1I/‘Oumuamua was detected on October 19, 2017, by the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1) located at Hawaii. Since the time of its discovery, speculations about its origin have never stopped. Observations showed no cometary activity, which is very odd since our understanding of planetary formation processes suggests that icy bodies are much easier to be ejected from their host systems than rocky bodies. While several processes have been proposed to explain ‘Oumuamua’s origins and peculiarities, none can naturally match its extreme shape (with an axis ratio > 6:1) and motion (tumbling and non-Keplerian). Some scientists even wondered if it was an alien probe.
Among the possibilities, tidal disruption followed by ejection is a possible way to eject rocky bodies. In space, some objects occasionally come very close to a bigger one. Tidal forces of the bigger one can tear these small ones apart, like the things happened to Comet Shoemaker-Levy 9 when it closely passed by Jupiter and Comet ISON flying by the Sun. But whether such process can explain ‘Oumuamua’s puzzling characteristics is highly uncertain.
" When you have eliminated the impossible, whatever remains, however improbable, must be the truth."-Sherlock Holmes
In the beginning of 2019, when ‘Oumuamua has disappeared for a year and its observational data have been well established, astrophysicist Douglas Lin and I decided to use our best numerical tool, PKDGRAV, to try this tidal formation idea. We simulated the structural dynamics of an object closely flying by a star. Promising results came out soon. We found that the star can dramatically split the object, if it comes enough close to the star, into elongated fragments and eject them into the interstellar space. The axis ratios of these fragments are up to 5:1, marginally matching ‘Oumuamua‘s shape. However, no matter how we adjusted the material and structural properties of the fly-by object, we couldn't produce a more elongated fragment.
In the meantime of deliberating on the shape issue, we began to look at the temperature evolution during the stellar close encounter. We found that the surface of fragments resulting from the disruption of the initial body would melt at very short distance to the star and re-condense at further distances. Like melting chocolate beans, the surface materials stick together and stop the further disintegration of fragments. We immediately ran some simulations to test the effect of enhanced material strength during the encounter. The axis ratio of the resulting fragments can be even larger than 10:1!!! And the cohesive strength ensures the structural stability of such elongated shape.
Heat diffusion during the stellar tidal disruption process also consumes large amounts of volatiles. These fragments' surfaces resemble ‘Oumuamua’s colors and lost visible coma. However, the temperature of their interiors only reached dozens of Kelvin. Some high sublimation temperature volatiles, like water ice, buried under the surface can remain in a condensed form. If ‘Oumuamua was produced and ejected by our stellar tidal disruption scenario, plenty of residual water ice could be activated during its Solar System passage. The resulting outgassing accelerations match ‘Oumuamua’s comet-like trajectory.
We have demonstrated a physical way to form one single ‘Oumuamua. But, the chance we were so lucky to discover ‘Oumuamua implies, on average, each planetary system should eject in total about a hundred trillion interstellar objects like ‘Oumuamua. Is the proposed scenario sufficiently efficient in producing this kind of objects?
Our calculations indeed certify the efficiency of stellar tides in producing ‘Oumuamua-like interstellar objects. Possible progenitors, including long-period comets, debris disks, and even super-Earths, can be transformed into ‘Oumuamua-size pieces during stellar encounters.
After all, we showed that all of ‘Oumuamua's characteristics can be consistently explained by a natural origin. No alien explanation is required. Our work highlights the prolificacy of ‘Oumuamua-like interstellar object population between stars. Since these objects pass through the domains of habitable zones, the prospect of transport of matter capable of generating life, called panspermia, by these objects cannot be ruled out. These interstellar objects could provide critical clues about how planetary systems form and evolve and how life started on the Earth.
The full article is accessible via this link: https://www.nature.com/articles/s41550-020-1065-8
Header image credit: Jing-Chuan Yu (Beijing Planetarium)