???? The ammonia trail leads to exoplanets

2023-11-30 07:00:05

Brown dwarfs, cosmic enigmas, and a major scientific breakthrough: this is what a recent study published in the journal Nature reveals. The James Webb Space Telescope (JWST) has turned its attention to the cool brown dwarf W1828, opening a window into the secrets of these stellar objects.

By pointing the James Webb Space Telescope (JWST) towards this object, a team of researchers, including researchers from DAp-AIM, was able to measure with the MIRI instrument and, for the first time, the isotopologues of ammonia in the atmosphere of a cold brown dwarf, opening the way to a better understanding of the formation of exoplanets.

Figure 1: Artist’s illustration of the cool brown dwarf WISE J1828, showing the water (H20), methane (CH4) and ammonia (NH3) molecules detected in the spectrum obtained with the JWST.
Credit ETH Zurich / Polychronis Patapis

Brown Dwarfs, these stars between planets and stars

Brown dwarfs are celestial bodies located on the boundary between stars and planets. Their mass is insufficient to initiate the thermonuclear fusion of hydrogen at their core, as stars do, but sufficient to initiate the fusion of deuterium (Deuterium (symbol 2H or D) is a natural isotope of hydrogen. Its atomic nucleus…), unlike the planets. In many ways, these stars resemble gas giant planets, making them excellent laboratories for the study of exoplanets.

The brown dwarf WISE J1828 is 32.5 light years from Earth, in the constellation Lyra. Its radius is only a third greater than that of Jupiter, for a mass 15 times greater. With a surface temperature of only 100 degrees Celsius, it is part of spectral class Y, whose atmospheres are dominated by absorption (In optics, absorption refers to the process by which the energy of a photon is taken by…) water, methane and ammonia. At these temperatures, the light output of these brown dwarfs peaks in the mid-infrared. The arrival of the JWST will revolutionize the study of these stars because its infrared sensor MIRI (Mid Infrared Instrument) covers their entire light range, which was previously difficult to observe.

Figure 2: Spectrum of WISE J1828 measured by the MIRI instrument on board JWST. The characteristic absorption bands of ammonia, water and methane molecules are clearly seen which cause signal attenuation in the wavelength range between 9 and 13 µm, 5 and 7 µm, and around of 7.6 µm respectively. The zoomed region of the spectrum shows an example of a 15NH3 absorption band identified with the resolution of the MIRI spectrometer.
Credit: ETH Zurich / Polychronis Patapis.

The ammonia isotope, a tracer of the formation of exoplanets

Isotopes are atoms that have the same number of protons but a different number of neutrons. Due to their different atomic mass (The atomic mass (or atomic molar mass) of an isotope of a chemical element is the mass…), isotopes of the same element have different physical properties, and therefore different spectral signatures. They are widely used on Earth. We think in particular of carbon 14 dating (Carbon 14 is a radioactive isotope of carbon, denoted 14C.), which makes it possible to estimate the age of bones or fossils. In astronomy, they occupy an increasingly important place. For example, the ratio of isotopes of carbon-12 (12C) and carbon-13 (13C) in the atmosphere of an exoplanet (An exoplanet, or extrasolar planet, is a planet orbiting around a…) can be used to infer the distance at which the exoplanet formed around its central star. Until now, 12C and 13C, linked in carbon monoxide, were the only isotopologues – molecules which differ only in the composition of their isotopes – which can be measured in the atmosphere of exoplanets. But for cold objects, it is very difficult to have access to these isotopic ratios.

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Thanks to this new study, the team of researchers demonstrated that it was also possible to use isotopologues of ammonia (NH3) as a tracer of the formation of exoplanets. Indeed, they detected for the first time in the atmosphere of a cold brown dwarf, serving here as a proxy for exoplanets, the spectral signature characteristic of the presence of the molecules 14NH3 (also written 14N) and 15NH3 (15N). Even though they only differ by one neutron in the nitrogen nucleus, we can clearly distinguish them in the observed spectrum (see Figure 2).

A new diagnostic tool for the formation of exoplanets

Gas giants such as Jupiter or Saturn do not only exist in our solar system, but they are also found in other exoplanetary systems. Some orbit very far from their star and the question of their formation arises. Were they formed in the proto-stellar disk like stars by gravitational instability or later in the protoplanetary disk (Stars form from a cloud of gas and dust whose central part…)? The 14NH3 / 15NH3 ratio is a tracer, that is, an indicator, which could be used in the future to study the formation of these planets.
Figure 3: This diagram summarizes different phases of star and planet formation and the relationship between ammonia (NH3) fractionation and the evolution of the 14N/15N ratio at different stages: inside a cloud molecular with pre-stellar cores (top left), during the formation of a protostar (top right) and in a circumstellar disk around a young star (bottom).
Credit: adapted from the article Barrado, D. et al. 15NH3 in the atmosphere of a cool brown dwarf. Nature (2023).

Indeed, as indicated in Figure 3, in a protoplanetary disk, the ratio of 14NH3 to 15NH3 depends on the distance from the star and increases sharply between the ice line of ammonia (NH3) and the ice line of molecular nitrogen (N2). This variation is still very qualitative; but the trend is there.

In this regard, ammonia and the amount of its isotopologues can not only provide information on how an exoplanet developed, but also on where on the protoplanetary disk it formed. The 14N/15N ratio can constrain the formation locations relative to the NH3 and N2 ice lines of the disk, making ammonia a new tool for understanding gas giant formation. This hypothesis can be tested on cold exoplanets far from their star, and therefore directly imageable by the JWST.

These results were published in the journal Nature.

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