A live broadcast promoted by the SETI Institute explored the discoveries made by the European Space Agency’s (ESA) Juice instrument about the interstellar comet 3I/ATLAS. The cosmic object was analyzed during its passage through the inner Solar System, offering a unique window into the study of material from other stellar systems.
Observing a celestial body formed around a star other than our own offers a rare opportunity to compare planetary systems. Furthermore, the campaign demonstrated how existing spacecraft can be adapted to generate unexpected scientific discoveries. Preliminary results indicate that although 3I/ATLAS has an extrasolar origin, its activity, coma and tails resemble those of many comets that orbit our own Sun.
The initiative also highlighted the importance of international collaborations between spacecraft and ground-based observatories. This union of efforts is crucial for the study of future interstellar visitors, maximizing the knowledge gained in their brief passages.
Scientists detected 3I/ATLAS in 2025, identifying it as the third interstellar object confirmed to cross the Solar System. This discovery triggered an immediate global mobilization, with telescopes from different parts of the world being aimed at the rare visitor.
In a recent conversation broadcast on SETI Live, SETI Institute planetary astronomer Dr. Franck Marchis spoke with planetary scientist Dr. Cecilia Tubiana about the opportunity to study this celestial body. However, a crucial phase of the comet’s journey presented a challenge: during perihelion, the moment of its greatest proximity to the Sun, ground-based observatories were unable to capture it, as the Earth was on the opposite side of the star.
Faced with the limitations of telescopes on the ground, the scientific community decided to use the Jupiter Icy Moons Explorer (JUICE) probe, from the European Space Agency. Although not designed to investigate interstellar comets, Juice was in an ideal spatial position to accompany 3I/ATLAS at this otherwise inaccessible stage of its trajectory.
Dr. Tubiana detailed that the scientific team readily recognized the chance to observe comet 3I/ATLAS from space, despite considerable logistical obstacles. Observation campaigns for deep space missions generally require about nine months of planning, but this endeavor had to be organized within a few months of the comet’s detection. She stated that the final approval of the campaign highlighted the great scientific interest in investigating material originating from another planetary system.
Assessing the chemical composition of an extrasolar comet
The information discussed during the SETI Live conversation is still considered preliminary, as the observation data arrived on Earth recently. The Juice probe collected its records between November 5 and 25, 2025, however, the spacecraft’s great distance meant that data transmission took approximately three months to complete. The researchers only began analyzing the observations in the second half of February 2026, and many aspects of this dataset remain under evaluation.
One of the main focuses of the study was to determine the comet’s water production rate. Using the MAJIS imaging spectrometer, scientists detected spectral signatures of water molecules in the comet’s coma, using their intensities to estimate the amount of water vapor released by the celestial body.
Initial analyzes indicate that the comet’s water emission rate is relatively high compared to that of some well-studied comets. It significantly surpasses that of the short-period comet 67P/Churyumov-Gerasimenko, target of ESA’s historic Rosetta mission, although it does not reach the extreme levels of activity observed for Halley’s Comet.
Dr. Tubiana pointed out that, although the activity of comets often correlates with the size of their nucleus, there are notable exceptions. She cited Comet Wirtanen as an example of a relatively small comet, but classified as “hyperactive”, demonstrating that size alone does not define a comet’s behavior.
A celestial visitor who appears familiar
In addition to measuring water production, the Juice probe used its Ultraviolet Spectrograph (UVS) to investigate the comet’s interaction with solar radiation. Interestingly, NASA’s Europa Clipper spacecraft has an identical UVS instrument, which enabled both missions to perform coordinated observations of comet 3I/ATLAS from different points in space during November 2025.
Images captured over five observation windows revealed that the comet exhibited an evolution already known to astronomers. As 3I/ATLAS approached the Sun, its brightness increased, its coma expanded, and the dust and ion tails became more visible.
