New data from the JWST has provided insights into the atmospheric composition of the exoplanet WASP-39B. What does this breakthrough mean for our understanding of other worlds and our search for life elsewhere in the universe?
The atmospheric composition of the hot gas giant exoplanet WASP-39 b has been revealed by NASA’s James Webb Space Telescope. This graphic shows four transmission spectra from three of Webb’s instruments operated in four instrument modes. All are plotted on a common scale extending from 0.5 to 5.5 microns. At upper left, data from NIRISS shows fingerprints of potassium (K), water (H2O), and carbon monoxide (CO). At upper right, data from NIRCam shows a prominent water signature. At lower left, data from NIRSpec indicates water, sulfur dioxide (SO2), carbon dioxide (CO2), and carbon monoxide (CO). At lower right, additional NIRSpec data reveals all of these molecules as well as sodium (Na). Image Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)
In another first for the James Webb Space Telescope (JWST), the pioneering instrument has peered deep within the atmosphere of a planet outside the solar system, painting a detailed picture of its skies.
In addition to developing a molecular and chemical portrait of this extrasolar planet — or exoplanet — the research points the way forward for our investigation of the atmospheres of other alien worlds and the hunt for the fingerprints of life that indicate we are not alone in the cosmos.
While astronomers and planetary scientists have investigated the atmosphere of exoplanets before, what sets the JWST’s findings apart is the clarity of the results it can deliver from a number of different molecules.
This sets the scene for the JWST’s forthcoming investigation of one of the most enticing planetary systems ever discovered, the TRAPPIST-1 system. This system features seven Earth-like rocky worlds, several of which exist in their star’s habitable zone, the region around a star that is neither too hot nor too cold to support the existence of liquid water.
Investigating hot-Saturns with the JWST
Before the JWST can turn its infrared eye to the terrestrial worlds of TRAPPIST-1, the telescope is probing the atmospheres of gas giants that orbit closer to their star than the habitable zone and are slightly easier to assess.
This is because the JWST examines a planet’s atmosphere in infrared as it passes in front of its host star. As this transit across the face of the star takes place, stellar light shines through the atmosphere with different chemicals and elements absorbing light at different frequencies.
This means that elements in a transiting exoplanet’s atmosphere leave tell-tale fingerprints in the light output of their star, and scientists can read these clues to determine what a planet’s atmosphere is comprised of.
The closer a planet is to its star and the larger its atmosphere, the easier this task is. This means the process is not as tricky for closer gas giants as it is for more distant orbiting rocky worlds or for close-proximity terrestrial planets which may have had their atmospheres stripped by harsh radiation from their nearby star.
NASA’s James Webb Space Telescope made the first identification of sulfur dioxide in an exoplanet’s atmosphere. Its presence can only be explained by photochemistry – chemical reactions triggered by high-energy particles of starlight. Photochemistry is essential to processes on Earth key to life like photosynthesis and the generation of our ozone layer. Image Credit: NASA, ESA, CSA, and STScI
These gas giant worlds are usually described as hot Jupiters or sometimes hot Saturns due to their similarity to the gas giant of the solar system, with the “hot” prefix indicating proximity to their host stars, something that grants these worlds' orbital periods that can be shorter than an Earth day or even a matter of hours.
These exoplanets are host to violent conditions which can result in temperatures so great that iron vaporizes on one side of the planet that permanently faces its star — described as tidally locked and analogous to how one side of the moon permanently faces Earth — and falls as molten metal rains on the planet’s cooler night-facing side.
Head in the Clouds: What did the JWST find around WASP-39B?
WASP-39B is a so-called ‘hot Saturn’ located around 700 light-years away from Earth. This exoplanet has been studied in the past by the Hubble Space Telescope and NASA’s now-retired Spitzer Space Telescope.
The JWST results, which can be found detailed in five separate papers available on the preprint server arXiv, top these previous studies by giving astronomers details of what clouds over WASP-39B may look like broken up rather than as a uniform blanket across the planet.
Examining the exoplanet with a variety of instruments, the JWST found an array of chemical signatures that would have been undetectable before the space telescope began science operations in July this year.
These results revolutionize the field. Not only can we now easily pierce through the atmospheres of giant planets around other stars and see what they are made of, but the instruments aboard JWST are performing so well that we will be able to get similar insights into smaller, rocky planets.
