Isotopic Evidence for a Cold and Distant Origin of the Interstellar Object 3I/ATLAS

Abstract

Interstellar objects provide the only directly observable samples of icy planetesimals formed around other stars, and can therefore provide insight into the diversity of physical and chemical conditions occurring during exoplanet formation. Here we report isotopic measurements of the interstellar comet 3I/ATLAS, which reveal an elemental composition unlike any Solar System body. The water in 3I/ATLAS is enriched in deuterium, at a level of D/H = (0.95 +- 0.06)%, which is more than an order of magnitude higher than in known comets, while its range of 12C/13C ratios (141-191 for CO2 and 123-172 for CO) exceeds typical values found in the Solar System, as well as nearby interstellar clouds and protoplanetary disks. Such extreme isotopic signatures indicate formation at temperatures $\lesssim30$ K in a relatively metal-poor environment, early in the history of the Galaxy. When interpreted with respect to models for Galactic chemical evolution, the carbon isotopic composition implies that 3I/ATLAS accreted roughly 10-12 billion years ago, following an early period of intense star formation. 3I/ATLAS thus represents a preserved fragment of an ancient planetary system, and provides direct evidence for active ice chemistry and volatile-rich planetesimal formation in the young Milky Way.

Figures (8)

• Figure 1: Spectrally integrated line flux maps for 3I/ATLAS observed using JWST NIRSpec: (a) H$_2$O at 2.7 $\mu$m, (b) CO$_2$ at 4.3 $\mu$m, (c) CO at 4.7 $\mu$m. Image axes are aligned with the equatorial (RA/decl.) grid. Molecular emission has been isolated by subtracting a polynomial fit to the adjacent continuum. Spatial coordinates are with respect to the brightest pixel in each map (the pseudo-nucleus). Inset panels (upper right) show the continuum-subtracted spectra within the image integration wavelength range, spatially integrated within a $3"$ diameter circular aperture centered on the pseudo-nucleus (corresponding to 3,900 km at the 1.8 au distance of the comet from the telescope). Lower left corner shows the direction of the (sky-projected) comet-Sun (S) and nucleus velocity ($v$) vectors.
• Figure 1: Best fitting production rates ($Q$) for H$_2$O, CO$_2$, CO and their minor isotopologues, as a function of sky-projected distance ($\rho$). Spectra were extracted and modeled within successive $0.2"$-thick annuli centered on the coma flux peak to generate $Q_{\rho}$. For $^{13}$CO$_2$ and $^{13}$CO, the models included a 5th-order polynomial baseline fit. Vertical error bars indicate $1\sigma$ statistical uncertainties, while the horizontal error bars indicate the radial extent of each spatial region.
• Figure 2: Observed NIRSpec molecular spectra of 3I/ATLAS integrated within a $d=3.6"$-diameter circular aperture centered on the pseudo-nucleus. Similarly spatially-integrated, best-fitting spectral models are overlaid. As shown by the grey filled regions in panel (a), the strong CO ($v=1-0$) lines were subtracted prior to fitting, to reduce spectral contamination when fitting H$_2$O. The grey region in panel (d) indicates wavelengths excluded (masked) from the fitting due to potential contamination by an unidentified feature around 4.42 $\mu$m. In panel (f), the regions surrounding the stronger CO lines (shown in grey) were masked during $^{13}$CO fitting, to avoid contamination by the CO model residuals, and to allow improved fitting of the higher-$J$ CO lines, some of which overlap $^{13}$CO. For panels (d) and (f), first-order baseline fits are shown; see Extended Data Fig. \ref{['fig:bases']} for the ensemble of baselines used in deriving our final range of $^{12}$C/$^{13}$C values for these species.
• Figure 2: High-resolution submillimeter-wave spectra of the CO (left) and HCN (right) line emission from 3I/ATLAS, observed using the ALMA ACA on 2025-Dec-22. Spectra were extracted at the common emission peak for both species, and are plotted on a velocity scale with respect to the rest frequencies of the CO $J=3-2$ and HCN $J=4-3$ lines, respectively. Best fitting spectral models are overlaid.
• Figure 3: Isotopic ratios observed in the coma of 3I/ATLAS compared with Galactic and Solar System observations for D/H (top) and $^{12}$C/$^{13}$C (bottom). Data are from the compilation of nom23, with additional D/H values for V883 Ori (gas-phase; tob23), and IRAS20126+4104, HOPS370 and L1527 (ice-phase; sla24sla25). D/H data are for H$_2$O unless otherwise specified. The "disks HCN" data represent the mean and range of observed D/H values across five protoplanetary disks. For $^{12}$C/$^{13}$C, the meteoritic "XCO$_3$" and "IOM" data are the mean values (and ranges) for carbonates and insoluble organic matter, respectively, in carbonaceous chondrites ale07ale10ale15; the TW Hya and HD 163296 values for CO are from yos22qi26; protostellar ice CO and CO$_2$ are the mean values from bru24; interstellar CO$_2$ ice is the average (and range) of values from boogert15; interstellar HCO$^+$ values for three Galactocentric distances are from luo24; super-Jupiter exoplanet CO data are from gan23; M dwarf GJ745 data are from cro19. Error bars are $1\sigma$ unless otherwise specified.