JWST detects rare oxygen and nitrogen isotopes in the cold atmosphere of COCONUTS-2b

Star system: L 34-26 | Distance from the Sun: 35.5 light years

COCONUTS-2b is one of the coldest known planetary-mass companions, orbiting far from its M dwarf host star. Using the JWST MIRI/MRS instrument, scientists obtained a detailed spectrum of its atmosphere at a temperature of about 480 K. For the first time, they detected rare isotopologues—molecules with different isotopes—of ammonia and water: ^{15}NH3, H2^{18}O, and H2^{17}O. This is the first clear evidence of oxygen isotopes in water in a cold planetary companion.

The study combined JWST and Gemini/FLAMINGOS-2 data to analyze the chemical composition and structure of COCONUTS-2b's atmosphere. The detection of these isotopologues opens new possibilities for tracing the formation history of cold gas giants by comparing their isotope ratios to those of their host stars. The results show that JWST can reveal the detailed chemical and isotopic makeup of even the coldest exoplanet atmospheres, providing unique clues to their origins.

Key findings
- JWST detected ^{15}NH3, H2^{18}O, and H2^{17}O in COCONUTS-2b's atmosphere
- First clear evidence of oxygen isotopes in water for a cold planetary companion
- Results enable comparison of isotope ratios with host stars
- Study demonstrates JWST's power for cold exoplanet atmospheric characterization
- Isotopic analysis helps trace planet formation scenarios

Scientific study

Oxygen and nitrogen isotopologues on cold COCONUTS-2b observed with MIRI/MRS (external link)