Recent research has discovered ammonia allotropes in the atmospheres of brown dwarfs, marking a major advance in astronomy. The James Webb Space Telescope discovery sheds new light on the formation of gas giants and exoplanets, challenging existing theories and highlighting alternative processes such as gravitational collapse.
The James Webb Space Telescope has detected an allotrope of ammonia in a brown dwarf, providing groundbreaking insights into the formation of gas giants and exoplanets, revealing potential alternative formation processes.
They reveal the age of bones and fossils and serve as medical diagnostic tools. Isotopes and isotopic homologues - molecules that differ only in their isotopic composition - also play an increasingly important role in astronomy. For example, based on the ratio of carbon-12 (12C) and carbon-13 (13C) isotopes in an exoplanet's atmosphere, scientists can infer how far the exoplanet orbits its central star.
So far, 12C and 13C combined in carbon monoxide are the only isotopes that can be measured in exoplanet atmospheres. Now, a research team has successfully detected an ammonia isotope in the atmosphere of a cool brown dwarf star. The research team just reported in the journal Nature that ammonia can be measured in the form of 14NH3 and 15NH3. Astrophysicists Polychronis Patapis and Adrian Glauser, members of the Department of Physics and the National Center for Planetary Research (NCCR), participated in the study, with Patapis being one of the first authors.
Brown dwarfs are somewhere between stars and planets: they are similar to gas giant planets in many ways, which is why they can be used as model systems for studying gas giants. In their work, Patapis and colleagues observed a brown dwarf star called WISEJ1828, which is 32.5 light-years away from Earth; in the night sky, it is located in the constellation Lyra.
WISEJ1828 is invisible to the naked eye: its effective temperature (that is, the temperature of a blackbody, which emits the same energy as the object being observed) is only 100°C, which is too cold for hydrogen fusion to occur and light to be transmitted all the way to Earth. In order to discover this ultra-cool dwarf star in the Y spectral class, the mirror of the James Webb Space Telescope (JWST) was turned toward the constellation Lydia last summer.
The Mid-Infrared Instrument (MIRI), an infrared detector installed on JWST, made it possible to reveal ammonia allotropes on WISEJ1828. MIRI's Moderate Resolution Spectrometer (MRS) recorded the brown dwarf's spectrum in the wavelength range of 4.9 to 27.9 μm. In addition to ammonia, the researchers also observed water molecules and methane molecules, each with characteristic absorption bands. In particular, ammonia causes attenuation of signals reaching the detector in the wavelength range between 9 and 13 μm.
Allotropes of ammonia can also be distinguished by their spectra: if the ammonia molecule is not composed of the most common nitrogen isotope 14N (14N is bonded to three hydrogen atoms), but is composed of 15N plus three hydrogen atoms, then the extra neutrons in the nitrogen nucleus will ensure that a bump appears in the spectrum, which can be explained by the presence of 15NH3.
The ratio of two isotopes of ammonia measured in WISEJ1828's atmosphere is particularly exciting: As Patapis and colleagues explain, the ratio of 14NH3 to 15NH3 is a tracer, an indicator that can be used in the future to study star and planet formation. It's a new tool that helps examine different, known mechanisms of gas giant planet formation.
Gas giants like Jupiter or Saturn are not unique to our solar system. These objects play an important role in exoplanet research: they appear early in star formation and are therefore a key factor in determining whether and how smaller, lighter planets form. So far, there is no definite answer to the question of how massive gas giant planets form. Experts have proposed different theories, but it is still unclear whether these planets formed through nuclear accretion like most other planets or whether they were the result of the gravitational collapse of a protoplanetary disk around a protostar.
The isotope ratios recorded by Patapis and colleagues could provide new clues. On Earth, there are 272 14N atoms for every 15N atom. The paper reports that the ratio of 14NH3 to 15NH3 measured in WISEJ1828's atmosphere was 670, meaning the brown dwarf accumulated less nitrogen-15 during its formation than other planets such as Earth and Jupiter. In fact, the abundance of 15N on WISEJ1828 is rarer than on any object in the solar system.
The process of so-called isotope fractionation, a change in the abundance of an isotope, is not fully understood, but comet impacts are thought to contribute to the enrichment of nitrogen-15 because comets have much higher 15N content. Comet impacts are also thought to be a fundamental building block of the planets in the solar system: comets contributed to the formation of Earth's atmosphere, although the extent of their contribution is not entirely clear.
The 15NH3 content in the spectrum of WISEJ1828 is very low, which indicates that this brown dwarf was not formed according to the usual planet formation method (i.e. core accretion), but was formed in a star-like manner. Therefore, this gravitational instability is likely to play an important role in the formation of gas giant planets, especially those that move in large orbits around stars.
In fact, this is another important issue discussed in the paper: the ratio of 14NH3 to 15NH3 appears to vary greatly depending on the distance between the gas giant planet and its star, which can be confirmed by simulating planets forming between the ammonia and molecular nitrogen ice lines. In astronomy, an ice line represents the minimum distance from a central star at which temperatures are low enough for certain volatile compounds to transform into a solid state.
Patapis and colleagues believe that the observed increase in the 14NH3 to 15NH3 ratio may indicate planetary accretion ice between the ammonia and nitrogen ice lines.
Astronomers have another tool in their study of directly observable exoplanets. Thanks to JWST, traces of ammonia have become clearly visible, once again confirming the great value and unparalleled performance of this space telescope.