Looking purely asteroidal, the object nevertheless presented several peculiarities, of which the most intriguing were the non-gravitational acceleration detected in its motion and a unique rotational light curve, exhibiting an enormous amplitude suggesting a peculiar shape, as well as small irregularities revealing rotational tumbling (Micheli et al. 2018; Drahus et al. 2018). Early observations of 1I\/\u2018Oumuamua did not reveal any traces of cometary activity. Even the deepest images showed a completely star-like object. The limits on dust production rates were as low as 2 g\/s or even 0.2 g\/s, depending on the assumed dust particle size distribution (Meech et al. 2017; Jewitt et al. 2017). The search for gaseous emission yielded no detections, though it was limited by the discovery circumstances. The object was discovered a few weeks after perihelion, around its closest approach to Earth, and faded extremely quickly as it receded from both Earth and the Sun.<\/p>\n
A search for OH 18 cm radio emission using the Green Bank Telescope resulted in a limit of Q[OH] < 1.7\u00d710\u00b2\u2077 molecules\/s, equivalent to a water production rate < 60 kg\/s (Park et al. 2018). However, this limit was obtained a few weeks after discovery, when the object was already much farther from both the Sun and Earth. Infrared observations with the Spitzer Space Telescope were carried out even later. No signal from \u2018Oumuamua was found, leading to a crude estimate of the water production limit < 1.8 kg\/s, assuming a 5:1 dust-to-gas ratio (Trilling et al. 2018).<\/p>\n
The mysterious non-gravitational acceleration, discovered in the astrometric data several months later when 1I\/\u2018Oumuamua was already too far away to be observed, could be explained by water outgassing at the level of ~10 kg\/s. However, this order-of-magnitude estimate required either dust production rates inconsistent with the observations or a bizarre, previously unobserved cometary dust size distribution lacking any particles smaller than a few hundred \u03bcm (Micheli et al. 2018). The lack of any apparent cometary activity, combined with the unresolved mystery of its non-gravitational motion, sparked numerous hypotheses about the nature of 1I\/\u2018Oumuamua. For example, it was speculated to be a molecular hydrogen iceberg (Seligman & Laughlin 2020), an N\u2082 ice fragment from the surface of an exo-Pluto (Jackson & Desch 2021), a piece of dark matter (Cyncynates et al. 2017), or even an artificial object, such as a solar sail (Bialy & Loeb 2018) or a part of a Dyson sphere. Another hypothesis proposed an H\u2082O-rich icy body releasing H\u2082 produced by irradiation of water ice (Bergner & Seligman 2023).<\/p>\n
![\"\"](\"https:\/\/www.hyades.oa.uj.edu.pl\/esa\/wp-content\/uploads\/2026\/11\/Fig_03-1024x335.jpg\")
A family portrait of the first identified interstellar interlopers. Left: Deepest image of 1I\/\u2018Oumuamua, obtained by Gemini North (Drahus et al. 2018, Nat. Astron. 2, 407). Middle: First color image of 2I\/Borisov, obtained by Gemini North (Guzik et al. 2020, Nat. Astron. 4, 53). Right: High-resolution image of 3I\/ATLAS, captured by Hubble; credit: NASA, ESA, STScI, D. Jewitt (UCLA); image processing: J. DePasquale (STScI).<\/p><\/div>\n
2I\/Borisov: a familiar comet from another star<\/strong><\/p>\n Two years later, in 2019, another interstellar interloper, comet 2I\/Borisov, was found as it passed through the Solar System. It turned out to be strikingly similar to our local comets, both in morphology and chemical composition (Guzik et al. 2020; Opitom et al. 2021). Thanks to its discovery a few months before perihelion and its relatively high brightness, it was studied in much greater detail than 1I\/\u2018Oumuamua. Spectroscopic studies revealed most of the typical cometary species, with relative abundances similar to those observed in Solar System comets. As is often the case for our native comets observed with ground-based optical telescopes, the first species detected in the spectrum of 2I\/Borisov was CN, followed shortly by C\u2082 (Fitzsimmons et al. 2019; Lin et al. 2020).