Water reservoirs in the Solar System exhibit a deuterium enrichment that links back to the physical environment at the time of stellar birth. Gas-phase and ice-grain deuterium enrichments occur through chemical processes that operate at low temperatures (<30 K) pointing towards an origin in the prestellar molecular cloud or in the outer parts of the protoplanetary disk. However, not all stars are born in environments similar to our Sun, nor do their subsequent evolutionary histories follow the same path. These environmental differences can be traced by the water deuterium-to-hydrogen (D/H) ratio. Here we use ALMA observations of the interstellar comet 3I/ATLAS to constrain the water D/H ratio in extrasolar cometary material. With a water D/H value of [D/H](H2O )> 6.6 & times;10(-3), 3I/ATLAS shows a deuterium enrichment exceeding Earth's ocean value by more than a factor of about 40 and typical Solar System cometary values by more than a factor of about 30. The elevated deuterium enrichment points to water that formed under colder, less irradiated conditions and from less thermally processed material, consistent with an origin in a planetary system that formed under different physical and chemical conditions than our own.

We report results from an analysis of the volatile composition and evolution of the main-belt comet (MBC) 133P/Elst-Pizarro using JWST NIRSpec and NIRCam observations and ground-based observations during its 2024 active apparition, and also assess the body of JWST MBC observations acquired to date. Using NIRSpec, we measure water vapor outgassing rates at two points in 133P's orbit, finding molecules s(-1) on UT 2024 June 12 (at a true anomaly of nu = 8 degrees and heliocentric distance of r(h) = 2.674 au), and molecules s(-1) on UT 2024 October 14 (at nu = 374 and r(h) = 2.747 au). These measurements nominally represent a decline of similar to 25% in between the visits, although they are also consistent with no change within uncertainties. We do not detect CO, CO2, or CH3OH, placing 133P's hypervolatile depletion () at a similar level found for previously observed MBCs. We find values for the three MBCs for which water vapor outgassing has been successfully detected that are consistent within uncertainties with an average value of . Lastly, we find no clear correlations of water production rates with nucleus size, semimajor axis, or heliocentric distance among MBCs observed by JWST so far, but would particularly encourage future JWST observations of additional MBCs interior to the 5A:2J mean-motion resonance with Jupiter and at high inclinations, as well as multiple observations of MBCs during single active apparitions to further investigate areas of interest identified from the current sample of JWST-observed MBCs.

We present high-resolution UVES+VLT observations of neutral nickel and iron atoms in the coma of the interstellar comet 3I/ATLAS taken after perihelion. Metal emission was strong shortly after perihelion and persisted at large heliocentric distances. At r_h ∼ 2 au the total metal production rate was found to be at least an order of magnitude larger than that of typical solar-system comets. Post-perihelion production rates exhibit pronounced asymmetry compared to the pre-perihelion behavior: production rates are higher after perihelion and decline more gradually with r_h, the difference being stronger for FeI. The NiI/FeI abundance ratio, initially anomalously large before perihelion, evolved toward values comparable to solar-system comets near 2 au, and shows a weaker r_h dependence after perihelion. To interpret these results, we revisited and extended the carbonyl hypothesis in which FeI and NiI are produced by the rapid photodissociation of Fe(CO)_5 and Ni(CO)_4 vaporized from the nucleus. Fits that include direct sublimation of carbonyls reproduce the observed rates and the high NiI/FeI line ratio, which is determined by the higher volatility of Ni(CO)_4. Desorption of carbonyls from sublimating CO_2 and H_2O ices is found to be negligible. The temperature profiles needed to reproduce the observations were found to be shallower than the equilibrium T ∝ r_h^-1/2 relation, suggesting that the sublimation could occur below the surface of the nucleus. Fits using temperature profiles from thermal models require sublimation from depths of several cm, especially post-perihelion. An additional transient heat source (T ≃ 100-140 K), possibly linked to the amorphous-crystalline ice transition, is proposed to explain the early NiI excess before perihelion.

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 formation1-3. 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 = (0.98 ± 0.06)%, which is more than an order of magnitude higher than in known comets, and 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 ≲30 K in a relatively metal-poor environment. When interpreted with respect to models for Galactic chemical evolution, the carbon isotopic composition implies that 3I/ATLAS may have accreted as long ago as 12 billion years, following a period of intense, early star formation. 3I/ATLAS thus represents a preserved fragment of an ancient planetary system.

