Interstellar comets are wandering celestial bodies, galactic nomads that do not orbit any star. Ejected from their original system during planet formation, they drift freely in interstellar space, subject only to the collective gravity of the Milky Way. Unlike "local" comets, which orbit the Sun in closed elliptical paths, they follow hyperbolic trajectories (eccentricity greater than 1). They cross our stellar neighborhood only once, by chance, never captured by the Sun's gravity, before returning to the depths of the Galaxy for hundreds of millions of years more.
N.B.: During the birth of a star system, gravitational perturbations from forming planets eject billions of small bodies into interstellar space. Our Sun itself released a considerable amount of such debris. These debris now wander the Galaxy, carrying the chemical imprint of their original system (minerals, ices, organic molecules) like so many "messages in a bottle" in the cosmic ocean.
On October 19, 2017, astronomer Robert Weryk (born 1982), working with the Pan-STARRS1 telescope in Hawaii, detected an object moving at an unusual speed, far greater than what solar gravity alone could explain. This object was designated 1I/'Oumuamua, a Hawaiian term meaning roughly "scout from afar arriving first." It was the first confirmed interstellar object ever detected in our solar system.
The object was detected as it was already moving rapidly away from the Sun after perihelion. Astronomers had only a few weeks to observe it before it became too faint. Spectroscopy attempts, which could have identified surface or outgassing molecules, were largely unsuccessful due to insufficient signal.
Its puzzling behavior immediately sparked intense scientific controversy. Its shape appeared highly elongated, with periodic brightness variations suggesting chaotic rotation. Even more surprisingly, its acceleration did not fully match gravitational predictions: a non-gravitational excess thrust was measured.
On August 30, 2019, Ukrainian amateur astronomer Gennady Borisov (born 1966) discovered an object with a distinctly cometary appearance from his observatory in Crimea. Its hyperbolic orbit was quickly calculated: an eccentricity of 3.356 left no doubt about its interstellar origin. The comet received the official designation 2I/Borisov, the second confirmed interstellar object, and the first to unambiguously display classic cometary activity.
Unlike 'Oumuamua, 2I/Borisov proved much more familiar. Spectroscopic analyses from the ground and space revealed the presence of carbon monoxide (CO), water (H₂O), and cyanide (CN), compounds commonly found in comets from our own solar system. Studies published in Nature Astronomy by Piotr Guzik and Bin Yang in 2020 showed that this comet had a particularly high CO ratio, much higher than the average for solar comets, suggesting formation in a cold environment, far from its original star.
For the first time, humanity could directly analyze the chemical composition of a celestial body formed in another star system, providing observational constraints on planetary formation processes on a galactic scale.
| Designation | Discovery date | Discoverer / Program | Eccentricity | Velocity \(v_\infty\) (km/s) | Cometary activity | Status |
|---|---|---|---|---|---|---|
| 1I/'Oumuamua | October 19, 2017 | Robert Weryk, Pan-STARRS1 (Hawaii) | 1.201 | about 26 | None detected | Confirmed interstellar |
| 2I/Borisov | August 30, 2019 | Gennady Borisov, Crimean Observatory | 3.356 | about 32 | Well-developed coma and tail | Confirmed interstellar |
Beyond scientific curiosity, interstellar comets are natural samples of extrasolar material. Their chemical composition reflects the physical and chemical conditions that prevailed in the protoplanetary disk of their original star: temperatures, isotopic ratios, molecular abundances. Each interstellar object is, in a way, the chemical fingerprint of an unknown stellar environment.
The idea that chemical materials circulate from one star system to another via such objects is closely linked to the concept of panspermia. If complex organic molecules, or even prebiotic compounds, can survive interstellar travel, the possibility of chemical exchange between star systems cannot be ruled out.
More fundamentally, the detection and study of these objects confirm that our solar system is not isolated in the Galaxy. It is immersed in a continuous flow of material from elsewhere, a permanent exchange of planetary debris that, over billions of years, weaves a kind of common chemical fabric between the star systems of the Milky Way.
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