Although the impressive images of the cosmos captured by the James Webb Space Telescope (JWST) have become famous in recent months, this great observatory of NASA and ESA has just obtained another first: the chemical fingerprint of an exoplanet’s atmosphere, where sulfur dioxide (SO2) is first found. Although the exoplanet is not believed to be habitable, this research does represent a new advance in the search for potential traces of life on planets that do meet the conditions for life.
The telescope’s highly sensitive instrument suite focused on the atmosphere of a “hot Saturn,” a planet as massive as Saturn that orbits a star about 700 light-years away, known as WASP-39b.
While Webb and other space telescopes, including Hubble and Spitzer, have previously revealed compounds isolated from this hot planet’s atmosphere, the new readings provide a full menu of atoms, molecules, and even signs of active chemistry and the presence of clouds. The new data also hints at what these clouds would look like up close: split up rather than a single, uniform layer over the planet.
“We are observing the exoplanet with multiple instruments that, together, provide a wide swath of the infrared spectrum and a panoply of chemical fingerprints inaccessible until JWST,” says Natalie Batalha, an astronomer at the University of California, Santa Cruz, who contributed and helped coordinate the new research.
The set of discoveries is detailed in a set of five new scientific articles, which will be published in a high impact magazine and are now available. Among the unprecedented revelations is the first detection in an exoplanet’s atmosphere of sulfur dioxide, a molecule produced from chemical reactions triggered by high-energy light from the planet’s parent star. On Earth, the protective ozone layer in the upper atmosphere is created in a similar way.
A peculiar signal and reaction
“In the first data we saw a very peculiar signal in the atmosphere of this planet whose origin we cannot understand. Now, with this analysis, we have been able to infer that it was the trace left by the sulfur dioxide produced by the high radiation that the planet receives from its star in the upper layers of the atmosphere”, indicates Jorge LilloBox, postdoctoral researcher at the Center of Astrobiology (CAB, CSIC-INTA) who has participated in the study.
According to Shang-Min Tsai, a researcher at the University of Oxford in the UK and lead author of the paper explaining the origin of sulfur dioxide in WASP-39 b’s atmosphere, “This is the first time we have seen concrete evidence of photochemical (chemical reactions initiated by energetic starlight) on exoplanets”. At an estimated temperature of 1,600 degrees Fahrenheit (900 degrees Celsius) and an atmosphere made mostly of hydrogen, WASP-39 b is not thought to be habitable.
The planet’s proximity to its host star, eight times closer than Mercury is to our Sun, also makes it a laboratory for studying the effects of radiation from host stars on exoplanets. A better understanding of the star-planet connection should lead to a deeper understanding of how these processes create the diversity of planets observed in the galaxy.
In addition to sodium, potassium, and water, the Webb telescope also saw carbon dioxide at high resolution, providing twice as much data as reported in its previous observations.
Meanwhile, carbon monoxide was detected, but the obvious signatures of methane and hydrogen sulfide were absent from Webb’s data. If present, these molecules are found at very low levels, a significant finding for scientists conducting inventories of exoplanet chemistry to better understand the formation and development of these distant worlds.
Three instruments: NIRSpec, NIRCam and NIRISS
Webb observes the universe in infrared light, at the red end of the light spectrum beyond what human eyes can see; that allows the telescope to pick up chemical signatures that cannot be detected in visible light. In total, three instruments have been used to characterize in depth the atmosphere of this planet in the infrared range: NIRSpec, NIRCam and NIRISS.
“You can really narrow down the properties of these planets by having such a broad spectrum,” says Adina Feinstein, a graduate student at the University of Chicago and first author of the paper focusing on spectrum observations using NIRISS, “so you start to get the full picture [de las atmósferas] that you couldn’t get before.”
To view the light from WASP-39 b, Webb tracked the planet’s path in front of its star, allowing some of the star’s light to filter through the planet’s atmosphere. Different types of chemicals in the atmosphere absorb different colors of the starlight spectrum, so the missing colors tell astronomers which molecules are present.
Having such a comprehensive list of chemical ingredients in an exoplanet’s atmosphere also gives scientists an idea of the abundance of different elements in relation to one another, such as carbon-to-oxygen or potassium-to-oxygen ratios.
Clues about the formation of the exoplanet
That, in turn, provides insight into how this planet, and perhaps others, formed from the disk of gas and dust that surrounded the parent star in its younger years. The chemical inventory of WASP-39 b suggests a history of crushing and merging of smaller bodies called planetesimals to create an eventual giant planet.
The new findings provide a good idea of the ability of Webb’s instruments to perform the wide range of investigations of exoplanets (planets around other stars) that the scientific community expects. That includes probing the atmospheres of smaller, rocky planets like those in the TRAPPIST-1 system.
“These results are a confirmation of the ability of the JWST instruments to explore the atmospheres of all types of exoplanets, including small and rocky worlds”, emphasizes Enric Pallé, a researcher at the Instituto de Astrofísica de Canarias (IAC) who has participated in the study.
For his part, David Barrado, a CAB researcher, points out that in the future others in orbit, such as PLATO, or on Earth, such as the ELT that is being built in Chile, will be added to the Webb telescope, projects in which the Astrobiology Center.