We present JWST NIRSpec and NIRCam observations of 457P/Lemmon-PANSTARRS, a main-belt comet that displayed activity around its 2020 perihelion and that was observed to regain activity during its 2024 perihelion by a ground-based observing campaign. The previous successful measurements of water production from two main-belt comets by the JWST NIRSpec instrument confirmed the hypothesis that H2O reservoirs are responsible for activity in dynamically stable main-belt comets. However, the main-belt comets observed with JWST thus far, 238P/Read and 358P/PANSTARRS, occupy orbits in the outer main belt, with main-belt comets with smaller semimajor axes not yet sensitively tested for H2O. We find that, despite clearly displaying dust activity in both ground-based and JWST imaging over a broad period, there were no corresponding H2O, CO, CO2, or CH3OH emissions within sensitive upper limits-notable given that 457P is the first main-belt comet with a semimajor axis within the 5:2 mean-motion resonance with Jupiter. We show that we were sensitive to production rates of gas predicted by the dust/gas ratios of 238P and 358P, and we hypothesize that 457P may be more depleted than its companions; QH2O must be less than 2 & times; 1024 molecules s-1, or 0.035 kg s-1. Further surveying of main-belt comets across the parameter space of semimajor axis and eccentricity will shed light on whether 457P represents an edge member of a spectrum or a distinct subclass of main-belt comets.

Cometary outbursts may be used as a means to infer the physical processes occurring on cometary nuclei. To that end, we studied eight outbursts of comet 7P/Pons-Winnecke identified between 2021 June 3 and 2021 August 31. The data analyzed consisted of optical images and derived photometry of the comet from the Las Cumbres Observatory network of telescopes. The outburst strengths relative to the ambient coma ranged from -0.2 to -1.1 mag, and the ejecta themselves had apparent brightnesses ranging from 17.4 to 13.3 mag. The morphologies of the ejecta varied, suggesting that the events may have originated from different sources across the nucleus. An order of magnitude estimation of the ejecta masses ranged from 10^5 - 10^6 kg, similar to other mini-outbursts of comets. The surface-area normalized outburst rate estimated during this time period is similar to comets 41P/Tuttle-Giacobini-Kresák, 9P/Tempel 1 and 46P/Wirtanen, but 10 times larger than that observed at comet 49P/Arend-Rigaux. However, a comparison to the mini-outburst rate of comet 67P/Churyumov-Gerasimenko reveals significant discrepancies between Rosetta spacecraft results and those from ground-based telescopes. We also investigate whether or not cometary outbursts from 7P in the 19th century are needed to explain outbursts in meteor shower rates observed in the 20th century.

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 = (0.79 +- 0.07)%, which is more than an order of magnitude higher than in known comets, while its range of 12C/13C ratios (144--196 for CO2 and 129--179 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 <~30 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.

We report a survey of molecular emission from cometary volatiles using the James Webb Space Telescope (JWST) toward interstellar object 3I/ATLAS carried out on UT 2025 December 22 and 23 at a heliocentric distance (rH) of 2.37-2.41 au. These measurements of CO, CO2, H2O, CH3OH, and CH4 sampled molecular chemistry in 3I/ATLAS as it receded from its encounter with our Sun and entered the vicinity of the H2O ice line-the region between rH = 2 and 3 au where the temperature becomes too low for H2O to vigorously sublime, and CO and CO2 begin to control the overall activity. CO was the most abundant molecule, followed by H2O and CO2, whose molecular abundances with respect to CO were (40.5 +/- 3.1)% and (41.6 +/- 0.3)%, respectively. This work presents spatial-spectral maps of column density and rotational temperature as a function of distance from the nucleus for all detected species. The spatial distributions of both quantities were highly anisotropic for the apolar species in the coma of 3I/ATLAS, yet were more nearly symmetric for the polar molecules. These results demonstrate how volatiles were segregated in the nucleus ices of 3I/ATLAS and reveal heating and cooling mechanisms in its coma. Derived maps of the ortho-to-para ratio (OPR) for H2O were flat with increasing distance from the nucleus and consistent with a coma-averaged value OPR = 2.7 +/- 0.2, slightly less than the expected equilibrium value of 3.

