Interstellar Comets: When the Universe Sends its Messengers

Interstellar Comets: Billions of Years of Wandering for a Single Passage

Wandering bodies ejected from their system of origin

Interstellar comets are wandering celestial bodies, galactic nomads that orbit no star. Ejected from their system of origin during the formation of the planets, they drift freely through interstellar space, subject only to the collective gravity of the Milky Way. Unlike so-called "local" comets, which move on closed elliptical orbits around the Sun, they follow hyperbolic trajectories (an open curve whose eccentricity is strictly greater than 1, precluding a closed orbit around the Sun) (eccentricity greater than 1).

Hyperbolic trajectories with no return

These objects cross our stellar neighborhood only once, by the pure chance of their trajectory, without ever being captured by the Sun's gravity, before departing toward the depths of the Galaxy for hundreds of millions of additional years.

N.B.:
During the birth of a stellar system, the gravitational perturbations of the forming planets eject billions of small bodies into interstellar space. Our own Sun released a considerable quantity of such debris. This debris now wanders through the Galaxy, carrying the chemical imprint of its system of origin (minerals, ices, organic molecules) like so many "messages in bottles" in the cosmic ocean.

The discovery of 'Oumuamua: a historic first

A chance detection by Pan-STARRS1

On October 19, 2017, astronomer Robert Weryk (born in 1982), working with the Pan-STARRS1 telescope in Hawaii, detected an object moving at an unusual speed, far greater than solar gravity alone can explain. The object received the designation 1I/'Oumuamua (first confirmed interstellar object, whose Hawaiian name means approximately 'scout sent from afar in advance'), a Hawaiian term meaning approximately "scout from afar". It was the first confirmed interstellar object ever detected in our solar system.

The object was detected while it was already moving rapidly away from the Sun, after its perihelion. Astronomers had only a few weeks to observe it before it became too faint. Spectroscopy attempts, which would have made it possible to identify molecules on its surface or outgassing, were largely unsuccessful, for lack of a sufficient signal.

An unexplained non-gravitational acceleration

Its baffling behavior immediately sparked intense scientific controversy. Its shape appeared highly elongated and showed a periodic variation in brightness suggesting a tumultuous rotation. Even more surprisingly, its acceleration did not fully match the gravitational predictions: an excess of thrust, non-gravitational in nature, was measured.

Borisov: the first confirmation of an interstellar comet

An amateur discovery from Crimea

On August 30, 2019, Ukrainian amateur astronomer Gennady Borisov (born in 1966) discovered, from his observatory in Crimea, an object with a distinctly cometary appearance. Its hyperbolic orbit was quickly calculated: its 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 display unambiguously classic cometary activity.

A revealing chemical composition

Unlike 'Oumuamua, 2I/Borisov proved to be much more familiar. Spectroscopic analyses conducted from the ground and from space revealed the presence of carbon monoxide (CO), water (H₂O) and cyanide (CN), compounds commonly found in the comets of our own solar system. Studies published in the journal Nature Astronomy by the teams of Piotr Guzik and Bin Yang in 2020 showed that this comet had a particularly high proportion of CO, far more than the average of solar comets, suggesting a formation in a cold environment, at a great distance from its star of origin.

For the first time, humanity could directly analyze the chemical composition of a celestial body formed in another stellar system, thereby providing observational constraints on the processes of planetary formation on a galactic scale.

Several criteria make it possible to identify an interstellar object

The four orbital and dynamical criteria

The identification of an interstellar object relies on several complementary physical and orbital criteria.

• An orbital eccentricity greater than 1: the orbit is open, hyperbolic, and the object cannot be gravitationally bound to the Sun.
• The residual velocity after the passage: an object bound to the Sun gradually slows down as it moves away, until it stops and turns back. An interstellar object, on the other hand, retains a positive velocity even at a very great distance: it never stops. For 'Oumuamua, this residual velocity was about 26 km/s, and for 2I/Borisov about 32 km/s.
• A positive orbital energy (\(\varepsilon >0\)): this energy measures the balance between what the object possesses (its kinetic energy \(\frac{v^2}{2}\)) and what the Sun "costs" it to escape (its potential energy \(\frac{GM_\odot}{r}\)). If the balance is negative, the object is a prisoner of the Sun. If it is positive, the object is free: this is the case of every interstellar comet.
• A consistent direction of origin: when tracing the trajectory backward, the object's radiant point must point toward interstellar space, with no connection to the known reservoirs of solar system comets such as the Oort cloud or the Kuiper belt.

Summary table of confirmed objects

The following table summarizes the two interstellar objects confirmed to date, with their main orbital and physical characteristics.

What interstellar comets teach us about the Galaxy

Natural samples of extrasolar matter

Beyond scientific curiosity, interstellar comets are natural samples of extrasolar matter. Their chemical composition reflects the physical and chemical conditions that prevailed in the protoplanetary disk of their star of origin: temperatures, isotopic ratios, molecular abundances. Every interstellar object is, in a way, the chemical imprint of an unknown stellar environment.

Panspermia and galactic chemical exchanges

The idea that chemical materials circulate from one stellar system to another via such objects is closely linked to the concept of panspermia (a hypothesis according to which microorganisms or their chemical precursors could be transported between stellar systems by meteorites, comets or other celestial bodies). If complex organic molecules, or even prebiotic compounds, can survive the interstellar journey, the possibility of a chemical exchange between stellar 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 bathes in a continuous flow of matter from elsewhere, a permanent exchange of planetary debris that weaves, over billions of years, a kind of common chemical fabric between the stellar systems of the Milky Way.