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James Webb illuminates the grey areas of astrophysics
The Universe still holds mysteries that astrophysicists and cosmologists are striving to solve with the help of ever more powerful telescopes. Since the commissioning of the James Webb Space Telescope in July 2022, this community of researchers has possessed an analytical tool of unmatched precision. Originally designed to study the oldest galaxies formed a few hundred million years after the Big Bang,1 the James Webb Space Telescope (JWST) is capable of exploring a much broader range of celestial bodies. With its immense mirror measuring 6.6 meters in diameter, the instrument developed by the U.S. National Aeronautics and Space Administration (NASA), in collaboration with the European (ESA) and Canadian (CSA) Space Agencies, possesses sensitivity that is one hundred time greater than its predecessor, the Spitzer Space Telescope.
Like the Spitzer, the JWST detects the infrared radiation emitted by the astronomical objects spread throughout the Universe: distant galaxies, supermassive black holes, planetary system in formation, and exoplanets are among its preferred targets. By scanning the early Universe, the telescope successfully identified an entire series of galaxies that are among the oldest ever observed, with the oldest, named JADE-z14-0, dating back 13.57 billion years. “While the discovery of galaxies from such a distant time was expected, we were surprised to observe that galaxies that are not massive but are abnormally bright–with a very high rate of new star formation–were ten times greater than predicted by our models,” notes David Elbaz, a Senior Researcher at the French Alternative Energies and Atomic Energy Commission (CEA), and a specialist on the formation and evolution of galaxies.
The role of cosmic filaments expands
The JWST’s observations focusing on the distant Universe also revealed the existence of a large number of supermassive black holes, whose mass equals a few million times that of the Sun. At first glance, these discoveries appear to go against the Standard Model of Cosmology, which predicts neither the formation of very bright galaxies nor that of giant black holes. As disconcerting as they may seem, these observations could nevertheless shine a light on the involvement of cosmic filaments. These thin strands of gas measuring millions of lightyears in length connect galaxies to one another, and could therefore play an essential role in the formation of these celestial objects: “At the dawn of the Universe, the purveyors of matter that are cosmic filaments most probably supplied galaxies with much greater effectiveness than we imagine today,” Elbaz emphasizes. “If this theory is confirmed in the future by very high-resolution observation instruments, such as the BlueMuse spectrograph that will soon equip the Very Large Telescope, it would help explain the almost spontaneous formation of supermassive black holes and galaxies almost as imposing as our Milky Way in the space of a few hundred million years.”
Among the many other missions assigned to the JWST is the study of planetary system in the process of formation, with a view to tracing the origins of the chemical complexity that led to the birth of our own Solar System. In keeping with this approach, the research of Olivier Berné, an astrophysicist at the Research Institute of Astrophysics and Planetary Science,2 focuses on nebulas, clouds of interstellar gas and dust where stars are born.
The prediction of a molecule confirmed
By aiming the telescope at the Orion Nebula located 1,350 lightyears from Earth, the international collaboration3 co-directed by the astrophysicist made a stunning discovery with the help, among others, of specialists in spectroscopy from the Molecular Science Institute of Orsay.4
The MIRI5 spectrometer successfully detected the infrared signature for methenium in the disk of gas and dust surrounding two of the nebula’s young stars. The identification of this chemical species of the formula CH₃⁺ represents a major scientific discovery, recognised by a publication in the journal Nature.6
Because it contains carbon, methenium could be behind the extraterrestrial organic chemistry that notably led to the appearance of life on Earth. “While the community of astrochemists predicted the existence of this molecule in the Universe in the 1970s, we did not expect to detect it in such environments conducive to the formation of future planets, which is to say protoplanetary disks,” relates Berné.
The presence of methenium within these gestating planetary systems actually stemmed from a very particular kind of chemistry, itself resulting from ultraviolet irradiation emitted by one or more massive and very bright stars located in the immediate vicinity. “Depending on the mass of the star located at the centre of the planetary system, we showed that this exogenous ultraviolet irradiation can foster the formation of planets as well as their destruction by dissipating the matter they are made of7.”
The origins behind the diversity of planetary systems
Between now and January 2025, Berné will have the opportunity to pursue his explorations on a much larger sample of planetary embryos. Thanks to the NIRSpec instrument,8 the group of scientists that he leads will gain simultaneous access to the spectrum for fifty protoplanetary disks surrounding low-mass young stars in the Orion Nebula. The data collected should make it possible to determine the chemical composition of various disks, as well as some of their physical properties such as temperature and matter density within each structure. This comes on top of the data obtained for over 250 protoplanetary disks already studied by the JWST in connection with other research programmes throughout the world. “These large samples should help us understand how the physicochemical processes shared by all these disks lead to the diversity of exoplanetary systems that we observe in the Universe,” adds Benoît Tabone, a CNRS researcher at the Space Astrophysics Institute,9 whose research also focuses on protoplanetary disks.