Interstellar objects are interlopers from other planetary systems, and their volatile compositions provide a glimpse into planet formation around their host star. We present near-infrared spectra of the coma of interstellar object 3I/ATLAS measured with the James Webb Space Telescope. Our results demonstrate an unexpectedly high D/H = (3.33±0.31)% for methane and represent an exceedingly rare detection of deuterated organic molecules in an interstellar object. This D/H ratio exceeds any other value for methane measured in the solar system, and is a factor of 14±2 higher than that measured in comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft, the only other comet for which CH_3D has been detected. Both 3I/ATLAS and 67P/Churyumov-Gerasimenko show a higher degree of methane dueteration compared to water, consistent with trends seen in other solar system bodies and the nearby interstellar medium, where deuteration in organics exceeds that of water by up to an order of magnitude. The D/H ratio in methane itself is observationally unconstrained in extrasolar sources to date, but the enriched ratio in 3I/ATLAS is similar to those measured in methanol and formaldehyde toward primitive environments. The elevated D/H ratio for methane in 3I/ATLAS is consistent with the hypothesis that its ices formed in a cold, low metallicity interstellar or protostellar environment under a high cosmic ray irradiation rate (Cordiner et al. 2026).

Constraining the sulfur reservoirs of interstellar objects provides essential clues to the nature of sulfur species in protoplanetary disks and interstellar molecular clouds and a potential solution to the cosmic sulfur depletion problem. Observations of interstellar objects, chemical messengers from these systems, allow us sensitive probes of their volatile and refractory content and, when placed in context with recent developments in laboratory astrophysics, allow us to propose hypotheses for their formation and initial conditions. We present Hubble Space Telescope Observations of the interstellar object 3I/ATLAS, taken in 2025 December and 2026 January, that show fluorescence emissions from the CO Fourth Positive Group that are well fit by production rates of 3.8 × 10 ^27 and 1.1 × 10 ^27 molec. s ^−1 , respectively. We also place stringent upper limits on the atomic sulfur column and show that 3I/ATLAS’s comae is significantly depleted in sulfur, with S/O 3 σ upper limits of 0.4% and 0.1% for the 2025 December and 2026 January epochs, respectively. We hypothesize that this sulfur depletion is the result of a physical and chemical formation environment that was extremely cold, UV-shielded, and/or sulfur-depleted and therefore not conducive to the formation of H _2 S or SO/SO _2 while still effectively forming OCS and depleting the H _2 S reservoir, with substantial material inherited from the dense interstellar cloud. That same process would also sequester sulfur into FeS and S _6 –S _8 allotropes, which would be refractory at the observed heliocentric distances and would not contribute to the atomic sulfur coma. These species, which are often highlighted as potential solutions to the cosmic sulfur depletion problem, are also theorized to be the source of excess atomic sulfur seen in solar system comets.

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)

Comet C/2025 K1 (ATLAS) reached perihelion at 0.33 au on 2025 October 8. Daily monitoring by the LCO Outbursting Objects Key Project revealed a major activity increase between November 2 and 4, accompanied by rapid changes in coma morphology. Serendipitous HST/STIS acquisition images obtained on November 8-10 captured the comet only days after this event and resolved five fragments, providing an early high-resolution view of a nucleus in the process of disruption. Fragment motions and morphologies indicate a hierarchical fragmentation sequence, including a slow secondary split of fragment II. Back extrapolation shows that both the primary and secondary breakups preceded their associated photometric outbursts by roughly one to three days. This consistent lag, together with the appearance of thin, short-lived arclets around fragment I in the first HST epoch, suggests that freshly exposed interior material warms rapidly but requires time before dust can be released efficiently. Given the comet's close perihelion passage, rotational instability driven by enhanced outgassing torques is a plausible contributor to nucleus disintegration and dust release, and may represent the primary source of the observed brightening. These combined ground- and space-based observations provide rare, time-resolved constraints on the thermal and structural evolution of a fragmented comet near perihelion and highlight the scientific value of capturing a nucleus within days of disruption, when thermal adjustment, dust mantle re-formation, and outgassing-driven torques jointly govern the onset of activity.

