The ammonia trail leads to exoplanets
With the help of the James Webb Space Telescope, a team of researchers led by the Centro de Astrobiología (CAB-CSIC-INTA), the Max-Planck-Institut für Astronomie and ETH Zurich - and including scientists from the University of Liège - has succeeded in measuring the isotopes of ammonia in the atmosphere of WISE J1828, a cold brown dwarf located 32.5 light-years from our Earth. This study, published in the journal Nature, demonstrates that the isotopic abundance of ammonia can be used to study the formation of giant gas planets.
They reveal the origin of wine, the age of bones and fossils, and are used as diagnostic tools in medicine: isotopes. Isotopes and isotopologues - molecules that differ only in the composition of their isotopes - are also playing an increasingly important role in astronomy. For example, the ratio between the isotopes of carbon-12 (12C) and carbon-13 (13C) in the atmosphere of an exoplanet can be used to deduce the distance at which the exoplanet formed around its central star.
Until now,12C and 13C, linked to carbon monoxide (CO), were the only isotopologues that could be measured in the atmosphere of exoplanets. A team of researchers from the European MIRI consortium - including Olivier Absil, FNRS Senior Research Fellow and head of the PSILab team of the STAR Institute (ULiège) - has succeeded in detecting ammonia (NH3) isotopologues for the first time, measured in the form of 14NH3 and 15NH3, in the atmosphere of a cold brown dwarf.
In search of ammonia
“Brown dwarfs are celestial bodies at the boundary between stars and planets, explains David Barrado, astrophysicist at CAB-CSIC-INTA and first author of the study. In many respects, they resemble giant gas planets, which is why they can be used as a model system for the study of gas giants".
In this particular case, the research group observed the brown dwarf WISE J1828, located 32.5 light-years from Earth in the constellation Lyra. With an effective temperature of just 100°C, WISE J1828 is not visible to the naked eye because it is far too cold for hydrogen fusion to take place and transmit visible light to Earth. Researchers, therefore, had to use the JWST to observe it. With the help of the MIRI (Mid-InfraRed Instrument, tested at the Centre Spatial de Liège - CSL) on board the JWST, the research team was able to detect isotopologues of ammonia using the infrared sensor. In the wavelength range between 4.9 and 27.9 μm, MIRI's Medium Resolution Spectrometer (MRS) recorded a spectrum of the brown dwarf in the mid-infrared. In addition to ammonia, water and methane molecules were observed in the atmosphere of WISE J1828, each with characteristic absorption bands.
“We have contributed to the determination of the physical structure and chemical composition of the atmosphere of WISE J1828, explains Olivier Absil. This is the focus of the thesis of Malavika Vasist, a PhD student at PSILab, who is developing a specific tool based on deep learning techniques to extract the atmospheric parameters of exoplanets and brown dwarfs from the infrared spectra obtained by the JWST.”
A new diagnostic tool for the formation of exoplanets
The ratio between the two isotopologues of ammonia measured in the atmosphere of WISE J1828 is particularly interesting: the ratio 14NH3 / 15NH3 is a tracer, i.e., an indicator that could be used in the future to study the formation of brown dwarfs and planets. It is a new tool that will make it possible to test different known mechanisms for the formation of gas giants.
Gas giants such as Jupiter and Saturn do not only exist in our solar system. These gigantic celestial bodies also play an important role among exoplanets: they form very early in the formation of stars. They could therefore be a decisive factor in determining whether and how the smaller, lighter planets in a solar system develop.
But how do gas giants form? Until now, there has been no clear answer to this question. Experts have developed various theories, but whether gas planets form by core accretion - like most other planets - or as a result of gravitational collapse in the protoplanetary disc around a progenitor star, has been unclear until now.
The isotope ratio now provides information on this subject. While on Earth there are 272 atoms of 14N (nitrogen) for each atom of 15N, the scientists involved in the study have calculated that the ratio 14NH3 to 15NH3 measured in the atmosphere of WISE J1828 is around 670. This means that the brown dwarf accumulated less 15N during its formation than the Earth. The abundance of 15N is even lower than that of all the celestial bodies in our solar system.
Spectrum of WISE1828 measured by the MIRI instrument on board the JWST. The features correspond to molecular absorptions mainly by water, methane or ammonia, while no indication of clouds in its photosphere was found. The zoomed-in region of the spectrum shows an example of an individual 15NH3 absorption feature that is identified with the resolution of the MIRI spectrometer. Credits: Polychronis Patapis, NASA/ESA/JPL
Different scenarios for planet formation
The processes of isotopic fractionation, i.e., the modification of isotope abundance, are not yet fully understood, but it is assumed that comet impacts contribute to the enrichment of 15N (nitrogen), given that comets have two to three times higher 15N content. Comet impacts, in turn, are considered to be a fundamental element in the construction of the planets in our solar system. Comet fragments, for example, allow light, Earth-like planets to retain an atmosphere during their early development phase, when they heat up and outgas.
A low content of 15NH3 in the spectrum of WISE J1828 therefore suggests that the brown dwarf did not form like a planet - i.e., by core accretion - but rather that its formation took place like a star, in the context of a gravitational collapse. However, star-like formation is the type of formation expected for brown dwarfs. This could therefore indicate that the ratio 14N/15N is a significant indicator of the formation history.
Gravitational instabilities may play an important role in the formation of gas giants, particularly those orbiting their star in a large orbit. Another result of the article is related to this: the ratio 14N/15N depends on the distance of a gas giant from its central star, and increases sharply between the so-called ammonia ice line and the molecular nitrogen ice line. "In this respect, ammonia and the frequency of its isotopologues can provide information not only on how an exoplanet developed but also on where in the protoplanetary disc it formed," concludes Paul Mollière (MPIA Heidelberg), co-first author of the paper.
Thanks to ammonia, astronomers will have an additional tool with which to study directly observable exoplanets. A tool that has only become tangible thanks to JWST, underlining once again the value and performance of the space telescope.
- Barrado, D., Mollière, P., Patapis, P. et al., 15NH3 in the atmosphere of a cool brown dwarf, Nature, 2023. DOI:10.1038/s41586-023-06813-y
- Zhang, Y. et al. The 13CO-rich atmosphere of a young accreting super-Jupiter, Nature, 2021. DOI: 10.1038/s41586-021-03616-x
- Wright, G. et al. Mid-infrared Instrument for JWST and its In-Flight Performance, Publications of the Astronomical Society of the Pacific, Volume 135, Issue 1046, id.048003, 20 p., DOI : 10.1088/1538-3873/acbe66