Could 1I/'Oumuamua be an icy fractal aggregate? (1902.04100v2)

Abstract: 1I/'Oumuamua is the first interstellar interloper to be detected, and it shows a non-gravitational acceleration that cannot be accounted for by outgassing, given the strict upper limits of outgassing evident from {\it Spitzer} observations, unless the relative abundances of the common volatiles are very different to those in comets. As an alternative, it has been suggested that its peculiar acceleration is due to radiation pressure, requiring a planar-sheet geometry of an unknown natural or artificial origin. Here we assess whether or not the internal structure of 1I/'Oumuamua, rather than its geometry, could support a radiation-pressure-driven scenario. We adopt a mass fractal structure and find that the type of aggregate that could yield the required area-to-mass ratio would have to be extraordinarily porous, with a density $\sim$ 10$^{-5}$ g cm$^{-3}$. Such porous aggregates can naturally arise from the collisional grow of icy dust particles beyond the snowline of a protoplanetary disk, and we propose that 1I/'Oumuamua might be a member of this population. This is a hypothesis worth investigating because, if this were the case, 1I/'Oumuamua would have opened a new observation window on to the study of the building blocks of planets around other stars. This could set unprecedented constraints on planet formation models.\end{abstract}

• The paper evaluates the radiation pressure hypothesis as a driver for 1I/'Oumuamua’s non-gravitational acceleration.
• It proposes that an extremely porous, icy fractal aggregate structure can create the high area-to-mass ratio observed.
• The study highlights implications for planetesimal formation, offering new insights into the early processes of exoplanetary development.

Could 1I/'Oumuamua be an Icy Fractal Aggregate?

The paper investigates the unusual case of 1I/'Oumuamua, the first interstellar object detected passing through our solar system. Upon observation, it was noted that 1I/'Oumuamua exhibited non-gravitational acceleration in its outbound trajectory that could not be solely attributed to typical factors such as outgassing, which lacked evidence based on Spitzer observations. This paper explores a novel hypothesis that 1I/'Oumuamua's unusual acceleration might be explained by radiation pressure, influenced by its internal structure rather than its geometry or composition alone.

Key Insights and Findings

1. Radiation Pressure Hypothesis: The paper evaluates the possibility that 1I/'Oumuamua's acceleration could be due to radiation pressure acting upon a body with a high area-to-mass ratio, a condition previously associated with thin natural or artificial planar structures, such as a lightsail. Bialy and Loeb proposed this scenario, estimating that such an area-to-mass ratio would necessitate an extremely porous structure.
2. Fractal Aggregate Composition: To achieve the needed area-to-mass ratio, the paper suggests that 1I/'Oumuamua could be a mass fractal aggregate composed primarily of icy particles. This structure would have a density as low as $\sim 10^{-5}$ g/cm$^3$, a property that is typical of fluffy aggregates formed beyond the snowline in a protoplanetary disk. The model suggests a fractal dimension conducive to optimal radiation pressure influence.
3. Implications for Planetary Formation: Should 1I/'Oumuamua fit this model, it implies an observational window into the fundamental building blocks of exoplanets and the processes involved in their formation. The paper posits that these aggregates could promote planetesimal growth by overcoming the radial drift and fragmentation barriers within protoplanetary disks.
4. Challenges and Future Directions: Several challenges are noted in validating this hypothesis, such as understanding the ejection mechanisms from the host system and the aggregates' survival during an interstellar journey. Furthermore, understanding their optical properties and structural integrity under environmental stresses remains critical.

Implications and Speculations

The proposed fractal aggregate model could potentially reshape our understanding of the diversity and characteristics of interstellar objects. Should further observational or experimental evidence support the presence of such low-density bodies, it could significantly impact theoretical models of planetary formation by introducing a new category of minor cosmic bodies characterized by substantial porosity and unique structural dynamics.

In summary, the paper provides a plausible alternative to outgassing-driven hypotheses for 1I/'Oumuamua's behavior by presenting a fractal aggregate model influenced by radiation pressure. This demands further investigation to confirm its viability and implications for broader astronomical and planetary sciences. The notion that 1I/'Oumuamua represents more than a solitary cosmic traveler but a window into planetary formation processes across the galaxy presents a scientifically rich avenue for future research.