📄 Showing the "Simple" version — "Advanced" is not ready yet. Add it to favorites to help prioritize it. Ask for the full read

A giant planet hangs in space in complete solitude, separated from its star by a distance 160 times greater than the path from the Sun to Pluto. The light from its parent star arrives so faintly that the surface temperature barely exceeds 200 degrees Celsius — about as warm as a well-heated sauna. This is COCONUTS-2b, one of the coldest worlds known beyond our Solar System. Until recently, such objects remained invisible to detailed study: their dim heat was lost in the noise of instruments. But with the arrival of the infrared space telescope, astronomers gained a chance to peer into the very essence of these icy giants.

A team of scientists pointed the MIRI instrument at COCONUTS-2b and obtained a spectrum — a rainbow strip where each color tells the story of the atmosphere's composition. For the first time for such a cold object, they managed not only to see the main gases but also to distinguish their isotopic variants. Isotopes are like identical building blocks but with different numbers of neutrons in the nucleus. For example, ordinary water H₂O contains oxygen-16, while heavy water H₂¹⁸O contains oxygen-18. In the atmosphere of COCONUTS-2b, they confidently detected three such rare versions at once: ammonia with nitrogen-15, as well as water with oxygen-18 and oxygen-17. The fraction of heavy water there turned out to be noticeably higher than in Earth's oceans.

This discovery turns COCONUTS-2b into a natural laboratory. Now, by comparing the isotopic composition of the planet and its star, we can reconstruct the history of their emergence. Perhaps, in the distant past, this giant migrated across the entire disk, gathering material from different zones like a snowball. Or maybe it was born right where it is now, but from a cloud with unusual chemistry. Future observations will provide the exact answer, but one thing is already clear: cold worlds are no longer featureless points. Their atmospheres hold the keys to understanding how planets assemble across the Galaxy.

Original abstract

as written by the authors arXiv:2604.26850

Linking the composition of gas giant planets to their formation paths has long been a goal in exoplanet science. Especially, cold gas giants with temperatures below $$\sim$$500K have been out of reach for detailed atmospheric characterization. With JWST, however, we can reach high signal-to-noise (S/N) spectra for such cool worlds and can can measure not only their main trace gas abundances, but even their isotopic content unlocking new possibilities in linking them to their formation paths. In this study, we present the spectrum of one of the coldest planetary-mass companions COCONUTS-2b ($$\mathrm{T_{eff}}\approx$$480K, separation of $$\sim$$6400 au from its M dwarf host star) obtained with the Mid-InfraRed Instrument Medium Resolution Spectrometer (MIRI/MRS). Combining the MIRI and archival Gemini/FLAMINGOS-2 data sets, we aim to characterize the chemical composition and physical structure of its frigid atmosphere, setting the stage to uncover insights on the formation of COCONUTS-2b. For the first time on a MIRI/MRS data set, we use the full spectral resolution of MIRI/MRS and perform atmospheric retrievals to unlock the search for faint absorption features by rare molecules and isotopologues. The latter are identified using a leave-one-out analysis and Bayes factor comparison. We robustly detect three isotopologues, namely $$^{15}$$NH$$_3$$, H$$_2^{18}$$O and H$$_2^{17}$$O in the atmosphere of COCONUTS-2b. We find the first clear evidence of oxygen isotopes in water in a cold companion. This data set demonstrates the capability of MIRI/MRS to characterize such cold planetary-mass companion's atmospheres with respect to their compositional and isotopic content. In the future, the constrained elemental and isotope ratios provide a unique avenue in comparing with the host star's abundances and eventually in tracing formation scenarios.