James Webb Space Telescope observes methane in exoplanet's atmosphere

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. 

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.

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.

"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."