We present a uniform, epoch-resolved analysis of soft X-ray observations of eight comets obtained with NICER, using Bayesian statistics to identify charge-exchange line components, measure relative ion fluxes, and infer nominal solar-wind freeze-in temperatures. The sample exhibits recurring spectral morphologies that fall into distinct families: carbon-dominated, intermediate, and nitrogen-/oxygen-dominated. Epoch-resolved flux ratios yield a robust separation between diagnostics: carbon-derived freeze-in temperatures cluster near T-freeze(C) approximate to 1.4-1.7 MK, while nitrogen-and oxygen-derived diagnostics are systematically higher, typically T-freeze(N, O) approximate to 2.0-2.3 MK. Short-timescale variability in inferred freeze-in conditions is common, indicating that instantaneous solar-wind charge-state fluctuations, rather than large changes in coma composition, dominate the spectral differences. We discuss instrumental and modeling limitations, demonstrate how our Bayesian fitting method mitigates degeneracies via physically motivated priors and Bayesian model selection, and recommend laboratory measurements and coordinated high-resolution X-ray observations to refine charge-exchange diagnostics and validate low-resolution inferences.

We review the current state of knowledge of the long-term evolution of the small bodies that give rise to comets and exocomets, as well as their reservoirs. The active cometary phase is only transitory, and bodies that become comets pass from a source population, such as the Kuiper Belt, Oort Cloud or their extra-solar analogues, through the active cometary phase, to eventual dormancy or destruction. We discuss dynamical delivery channels that can move comets from their source reservoirs to orbits with small periapsides, and the depletion of these reservoirs by dynamical and collisional means. We also discuss the physical evolution of cometary nuclei, especially in light of recent advances from missions to Solar System comets such as Rosetta's visit to 67P. We then describe our current knowledge of interstellar objects, which can originate from the same source regions as exocomets but be amenable to detailed study when they enter the Solar System. We include a summary of stellar winds emanating from different types of stars, which become increasingly strong once stars leave the Main Sequence. This is followed by a description of how small bodies are affected by stellar evolution, and the range of comet-like phenomena observed towards white dwarf stars. Overall, while we have an increasingly good picture of the physical and dynamical evolution of Solar System comets, a number of large gaps remain in our knowledge of the physics of exocomets, related to our inability to directly probe these bodies and many of the planets that might be affecting their orbits.

We report on the observation and measurement of astrometry, photometry, morphology, and activity of the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object's region of the sky during routine commissioning. Facilitated by Rubin's high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of similar to 70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of similar to 3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at similar to 0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-sectional ratio of eta greater than or similar to 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g - r = (0.657 +/- 0.013) mag, r - i = (0.235 +/- 0.018) mag, i - z = (0.147 +/- 0.042) mag, and z - y = (0.047 +/- 0.052) mag. These data represent the earliest observations of this object by a large (greater than or similar to 8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin's Legacy Survey of Space and Time will provide after it begins in early 2026.

Despite decades of observations, the physical processes governing mass loss from small bodies beyond our Solar System remain poorly constrained. These “exocomets” are often treated as analogs of Solar System comet, yet the stellar environments they inhabit spans a wide range in terms of luminosity, stellar wind intensity, and evolutionary stage, leading to potentially very diverse physical behaviors. Within our Solar System, small bodies lose gas and dust through a range of mechanisms, including sublimation, desorption, impacts, and/or sputtering. Once released, the composition and dynamics of the ejecta are then altered by additional processes, such as dust sublimation, ionization, and radiation pressure. In extrasolar systems, these mechanisms unfold under vastly different radiative and plasma conditions, leading to a rich diversity of mass-loss pathways and observable signatures. This work reviews our understanding of the mechanisms driving mass loss from small bodies and the subsequent evolution of ejecta in diverse stellar environments. We compare the physical and chemical mechanisms that drive gas and dust production, such as sublimation, thermal and photon desorption, and investigate how they scale with stellar luminosity, temperature, and activity. We then examine the processes that modify the composition of the ejecta (e.g., dust sublimation, dissociation, or ionisation) and its dynamics (e.g., radiation pressure or stellar winds). To illustrate how these processes vary across different stellar environments, we use four well-studied planetary systems as case studies: the Sun, Pictoris, AU Microscopii, and WD 1145+017. By exploring how cometary tails behave under such diverse conditions, this work provides a physical framework for interpreting exocometary activity and sheds light on why A-type stars, such as the famous -Pictoris, are over-represented in the population of exocomet-hosting stars.

