also, since the planet is much smaller and colder than its host star, the depth of an eclipse is much smaller than the depth of a transit.
The initial observations the team made need to be transformed into something we call a spectrum; this is essentially a measurement showing how much light is either blocked or emitted by the planet’s atmosphere at different colors (or wavelengths) of light. Many different tools exist to transform raw observations into useful spectra, and sometimes different tools give different results, so they used two different approaches to make sure their findings were robust to different assumptions.
Next, the team interpreted this spectrum using two kinds of models to simulate what the atmosphere of a planet under such extreme conditions would look like. The first type of model is entirely flexible, trying millions of combinations of methane and water abundances and temperatures to find the combination that best matched their data.
The second type, called “self-consistent models,” also explores millions of combinations but uses existing knowledge of physics and chemistry to determine the levels of methane and water that could be expected. Both model types reached the same conclusion: a definitive detection of methane.
“Before JWST, methane had remained largely undetected, despite expectations that it could have been detected with Hubble Space Telescope in planets where it should have been abundant. These lack of detections generated a flurry of ideas ranging from the intrinsic depletion of carbon to its photochemical destruction to the mixing of deep methane depleted gas,” Line said.
To validate their findings, the researchers used robust statistical methods to evaluate the probability of their detection being random noise. In the astronomy field, astronomers regard 5-sigma detections as the “gold standard,” meaning the odds of a detection being caused by random noise are one in 1.7 million.
Meanwhile, Welbanks and Bell detected methane at 6.1-sigma in both the transit and eclipse spectra, which sets the odds of a spurious detection in each observation at one in 942 million, surpassing the 5-sigma “gold standard” and reinforcing their confidence in both detections.
With such a confident detection, not only did the researchers find a very elusive molecule, but they will now start exploring what this chemical composition tells us about the planet’s birth, growth and evolution.
“Methane is important as it is the main carbon reservoir in cooler (less than 1,000 K) giant planets, much like our own solar system giants, Jupiter and Saturn. If we want to understand atmospheric composition and chemistry in these cooler regimes, detecting and constraining the abundance of methane is absolutely essential,” says Line, associate professor at the School of Earth and Space Exploration.
For example, by measuring the amount of methane and water in the planet, researchers can infer the ratio of carbon atoms to oxygen atoms. This ratio is expected to change depending on where and when planets form in their system. Thus, examining this carbon-to-oxygen ratio can offer clues as to whether the planet formed close to its star or further away before gradually moving inward.
Another thing that has Welbanks, Bell and the team excited about this discovery is the opportunity to finally compare planets outside of our solar system to those in it.
NASA has a history of sending space probes to the gas giants in our solar system to measure the amount of methane and other molecules in their atmospheres. Now, by having a measurement of the same gas in an exoplanet, researchers can start to perform an “apples-to-apples” comparison and see if the expectations from the solar system match what they see outside of it.
Finally, as scientists look toward future discoveries with JWST, this result reveals that they are at the brink of more exciting findings.
Additional MIRI and NIRCam observations of WASP-80 b with JWST will allow scientists to probe the properties of the atmosphere at different wavelengths of light. The findings from this international group of researchers led them to think that they will be able to observe other carbon-rich molecules, such as carbon monoxide and carbon dioxide, enabling them to paint a more comprehensive picture about the conditions in this planet’s atmosphere.
Additionally, as more methane and other gases in exoplanets are observed, scientists like Welbanks, Line and Bell will continue to expand their knowledge about how chemistry and physics work under conditions unlike what we have on Earth, and maybe sometime soon, in other planets that remind us of what we have here at home.
“Not only is methane an important gas in tracing atmospheric composition and chemistry in giant planets, it is also hypothesized to be, in combination with oxygen, a possible signature of biology. One of the key goals of the Habitable Worlds Observatory, the next NASA flagship mission after JWST and Roman, is to look for gases like oxygen and methane in Earth-like planets around sun-like stars,” Welbanks said.
“Understanding the physical processes that dictate its presence over a broad range of planetary conditions will be critical to providing context to these future observations.”
This work is supported by funding from the NASA Next Generation Space Telescope Flight Investigations program, NASA: Goddard Space Flight Center, The Space Telescope Science Institute, The Association of Universities for Research in Astronomy Inc, University of Arizona (UA), STScI grant and JSPS Overseas Research Fellowship.
This press release was written by Thaddeus Cesari, NASA Goddard Space Flight Center, with contributions from Kim Baptista from ASU’s School of Earth and Space Exploration.
ASU’s Competitive Statecraft Initiative will produce Inter Populum, a new academic journal dedicated to issues surrounding irregular warfare
Arizona State University has long had the reputation and designation as a military-friendly school. From the celebrated McCain Institute to the Pat Tillman Veterans Center to the Global Security Initiative, the university’s connection to the United States military runs deep.
Ryan Shaw, who served as an officer in the U.S. Army and earned his commission at West Point, is working to add more components to ASU’s mission. In addition to his direction and leadership over ASU’s Competitive Statecraft Initiative, he recently helped launch Inter Populum: The Journal of Irregular Warfare and Special Operations, a new peer-reviewed academic journal that will be published twice a year.
