Since its discovery in 2017, 1I / ʻOumuamua has cultivated the mystery around its nature and its origins. A new study suggests that the interstellar object is made up largely of dihydrogen ice. According to this hypothesis, ʻOumuamua would have formed in a giant molecular cloud.
1I / ʻOumuamua was discovered on October 19, 2017 by the Pan-Starrs program and was initially classified as a comet due to its trajectory quickly identified as hyperbolic. However, the absence of cometary activity observed – neither hair nor tail – led to his prompt reclassification among the minor planets (asteroids and similar) before he inaugurated the newly established list of interstellar objects.
Despite the absence of observed activity, astrometric measurements showed that ʻOumuamua underwent a non-gravitational acceleration on its exit path from the Solar System. Such acceleration is usually observed when matter escapes from a comet, which pushes the nucleus through an action-reaction effect. However, the data show that the acceleration of ʻOumuamua is incompatible with a jet caused by sublimation of water ice as can be observed for comets in the Solar System.
A visitor made of hydrogen ice
In a new study accepted for publication in the Astrophysical Journal Letters (available in prepublication on arXiv), Darryl Seligman, postdoctoral researcher in the geophysics department of the University of Chicago, and Gregory Laughlin, professor in the department of astronomy of the university Yale, propose that ʻOumuamua contained a significant fraction of dihydrogen ice (H2). According to them, this could explain all the observed properties of ʻOumuamua. Sublimation of dihydrogen at a rate proportional to the incident solar flux produces a jet covering the surface which reproduces the observed acceleration. The loss of mass by sublimation leads to a monotonous increase in the ratio between the axes of the body, explaining the form of ʻOumuamua. Tracing the trajectory of ʻOumuamua through the Solar System makes it possible to calculate its mass and its aspect ratio before meeting the Sun.
Seligman and Laughlin show that dihydrogen-rich bodies can form in the coldest dense nuclei of giant molecular clouds, where densities are in the order of one hundred thousand to one million molecules per cubic centimeter and temperatures approach 3 Kelvin (-270 ° C) from the cosmic microwave background. For comparison, one cubic centimeter of air under usual conditions (20 ° C and 1 bar) contains around 250 billion billion molecules.
The researchers explain that exposure to galactic cosmic rays after the formation of ʻOumuamua implies an age of around 100 million years.
Are the mysterious origins of ʻOumuamua finally resolved?
Article by Laurent Sacco published on 04/18/2020
Numerical simulations, using a scenario involving the tidal forces of a red dwarf, reveal for the first time the strange characteristics of ʻOumuamua. The interstellar object would be the fragment of a celestial body destroyed and deformed by these forces when it passed too close to its host star.
We remember the growing surprise of astronomers following the detection in October 2017 of the passage of 1I / 2017 U1, now known as ʻOumuamua, a name of Hawaiian origin, partly because it had been identified using the Pan-Starrs (acronym for Panoramic Survey Telescope And Rapid Response System), an instrument present at the top of Haleakalā, in Maui (Hawaii).
Its speed and trajectory necessarily implied that it was an interstellar traveler, a celestial body formed beyond the Solar System. But what was most amazing with ʻOumuamua was not so much the fact that he was on a hyperbolic trajectory leading him to leave the Solar System forever at almost 90 km / s. No, it was its totally new form that was amazing.
Of reddish appearance, ʻOumuamua seemed to be visibly cigar shaped with a length 10 times greater than its width – some estimates attributed it to a length of 800 meters for a width of about 80 meters. This peculiarity was such that we wondered at one point if it could not be an artifact of an advanced extraterrestrial civilization, a bit like in the famous novel by Arthur Clarke, Rendez-vous with Rama. But the Seti program did not discover anything about it while trying to listen to it make radio transmissions and, in the opinion of the scientific community, we were in the presence of an object similar to an asteroid, or even a comet, but very depleted in volatile elements, it seemed.
ʻOumuamua, a fragment of a comet destroyed by tidal forces?
To try to account for its strange shape, never seen for an asteroid or a comet in the Solar System, Sean Raymond, from the Astrophysics laboratory of the University of Bordeaux, had published with his colleagues Philip J. Armitage and Dimitri Verasun an article that can be viewed on arXiv. The three astrophysicists involved the tidal forces of a giant planet around another star, near which a cometary body would have passed too close and which would have been fragmented a little as in the case of the famous comet Shoemaker-Levy 9 before one of the fragments is ejected into interstellar space as explained in more detail by Sean Raymond in the video below.
Today, it is a similar scenario which has just been exposed in an article of Nature Astronomy by two colleagues of Sean Raymond, Yun Zhang of the Lagrange laboratory (Université Côté d’Azur – Observatoire de la Côte d’Azur – CNRS ) and Doug Lin, of the Department of Astronomy and Astrophysics, of the University of California at Santa Cruz. The two researchers conducted simulations which, for the first time, seem to account for all of the amazing characteristics of ʻOumuamua.
Recall that ʻOumuamua has not been observed degassing directly like a comet when we might have expected. Indeed, according to the example of the Solar System and the calculations made, it is easier and more frequent to eject a cometary body on a very eccentric orbit having led it to pass too close to a giant planet or a star than an asteroid. It was therefore to be expected that ʻOumuamua was a cometary body rich in water and volatile elements. This does not seem to be the case.
A red dwarf at the origin of ʻOumuamua?
To solve this riddle and account for the very elongated shape of ʻOumuamua, Yun Zhang and Doug Lin involved the very close passage of a body made up of an aggregate of rocks but also of ice, a situation often encountered in the System solar, with the host star of its original planetary system. Its mass is half that of the Sun in the numerical simulations carried out, which makes it a red M-type dwarf, the most numerous stars in the Milky Way. Above all, the theory of stellar structure tells us that such a star is particularly dense, which endows it with an extended region where its tidal forces can destroy a celestial body. It is therefore a situation which should be the most frequent in a scenario with destruction, then ejection of a small celestial body by the force of gravity of a star.
