1I/‘Oumuamua as an N₂ Ice Fragment of an exo‐Pluto Surface: I. Size and Compositional Constraints

Abstract

when it was only 0.22 au from Earth and briefly "brightened" to a 20th-magnitude object. Its heliocentric orbit was soon found to be hyperbolic, with eccentricity e = 1.2, making 'Oumuamua the first definitive interstellar object discovered (Meech et al., 2017). In August 2019, a second object on a hyperbolic orbit (e = 3.36) was discovered from a home observatory, the interstellar comet 2I/Borisov. These two objects have been traveling through our solar system for centuries and must be part of a population of millions of such interstellar objects currently passing through the Solar System. This newly discovered population of objects provides an opportunity to probe the compositions and physical properties of analogs to comets and asteroids in extrasolar systems. In addition, they offer the opportunity to test whether the same processes that occurred in our solar system have occurred in other planetary systems.These two interstellar objects appear to very different. 2I/Borisov is readily recognised as a comet like those in our own solar system, actively outgassing species like CN (Fitzsimmons et al., 2019). 1I/'Oumuamua on the other hand appears very different from solar system comets and has been much more resistant to explanation. Several oddities of 'Oumuamua were compiled by Bialy and Loeb (2018), although most of its unusual properties are not as mysterious as they seem at first glance, as made clear in the review by 'Oumuamua ISSI Team et al. (2019). We review these here.

1I/‘Oumuamua as an N2 Ice Fragment of an Exo‐Pluto Surface II: Generation of N2 Ice Fragments and the Origin of ‘Oumuamua

“…We hypothesize here and in the companion paper (Jackson & Desch, 2021) that 'Oumuamua was originally a chunk of N 2 ice with mass ∼2.4 × 10 8 kg and mean radius ∼40 m, generated by collisions on the surface of a Pluto-like body and then ejected from another stellar system. To judge whether this is a plausible scenario, we first calculate the likelihood that such a body would be generated and ejected from our own Solar System.…”

“…Survival of fragments is not guaranteed, though, as comets in the Oort cloud, beyond the heliosphere boundary at ∼100 AU, would be eroded by GCRs. Over the 4.5 Gyr lifetime of the Solar System, N 2 ice fragments would have eroded as much as 260 m in radius, and H 2 O ice fragments as much as 30 m (Jackson & Desch, 2021), implying that objects with initial D < 60 m (if H 2 O) or D < 520 m (if N 2 ) could not survive to the present day to be observed as long‐period comets, although fragments with initial D > 1 km could. This will lead to an uncertain reduction in the number of fragments among long‐period comets, but if the fragment size distribution has a slope of q = 6, then fragments with D > 1 km would be only ∼20 −5 = 3.1 × 10 −7 times as numerous as those with D > 50 m, and collisionally generated fragments would therefore represent at most (0.2 × 2.7 × 10 15 × 3.1 × 10 −7 )/(0.2 × 6.7 × 10 11 ) ∼ 0.13% of all long‐period comets > 1 km in diameter, 1/2 of them, or 0.07% (one out of 1,400) being N 2 ice.…”

“…Jackson and Desch (2021) concluded that ‘Oumuamua was consistent with a fragment of N 2 ice produced by collisions with the surface of an exo‐Pluto in another stellar system. A fragment tens of meters in size, made of N 2 ice with trace amounts of CH 4 , would match ‘Oumuamua's size, albedo, color, constraints on outgassed species, and especially the observed nongravitational acceleration.…”

“…However, numerous models have been developed to explain how such a large number of objects could be produced and ejected from solar systems, summarized by ('Oumuamua ISSI Team et al, 2019). Jackson and Desch (2021) estimated that 'Oumuamua's mass upon entry to the Solar System was ∼1 × 10 8 kg, and could have been ∼2.4 × 10 8 kg upon ejection from its Solar System, before passage through the interstellar medium (ISM). Using the latter mass, this implies a density of 'Oumuamua-like objects in the ISM of 0.0006-0.08 M E pc −3 .…”

Interstellar planetesimals: potential seeds for planet formation?

“…We also need to take into account how the evaporation, processing, accretion and sputtering of interstellar objects in the different phases of the interstellar medium can affect the population of planetesimals. This is of interest regardless of the nature of the objects, but is particularly important if they are highly porous or are a fragment of H 2 ice (Seligman & Laughlin 2020) or N 2 ice (Jackson & Desch 2021), as it has been proposed for 1I/'Oumuamua. And generally, for all interstellar objects originating from planetary systems, it is important to understand how their ejection and entry (with a wide range of dynamical histories) can alter their physical properties (Raymond et al 2020).…”