According to Shaw, a professor of practice in history and strategy, managing director of strategic initiatives and senior advisor to ASU President Michael Crow, Inter Populum will explore everything from lessons learned through historical case studies to current best practices to the nature of future conflict.
Shaw spoke to ASU News about the academic journal, irregular warfare and what it means for the world going forward.
Editor’s note: Answers have been edited for length and clarity.
Ryan Shaw
Question: Most people’s first question would be a pretty simple one: What is irregular warfare?
Answer: Actually, that’s not a simple question at all. The U.S. military just approved a new doctrinal definition for irregular warfare (IW).
To my mind, there are three important factors that any good definition should account for. First is the “who” — if it’s only professional soldiers wearing uniforms with flags on their soldiers, it’s probably not IW. IW usually involves insurgents, criminal gangs, proxies or others that can be called “non-state actors.”
Second is the “what” — if it’s force-on-force battles like what you think of with the two world wars, that’s conventional warfare. IW involves less direct forms of combat, like guerilla tactics, terrorism, subversion, sabotage and resistance.
But the third factor is, I think, the most important one for defining irregular warfare: the “why.” Conventional war is usually a fight to destroy another military or to seize a piece of terrain. But the “target” of irregular warfare is the people themselves — more specifically, their loyalty and their perceptions of the legitimacy of their government.
Of course, it’s much harder to measure success when the target is psychological rather than physical. That’s why IW is more complex than conventional war — not harder, necessarily, but definitely more complex. And that’s why the journal is named “Inter Populum.” That’s Latin for “among the people.” IW is sometimes referred to as war among the people — it’s the human domain that matters most.
Q: Are there elements of irregular warfare going on right now with Ukraine-Russia, China-Taiwan and Hamas?
A: For sure. Hamas is a terrorist organization employing irregular tactics. And they, like Hezbollah, are Iranian proxies, so from an IW perspective, that conflict is much bigger than just Gaza. Many elements of the war in Ukraine are conventional, but the Wagner Group is a private company that has been stirring up trouble in Ukraine on behalf of the Russian state for a decade now. And both sides there are working to build resistance movements in the territory they consider their own. Taiwan is doing the same as they feel increasingly threatened by the Peoples Republic of China. And while China knows it would be folly to provoke a conventional war with the U.S., either directly or by bullying our allies, they consistently use irregular means to chip away at our influence and advantages without sparking a major war. Irregular warfare is alive and well.
Q: Why did you take on this additional role?
A: Because the world needs this journal, and because ASU is the ideal home for it. That sounds grandiose, but I believe it’s true. There is a real need for more rigorous scholarship around these issues.
The U.S. spent two decades tangled up in counterinsurgencies in Iraq and Afghanistan. Those wars cost us trillions of dollars and thousands of American lives — many of them people I knew and served with. That was IW — and to say the least, it didn’t work out like we’d hoped. Now, all the attention is on preparing for a big, conventional war with China or Russia and everybody thinks that means we can forget about IW. But IW is never going away. As long as we can deter our adversaries from taking us on in conventional or nuclear war, they will try to chip away at our strategic advantages through IW. And the terrorism problem isn’t going anywhere, either. Getting these things right doesn’t cost big money like battleships and fighter jets — the real investment required is intellectual. That’s why a journal like this is important.
And I do think ASU is the right home. We’re already offering graduate programs specifically in IW — few universities do that. We’re ranked No. 1 in the U.S. for transdisciplinary research, and IW is inherently transdisciplinary. We’ve got excellent faculty with the right expertise and vast experience, and we have deep connections into the military and special ops communities. Besides that, our publishing team is exceptional, and they’ve been very supportive.
Q: Who is Inter Populum’s intended audience?
A: We aim to make it useful to both scholars and practitioners. That is, people who study these things in universities and at think tanks, and also those who are “doing” irregular warfare, whether in the military, defense civilians in the intelligence community and law enforcement, and even policymakers who write the laws and provide the funding to make IW work.
And not just in the U.S. One of ASU’s great assets in this space is Security & Defense PLuS, our partnership with King’s College London and the University of New South Wales in Australia. … Those two universities have excellent programs and deep ties to their defense ministries. The U.S. doesn’t go to war without allies, and ASU doesn’t start a venture like this without our partners.
Q: What do you hope to achieve with this journal?
A: T.E. Lawrence wrote that “guerrilla war is far more intellectual than a bayonet charge.” That was his pithy way of saying what I said before, that IW is just tremendously complex. So it requires rigorous thought and open debate. But while there are great professional journals for nearly every branch and function of the military, there has not been one dedicated specifically to the study of irregular war — until now. … Scholarly journals are vital for getting new and diverse ideas out into the wild. That’s our ambition for Inter Populum — to be the central medium for discussion, debate and the exchange of ideas among scholars and practitioners. And ultimately, of course, all this will result in better approaches to security and defense, and a freer, more peaceful world.
Top photo courtesy Pixabay