The effect of these forces was therefore tested on a computer by taking a celestial body of about 100 meters in diameter on a very elliptical trajectory, therefore with a strong eccentricity, and whose average distance to the star is several thousand times the distance from Earth to the Sun, which is necessary for the fragments produced by its destruction to be ejected into the interstellar medium as explained in a press release from the Lagrange laboratory.
The calculations show that the tidal forces will then first stretch, then fragment the celestial body which will then be dislocated by giving bodies of the shape attributed to ʻOumuamua. Better, the modeling also indicates that the passage close to the red dwarf will cause the fragments to heat up enough for the rocks to melt on the surface in addition to producing a vaporization of the ice. In the end, a few hours after the equivalent of the passage to the perihelion of the Sun, the surface cools giving a silicate crust devoid of water and depleted in volatile elements. It then resembles more that of an asteroid than that of a comet, especially due to the sintering of silicates in the crust, the cohesion of the object is increased when it continues to be deformed and that the rocks are not yet fully cooled.
In the end, there would still be a reservoir of ice, be it water or carbonaceous compounds such as monoxide (CO) and carbon dioxide (CO2) as soon as we descend to more than a meter deep under the crust. Even if ʻOumuamua did not present a cometary activity under the gaze of the telescopes, the celestial mechanics had all the same noticed a small abnormal acceleration, of which the attraction of the bodies of the Solar System could not account for when the interstellar object had approached the Sun. We can explain this anomaly if a slight degassing still occurred, which is consistent with the scenario still making ‘Oumuamua a cometary body. There is no need to revive the hypothesis of the interstellar probe E.T as some have proposed. Artist’s view of the formation of ʻOumuamua based on the script by Zhang and Lin. © YU Jingchuan from Beijing Planetarium
Myriads of ʻOumuamua in the Milky Way and bearers of life?
If the researchers are right, ʻOumuamua would be the tip of a population of interstellar objects which would have originated in the equivalents of the famous cometary belt of Oort around the Sun, but around the red dwarfs. They will therefore initially be cousins of long-lived comets observed in the Solar System, kilometer-sized bodies born far from the Sun and which are projected towards it by gravitational disturbances during the close passage of a star as was the case there 70,000 years ago, a small binary star called the Scholz star, which would have passed 0.8 or even 0.6 light years from the Sun.
On average, each planetary system is expected to eject a hundred billion objects in total like ʻOumuamua. According to the researchers, some of the parent bodies could also be superterres or mini-Neptune, also destroyed there by tidal forces. As these objects are very numerous and they must pass by perhaps inhabited exoterres, as was the case for ‘Oumuamua with the Earth, it is possible to question their role within the framework of the theory panspermia.
“It’s a whole new area. These interstellar objects could provide critical clues to how planetary systems form and evolve, “says Yun Zhang, and according to Doug Lin,” ʻOumuamua is just the tip of the iceberg. We anticipate that many other interstellar visitors with similar traits will be discovered by an upcoming observation with the future Vera C. Rubin Observatory. ”
The interstellar object “Oumuamua would be a” dust sheep “
INSU article published on 12/01/2019
A study suggests that ‘Oumuamua, the first interstellar object discovered, could be formed from an aggregation of micrograins. It could be the most porous celestial body ever observed in our Solar System.
In October 2017, an observatory in Hawaii detected for the first time a celestial body moving on a hyperbolic trajectory, unlike all the objects observed until then in the Solar System such as comets, planets and asteroids which are moving in closed orbits around the Sun. Proof that the mysterious object comes from elsewhere, from another star.
Called ‘Oumuamua which in Hawaiian means’ guest’, ‘messenger’ or ‘scout’, it has been established that 1I / 2017 U1 (official classification) is approximately 400 meters long and was, when it was discovered, about to get out of the Solar System and never come back.
A speed unexplained by gravity in the Solar System
The exotic guest carries information about star systems that may be completely different from ours. One of the many unanswered questions is related to its particular movement. Indeed, in addition to following an open trajectory, ‘Oumuamua also travels at a speed a little faster than what can be explained by the known gravity in our Solar System.
Leaving aside the speculation that it could be an extraterrestrial spacecraft, only solar radiation can explain the additional power necessary to justify the trajectory observed. The hypothetical degassing reaction forces were indeed evaluated as too weak for this object which did not have a cometary tail.
Now, if the radiation pressure of the Sun is indeed the cause of this non-gravitational acceleration, the mass density of ‘Oumuamua must be extremely small, not exceeding 1% of the mass density of air on the surface of the Earth, about 10 grams per cubic meter. This extremely low density makes the interstellar host the most porous object ever observed in our Solar System – and therefore constitutes a particularly interesting object of study.
A fractal structure
According to a study published on November 11, 2019 in The Astrophysical Journal Letters, ‘Oumuamua could be a large aggregate of dust. More precisely, it would have a fractal structure: that is to say that it would include holes of all sizes, as illustrated in the figure above. Fractal asteroids have previously been regularly identified after falling on Earth or in comet tails, but are usually fragments of millimeter or centimeter size.
According to the authors of this research, even objects with an extremely low density would survive the forces to which they were exposed in our Solar System: the branched structure of ‘Oumuamua, potentially formed by the entanglement of dust particles at the beginning of evolution from another star system, would collapse immediately if it were on the surface of the Earth. But far from our planet, such an object would be able to preserve its structure in the face of the gravity of the Sun, the forces of radiation and, moreover, the forces resulting from its own rotation. In a sense, this study shows that space is a very soft place.