A group of scientists, including Michiel Min (SRON), have for the first time observed ammonia isotopologues in the atmosphere of a celestial body. They used the James Webb Space Telescope. It gives astronomers new tools to deduce how gas giants are formed. Publication in Nature.
isotopologues
They reveal the origin of wine, the age of bones and fossils, and help diagnose diseases: we are talking about isotopes. Isotopes and isotopologues—molecules that contain isotopes—also play an increasingly important role in astronomy. For example, the ratio of the isotopes carbon-12 (12C) to carbon-13 (13C) in an exoplanet’s atmosphere reveals how far away from its parent star it was formed.
ammonia
Thusfar carbon monoxide—with 12C and 13C—has been the only isotopologue ever measured in the atmosphere of an exoplanet. A group of researchers from the MIRI European Consortium have now for the first time found ammonia isotopologues in the atmosphere of a cold brown dwarf. They measured both 14NH3 and 15NH3.
brown dwarf WISE J1828
Brown dwarfs are celestial bodies at the interface between stars and planets. They can therefore serve as a model system to study giant gas planets. In this case, the research group observed the brown dwarf WISE J1828, located 32 light-years from Earth. In the night sky it is located in the constellation Lyra, but you cannot see it with the naked eye because its surface is only 100 °C. The MIRI instrument on the James Webb space telescope is sensitive enough to observe it in infrared.
New indicator for planet formation
It is still unclear whether gas giants form like a snowball attracting more and more material, or through a local gravitational collapse in the protoplanetary disk around a baby star. Rocky planets seem to play out the snowball scenario.
The ratio between the two isotopologues 14NH3 to 15NH3 now appears to be an indicator for the formation of gas giants. While on Earth there are 272 14N atoms for every 15N atom, the scientists in this study calculated that the 14NH3/15NH3 ratio in WISE J1828’s atmosphere is approximately 670. This means that the brown dwarf accumulated less 15N during its formation than Earth.
The process that controls the amount of isotopes is still incompletely understood, but it is believed that comet impacts enrich a planet with 15N because comets have two or three times more 15N. Comets appear to be the basic building blocks of (at least rocky) planets. For example, they ensure that rocky planets maintain an atmosphere during their hot early stages of development.
A low amount of 15N in WISE J1828’s spectrum indicates that it was not formed as a snowball—like rocky planets—but by a gravitational collapse—like stars. This is also in line with expectations, because brown dwarfs are in fact stars. The 14NH3/15NH3 ratio therefore seems to be a good indicator of the formation history of a gas giant.
Publication
‘15NH3 in the atmosphere of a cool brown dwarf’, David Barrado, Paul Mollière, Polychronis Patapis, Michiel Min, Pascal Tremblin, Francisco Ardevol Martinez, Niall Whiteford, Malavika Vasist, Ioannis Argyriou, Matthias Samland, Pierre-Olivier Lagage, Leen Decin, Rens Waters, Thomas Henning, María Morales-Calderón, Manuel Guedel, Bart Vandenbussche, Olivier Absil, Pierre Baudoz, Anthony Boccaletti, Jeroen Bouwman, Christophe Cossou, Alain Coulais, Nicolas Crouzet, René Gastaud, Alistair Glasse, Adrian M. Glauser, Inga Kamp, Sarah Kendrew, Oliver Krause, Fred Lahuis, Michael Mueller, Göran Olofsson, John Pye, Daniel Rouan, Pierre Royer, Silvia Scheithauer, Ingo Waldmann, Luis Colina, Ewine F. van Dishoeck, Tom Ray, Göran Östlin & Gillian Wright, Nature