Important milestone: Astronomers have analyzed the atmosphere of an exoplanet more comprehensively than ever before – and gained important insights into the chemistry and formation of such planets. Using spectral data from the James Webb Space Telescope, they also detected sulfur dioxide on the hot gas giant WASP-39b for the first time, in addition to water, carbon dioxide, carbon monoxide, sodium and potassium. The enrichment of the atmosphere with heavy elements and oxygen also provides indications of how and where this planet once formed.
The atmosphere of a planet contains a wealth of information: it reveals the nature of its gas envelope and surface, what processes are taking place on the planet, how it developed and, last but not least, whether life exists on it. While some elements and molecules such as water or sodium have been detected on exoplanets before, that was enough
The resolution of the telescopes is not enough to break down the atmospheres more precisely.
Only with the new one James Webb Space Telescope this has changed. In particular, the spectrograph NIRSpec and the NIRISS instrument are designed to create high-resolution near-infrared spectra of exoplanets and their gas envelopes. When such a planet moves in front of its star, the atoms and molecules in their gas shells leave behind characteristic absorption lines in the light spectrum. As early as August 2022, astronomers had succeeded in doing this for the first time Signature of carbon dioxide (CO2) to be detected at an exoplanet.
Triple view of an exoplanet
Now the astronomers are following suit: in five specialist articles they provide the most comprehensive inventory of an exoplanet atmosphere to date. As with the CO2 detection, the exoplanet WASP-39b, around 700 light-years away, was the focus of the observations. This hot gas giant has the mass of Saturn but is blown up to 1.3 times the size of Jupiter. As a result, its far-reaching gas envelope is particularly easy to recognize in transit spectra.
For their study, the teams of astronomers targeted gas giant WASP-39b in the summer of 2022 using three of the Webb telescope’s four instruments: NIRCam, NIRSpec, and NIRISS. These created spectra of the exoplanetary atmosphere in different sections of the near-infrared wavelength range. Together they now provide the most comprehensive characterization of an exoplanet atmosphere to date.
No methane and the first detection of sulfur dioxide
Specifically, the observations showed that the atmosphere of WASP-39b contains carbon monoxide, sodium and potassium in addition to the carbon dioxide and water already detected. The typical absorption lines of these elements and molecules were clearly visible in the spectra of almost all instruments. It also became clear that the gas envelope of WASP-39b does not appear to contain any methane – unlike some gas giants in our solar system.
Also striking: The spectra of the instruments NIRCam and NIRSpec showed a clear absorption line at a wavelength of 4.05 micrometers, which could not be assigned at first. But based on comparisons with model spectra, the astronomers concluded that this signature comes from another molecule not previously detected in exoplanets: sulfur dioxide (SO2).
Indication of photochemical processes
“While sulfur dioxide is ubiquitous on terrestrial worlds such as Earth, Venus or Jupiter’s moon Io and is mostly outgassed by volcanoes, this is fundamentally different for gas giants,” explain Shang-Min Tsai from the University of Oxford and his colleagues. In the thermochemical equilibrium of the lower atmosphere, sulfur comes to this planet primarily in reduced form as hydrogen sulfide (H2S) before.
The amount of sulfur dioxide detected in WASP-39b is therefore significantly higher than expected. This suggests that active chemical processes are taking place in the gas envelope of this hot gas giant, which shift the equilibrium and oxidize the sulfur. Tsai and his team suspect that the intense UV radiation from the planet orbiting very close to its star triggers these reactions. Accordingly, the UV radiation generates hydrogen and hydroxyl radicals (OH), which then break down the hydrogen sulfide over several steps and produce sulfur dioxide.
How did WASP-39b come about?
Also of interest: The sulfur dioxide along with the ratios of various elements to oxygen suggest that WASP-39b’s atmosphere contains 10 to 30 times more heavy elements than, say, its host star or the Sun. In addition, the proportion of volatile elements is remarkably low, while that of metals and silicates with high melting and boiling points is relatively high, as the researchers determined.
This provides crucial clues to the formation history of this exoplanet. According to models, it would be possible for large gas planets to form directly from the collapse of dense clumps of gas in a star’s protoplanetary disk. An indication of this is an increased proportion of volatile, light elements. “In contrast, the enrichment with solids, as in the accretion of planetesimals, leads to an increased proportion of semi-volatile elements,” explain Adina Feinstein from the University of Chicago and her team.
The result is that WASP-39b probably came into being in a similar way to Saturn and Jupiter in our solar system: First, a solid planetary core grew through the agglomeration of many smaller and larger chunks. Then its increasing gravity drew in large amounts of gas. The spectral data also suggest that WASP-39b grew further out in the protoplanetary disk and then later migrated inward near its star.
“Such dates are a turning point”
Together, these new data have given astronomers a more detailed look at an exoplanet’s atmosphere than ever before. “Data like this is a game changer,” says Natalia Batalha of the University of California, Santa Cruz. Because such data provide completely new insights into the chemistry, physics and development of extrasolar worlds – and could one day even lead to the discovery of extraterrestrial life.
At the same time, the results underline that the James Webb telescope has so far more than fulfilled the high expectations. “These early observations are a foretaste of all the further amazing scientific results that can be expected from the JWST,” comments Laura Kreidberg, Director of the Max Planck Institute for Astronomy in Heidelberg. (Nature, in press)
Source: Nature, Max Planck Institute for Astronomy