<\/p>\n Subsequent detections of O I, NH\u2082, OH and HCN were also consistent with our knowledge of Solar System comets (McKay et al. 2020; Xing et al. 2020; Cordiner et al. 2020; Opitom et al. 2021). The somewhat high abundance of CO also fell within the broad range of values previously observed in carbon monoxide-rich comets (Cordiner et al. 2020; Bodewits et al. 2020). Even the unexpected detection of gaseous atomic nickel in the coma of 2I\/Borisov turned out to be a common but previously overlooked feature of comets native to the Solar System (Guzik & Drahus 2021; Manfroid et al. 2021). On its way out of the inner Solar System, 2I\/Borisov underwent outbursts and nucleus fragmentation\u2014yet another relatively common characteristic of ordinary comets (Drahus et al. 2020; Jewitt et al. 2020)!<\/p>\n 3I\/ATLAS: unlocking the isotopic secrets of interstellar water<\/strong><\/p>\n The third and most recent interstellar interloper, 3I\/ATLAS, was discovered in 2025, entering the Sun\u2019s gravitational influence at a frightening speed of 58 km s\u207b\u00b9 (Bolin et al. 2025). Since its discovery, 3I\/ATLAS has been intensively observed by major astronomical facilities, including JWST, Hubble, ALMA, ESO\/VLT, Gemini, Palomar, IRAM, JCMT, and many others. Early spectroscopic observations, obtained by JWST at a heliocentric distance of 3.2 au, revealed strong CO\u2082 outgassing, with the other major cometary species H\u2082O and CO also present at lower levels (Cordiner et al. 2025). Meanwhile, the VLT detected emission from the common cometary trace species Ni I and CN as far as 3.65 au from the Sun (Rahatgaonkar et al. 2025).<\/p>\n A major breakthrough came with measurements by JWST and ALMA, which enabled the first determination of the deuterium-to-hydrogen ratio in the water of an interstellar comet (Cordiner et al. 2026; Salazar Manzano et al. 2026). These results reveal a dramatically elevated D\/H ratio, clearly setting 3I\/ATLAS apart from Solar System comets and pointing to a non-local origin. JWST measured approximately 63 times the terrestrial VSMOW value, while ALMA independently established a lower limit of D\/H > 6.6\u00d710\u207b\u00b3. Such a high level of deuteration implies that the bulk of the object\u2019s water ice formed at very low temperatures below 30 K, with little subsequent reprocessing at higher temperatures. Together with elevated carbon isotopic ratios, this favours an ancient origin, potentially as long ago as 12 billion years, although the age remains model-dependent (Cordiner et al. 2026).<\/p>\n What secrets will the next visitors reveal?<\/strong><\/p>\n While we see an evident progression, from the molecular non-detections in 1I\/\u2018Oumuamua, through the first molecular detections in 2I\/Borisov, to the detailed isotopic investigation of 3I\/ATLAS, this evolution has been enabled largely by the somewhat surprising increase in brightness from one visitor to the next. Evidently, the remarkable results for 3I\/ATLAS required the combination of the object\u2019s almost unbelievably high brightness with the shockingly high D\/H ratio in its water. With this in mind, a tempting question arises: would the previous interstellar comet, 2I\/Borisov, still be considered a twin of Solar System comets if we had the means to unlock its isotopic composition?<\/p>\n Many more discoveries are expected from LSST, but most new visitors are likely to be much fainter than 3I\/ATLAS (Hoover et al. 2022). Each such body is a unique piece of material from another planetary system, which can be investigated up close using the well-established methods of planetary astronomy. However, there is a burning need for a dedicated instrument able to rapidly observe newly discovered interstellar objects and detect even small amounts of gas. Such observations could help resolve the next \u2018Oumuamua-like mystery and extend the pioneering D\/H measurements in 3I\/ATLAS to other visitors from beyond our Solar System.<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" According to solar system formation models, most of the unused primitive material was ejected into interstellar space by gravitational instabilities shortly after the formation of gas giants (Raymond et al. 2018). Similar processes are expected to occur in countless other planetary systems, populating the Galaxy with myriads of wanderers. Although […]<\/p>\n","protected":false},"author":4,"featured_media":46,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_monsterinsights_skip_tracking":false,"footnotes":""},"class_list":["post-239","page","type-page","status-publish","has-post-thumbnail","hentry"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t