Merecedes Lopez-Morales, Lecturer / SAO Astrophysicist, Harvard University
Among the data, described as a “game-changer” by JWST data coordinator and University of California, Santa Cruz, astronomer Natalie Batalha, was the unprecedented detection of sulfur dioxide in the atmosphere of an exoplanet.
This molecule is created when high-energy light from an exoplanet’s parent star triggers chemical reactions in the world’s atmosphere, similar to how Earth’s ozone layer was created.
This result confirms that just as light-triggered chemical reactions — or “photochemistry” — shape our planet’s atmosphere, it also shapes the atmosphere of extreme exoplanets like hot Saturns.
The JWST also detected sodium, potassium, water vapor, and carbon dioxide in the WASP-39 b data, but conspicuous in their absence were traces of methane and hydrogen sulfide.
WASP-39 b has a surface temperature of an estimated 1,600 degrees Fahrenheit and an atmosphere almost entirely composed of hydrogen, so it's a good example of why hot Jupiters and hot Saturns aren’t primarily our focus when looking for traces of life elsewhere in the universe. Life as we understand it simply could not survive on these worlds; it probably couldn’t even get started.
That means our hunt for life turns to less extreme but no less interesting planets that have more in common with the only planet we know to host life — Earth. And with its abundance of Earth-like worlds, this makes the TRAPPIST-1 system a prime target for the JWST.
Getting the JWST ready for TRAPPIST-1
Astronomers are salivating over an in-depth study of TRAPPIST-1, a system of seven planets located around an ultra-cool dwarf star first discovered in 1999 by astronomer John Gizis and his team.
Observations from the Spitzer Space Telescope in 2017 and 2018 indicate that all of these seven worlds are rocky terrestrial planets just like Earth. This conclusion has been further verified by the fact that other investigations of these worlds have indicated atmospheres with very little hydrogen, implying that they are not gas giants.
What is even more exciting than this is that in 2017, astronomers discovered that at least three of the exoplanets in the system are in the habitable zone around the star Trappist-1. This means they could potentially host liquid water, one of the key ingredients for life.
The TRAPPIST-1 exoplanets are all much closer to their star than Earth is to the sun, with the furthest planet around the star closer to it than Mercury, the innermost planet of our solar system, is to our sun.
Usually, this would mean the planets are not suitable for life, as such proximity to a star like the sun creates violent and inhospitable conditions such as those we see on Mercury. But, as Trappist-1 is so cool and puts out only .05 percent of the amount of energy radiated by the Sun, the amount of radiation its close planets receive is vastly diminished.
This means that the furthest planet out, despite being closer to TRAPPIST-1 than Mercury is to the sun, receives so little radiation that astronomers believe it is cold enough to be covered by a shell of ice.
Space enthusiasts may not have to wait long for the JWST TRAPPIST-1 results. The planets were one of the primary targets of the space telescope’s first operating cycle.
In particular, the JWST will be zooming in on TRAPPIST-1e, which is considered to be one of the most potentially habitable exoplanets ever discovered. Astronomers believe that the exoplanet is rich in water and its skies are in perpetual twilight.
By performing well beyond expectations in its exploration of WASP-39 b, the JWST has nicely set the scene for investigation of the TRAPPIST-1 worlds and the detections of elements that, while not providing conclusive proof of life, will bring astronomers closer to this goal of finally understanding humanity’s place in the cosmos.
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References and Further Reading
Fienstein. A. D., et al, ‘Early Release Science of the exoplanet WASP-39b with JWST NIRISS,’ (2022), [https://arxiv.org/abs/2211.10493]
Tsai. S-M., et al, ‘Direct Evidence of Photochemistry in an Exoplanet Atmosphere,’ (2022), [https://arxiv.org/abs/2211.10490]
Ahrer. E-M., et al, ‘Early Release Science of the exoplanet WASP-39b with JWST NIRCam,’ (2022), [https://arxiv.org/abs/2211.10489]
Anderson. L., et al, ‘Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H,’ (2022), [https://arxiv.org/abs/2211.10488]
Rustamkulov. Z., et al, ‘Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM,’ (2022), [https://arxiv.org/abs/2211.10487]
NASA telescope reveals largest batch of Earth-size, habitable-zone planets around single star, NASA, [https://exoplanets.nasa.gov/news/1419/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around-single-star/]
Astronomers Confirm Orbital Details of TRAPPIST-1’s Least Understood Planet, [https://www.nasa.gov/feature/ames/kepler/astronomers-confirm-orbital-details-of-trappist1-least-understood-planet]
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