Using special ultraviolet filters, the Janus camera team was able to map both a conventional dust tail and a distinct ion tail, generated by ionized gases that are swept away by the solar wind. The researchers also recorded plasma interactions that notably modified the structure of the ion tail.
Evidence suggests that comet 3I/ATLAS behaves very similarly to comets found in our Solar System, despite its origin around another star. Rather than revealing a completely unknown type of object, the findings indicate that many of the physical processes that govern cometary activity may be common to diverse planetary systems. This similarity allows scientists to compare the formation and evolution of small bodies beyond our Solar System with those already studied closer to us.
For Dr. Tubiana, this striking similarity was one of the most significant revelations of the campaign. In his words, the object is “different, but not exotic”, sharing a fundamental affinity with the comets that astronomers already know.
Consolidating an observation network for interstellar objects
Although Juice’s observations revealed that comet 3I/ATLAS behaves very similarly to comets in our Solar System, they also highlighted an important difference. Dr. Tubiana explained that measurements from the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope indicate that the comet has a high deuterium-to-hydrogen (D/H) ratio. In simpler terms, a larger fraction of the hydrogen in its water molecules is replaced by deuterium, a heavier isotope of hydrogen, suggesting it formed in a colder environment than our Solar System.
According to Dr. Tubiana, this high D/H ratio indicates that the comet likely originated in a colder environment than the one where our own Solar System was formed. She highlighted that this is the first interstellar object for which such a measurement has been feasible, offering scientists a valuable clue about conditions existing in another planetary system.
The discussion also emphasized how the study of interstellar objects has progressed over the past decade. When 1I/’Oumuamua was discovered in 2017, astronomers had little time to organize coordinated observations, and many questions were left unanswered due to the object’s rapid exit from the Solar System.
With the arrival of 2I/Borisov in 2019 and 3I/ATLAS in 2025, the scientific community had already established a much more organized approach. Researchers around the world now quickly share data, jointly submit observation proposals, and combine data from spacecraft and ground-based facilities to maximize the scientific return from these rare visitors.
During the discussion, Dr. Marchis highlighted how quickly scientists formed an international observation community for comet 3I/ATLAS. They shared data, coordinated proposals, and combined observations from spacecraft and ground-based telescopes, a collaborative approach that allowed them to maximize the scientific return during the comet’s brief passage through the Solar System.
This collaborative strategy is especially crucial because interstellar objects remain for a limited time in the Solar System before continuing their journey through interstellar space. This way, each observation represents a unique chance that cannot be replicated.
For the future, Dr. Tubiana expressed optimism that upcoming observatories, like the Vera C. Rubin, will discover many more interstellar objects. With increased detection rates, scientists will have the ability to study a wider and more diverse population of visitors from other planetary systems, rather than relying on just a few examples.
The 3I/ATLAS campaign demonstrated that spacecraft already operating in deep space can be quickly adapted to observe unexpected discoveries. This strategy, combined with coordinated observations from ground-based observatories and state-of-the-art facilities, offers a robust framework for studying material formed around other stars and for deeper understanding of how planetary systems form and evolve across the galaxy.
Understanding the frequency of interstellar objects in our system
Although few interstellar objects have been confirmed so far, astronomers believe that many others cross the Solar System without being identified. Advances in survey technology and next-generation observatories are expected to dramatically increase the rate of discoveries in the coming years.
The importance of the Juice probe’s observations of comet 3I/ATLAS
The Juice probe was fundamental because ground-based telescopes were unable to observe the comet during its perihelion, as the Earth was positioned on the opposite side of the Sun in relation to the object. Juice’s unique position in space allowed scientists to study the comet during this phase, which would have been inaccessible by other means.
What interstellar comets can reveal about distant planetary systems
Interstellar comets preserve material from the early stages of planet formation around other stars. Studying its composition helps scientists compare how planets and smaller bodies form in different stellar environments across the galaxy, enriching our knowledge of cosmic diversity.