With the infrared spectrums detected by the MIRI and NIRSpec instruments, the scientist closely studied the inner disk, located less than a dozen astronomical units from their star (an astronomical unit equals the Earth-Sun distance, or 150 million kilometres). His goal is to characterise the chemical composition for this region, where most exoplanets form. To this end, the scientist established partnerships with numerous research groups based in Europe and the United States. The research conducted as part of the Minds European collaboration10 (MIRI mid-Infrared Disk Survey), in which Tabone took part, demonstrated that there is great diversity among inner disks. By analysing the infrared light coming from the gas phase of a disk of matter gravitating around a small star formed 3 million years ago, the consortium also revealed the presence of a very large quantity of acetylene (C2H2). Aside from this simple and highly reactive hydrocarbon molecule, the researchers discovered benzene (C6H6) and diacetylene (C4H2), two hydrocarbons that were previously undiscovered in protoplanetary disks.11
A cocktail of carbon molecules
One year later, the same consortium successfully identified a mix containing a dozen carbon molecules near a star with similar characteristics.12 This abundance of carbon in the gaseous portion of the disk could result from the star’s intense activity. By destroying dust grains rich in carbon, the celestial body could spark the transfer of this chemical element in the gaseous part of the disk. “Like the Earth, the rocky planets formed from dust grains belonging to this category of disk could have a mineral composition low in carbon,” Tabone extrapolates. Successfully determining the link between the composition of the disk and the characteristics of the future planets it will give rise to is a challenge that the researcher now aims to explore by using the JWST’s exceptional resolution in near- and mid-infrared.
The telescope’s unrivalled sensitivity also proved its worth in a fledgling domain, studying the atmospheric composition of exoplanets. Of the 5,700 known planets located outside the Solar System, an international team took close interest in WASP-39 b, a Hot Jupiter gas giant located 600 lightyears from our Solar System. Thanks to the NIRSpec spectrograph, the scientists were able, for the first time ever, to detect the infrared signature of carbon dioxide in the atmosphere of an exoplanet. Very recently, the NIRISS instrument13 revealed that the atmosphere of exoplanet GJ 9827 d, located 100 lightyears from Earth, consists almost entirely of water vapour. These two remarkable discoveries offer a glimpse of the possibility of detecting and quantifying these same molecules in the thinner atmospheres of potentially habitable rocky planets.
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- 1. • Event comparable to a giant explosion that occurred 13.8 billion years ago, and that was behind the expansion of the Universe.
- 2. CNRS/Cnes/Université Toulouse Paul Sabatier.
- 3. This consortium, named PDRs4All, includes over 170 researchers from across the globe.
- 4. CNRS/Université Paris-Saclay.
- 5. Acronym for Mid-infrared instrument. The MIRI instrument consists of a camera and a spectrometer both operating in mid-infrared.
- 6. “Formation of the methyl cation by photochemistry in a protoplanetary disk,” O. Berné et al., Nature, 26 June 2023: https://doi.org/10.1038/s41586-023-06307-x
- 7. Intervening in the field of near infrared, this spectrograph can simultaneously observe one hundred light sources emitted by galaxies, planets, and protoplanetary disks.
- 8. For “Near-Infrared Spectrograph.” See “A far-ultraviolet–driven photoevaporation flow observed in a protoplanetary disk,” O. Berné et al., Science, 29 February 2024: https://www.science.org/doi/abs/10.1126/science.adh2861
- 9. CNRS/Université Paris-Saclay.
- 10. https://minds.cab.inta-csic.es
- 11. “A rich hydrocarbon chemistry and high C to O ratio in the inner disk around a very low-mass star,” B. Tabone et al., Nature Astronomy, 11 May 2023: https://www.nature.com/articles/s41550-023-01965-3
- 12. “Abundant hydrocarbons in the disk around a very-low-mass star,” A. M. Arabhavi et al., Science, 6 June 2024: https://www.science.org/doi/10.1126/science.adi8147
- 13. NIRISS includes an imager and a near-infrared spectrograph. By analysing the light from stars that cross through the atmosphere of exoplanets, these instruments can determine their chemical composition.
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