We report the detection of water activity in the third confirmed interstellar object, 3I/ATLAS, based on ultraviolet imaging with the Neil Gehrels Swift Observatory’s Ultraviolet/Optical Telescope. Assuming a reddening of 29% between 3325.7 and 5437.8 Å, measurements on 2025 July 31–August 1 yielded a first, marginal detection of OH (A ^2 Σ–X ^2 Π) emission near 3085 Å, corresponding to a water production rate of (0.74 ± 0.50) × 10 ^27 molecules s ^−1 . The subsequent visit on 2025 August 18–20 revealed a clear OH detection, implying a higher water production rate of (1.36 ± 0.35) × 10 ^27 molecules s ^−1 (40 kg s ^−1 ) at a heliocentric distance of 2.90 au. This places 3I/ATLAS among the few comets with confirmed OH emission beyond 2.5 au, where water ice sublimation from the nucleus is typically inefficient. The inferred production rate at 2.9 au implies an active area of at least 7.8 km ^2 , assuming equilibrium sublimation. This requires that over 8% of the surface is active, which is larger than activity levels observed in most solar system comets. Contemporaneous near-infrared spectroscopy indicated the presence of icy grains in the coma, which may serve as an extended source of water vapor.

We report the detection of water vapor associated with main-belt comet 358P/PANSTARRS on UT 2024 January 8-9 using the NIRSPEC instrument on board JWST. We derive a water production rate of QH2O=(5.0 +/- 0.2)x1025 molecules s-1, marking only the second direct detection of sublimation products of any kind from a main-belt comet, after 238P/Read. Similar to 238P, we find a remarkable absence of hypervolatile species, finding QCO2<7.6x1022 molecules s-1, corresponding to QCO2/QH2O<0.2 %. Upper limits on CH3OH and CO emission are also estimated. Photometry from ground-based observations shows that the dust coma brightened and faded slowly over similar to 250 days in 2023-2024, consistent with photometric behavior observed in 2012-2013, but also indicates a similar to 2.5x decline in the dust production rate between these two periods. Dynamical dust modeling shows that the coma's morphology as imaged by JWST's NIRCam instrument on 2023 November 22 can be reproduced by asymmetric dust emission from a nucleus with a midrange obliquity (epsilon similar to 80 degrees) with a steady-state mass-loss rate of similar to 0.8 kg s-1. Finally, we find similar Af rho-to-gas ratios of log10(Af rho/QH2O)=-24.8 +/- 0.2 for 358P and log10(Af rho/QH2O)=-24.4 +/- 0.2 for 238P, suggesting that Af rho could serve as an effective proxy for estimating water production rates in other active main-belt comets. The confirmation of water vapor outgassing in both main-belt comets observed by JWST to date reinforces the use of recurrent activity near perihelion as an indicator of sublimation-driven activity in active asteroids.

Small bodies exist in distinct populations within their planetary systems. These reservoir populations hold a range of compositions, which to first order are dependent on formation location relative to their star. We provide a general overview of the nature of the reservoirs that source exocomets, from the influence of the stellar environment through planetesimal formation to comparisons with Solar System populations. Once transitioned from a young protoplanetary disc to a debris disc, a star can expect to be rained with exocomets. While exocomets are predominantly detected to date at A-type stars, planetesimals plausibly exist across a range of stellar masses, based on exoplanet abundance, debris disc occurrence and white dwarf infall.

Water ice is a fundamental building material of comets and other bodies in the outer solar system. Yet the properties of cometary water ice are challenging to study, due to its volatility and the typical distances at which comets are observed. Cometary outbursts, impulsive mass-loss events that can liberate large amounts of material, offer opportunities to directly observe and characterize cometary water ice. We present a study of comet 243P/NEAT, instigated by a −3 mag outburst that occurred in 2018 December. Optical images and a 251-day light curve were examined to characterize the outburst and the comet’s quiescent activity. Variations in the quiescent light curve appear to be dominated by coma asymmetries, rather than changing activity levels as the comet approached and receded from the Sun. Furthermore, the light curve shows evidence for one to two additional small outbursts (–0.3 mag) occurring in 2018 September. The large 2018 December outburst likely ejected water-ice grains, yet no signatures of ice were found in color photometry, a color map, or a near-infrared spectrum. We discuss possible dynamical and thermal reasons for this nondetection. In this context, we examined the comae of comets 103P/Hartley 2 and C/2013 US _10 (Catalina), and we show that a one-to-one mapping between continuum color and the presence of water ice cannot be supported. We also discuss possible causes for the large outburst, and we find that there is an apparent grouping in the kinetic energy per mass estimates for the outbursts of five comets.