Constraints on the Occurrence of ‘Oumuamua-Like Objects

Abstract

At present, there exists no consensus in the astronomical community regarding either the bulk composition or the formation mechanism for the interstellar object 1I/2017 U1 (‘Oumuamua). With the goal of assessing the merits of the various scenarios that have been suggested to explain ‘Oumuamua's appearance and observed properties, we report a number of new analyses and provide an up-to-date review of the current hypotheses. We consider the interpretations that can reconcile ‘Oumuamua's observed non-Keplerian trajectory with the nondetection of traditional cometary volatiles. We examine the ability of these proposed formation pathways to populate the galaxy with sufficient interstellar objects such that the detection of ‘Oumuamua by Pan-STARRS would be statistically favored. We consider two exotic ices, hydrogen and nitrogen, showing that the frigid temperature requirement for the former and the necessary formation efficiency of the latter pose serious difficulties for these interpretations. Via order-of-magnitude arguments and hydrodynamical cratering simulations, we show that impacts on extrasolar Kuiper Belt analogues are not expected to generate N2 ice fragments as large as ‘Oumuamua. In addition, we discuss observational tests to confirm the presence of these ices in future interstellar objects. Next, we examine the explanations that attribute ‘Oumuamua's properties to other compositions: ultraporous dust aggregates and thin membranes powered by solar radiation pressure, among others. While none of these hypotheses are perfectly satisfactory, we make predictions that will be testable by the Vera Rubin Observatory to resolve the tension introduced by ‘Oumuamua.

Identifying Interstellar Object Impact Craters

“…We acknowledge that 'Oumuamua was likely not composed of water ice, and that 2I/Borisov was depleted in H 2 O. The typical composition of ISOs remains debated, however, and all recent hypotheses have specific, production-limiting aspects (Levine et al 2021). Nevertheless, 'Oumuamua's anomalous acceleration probably implies a significant volatile component, either in the form of common ices (e.g., H 2 O and CO), exotic ices (e.g., H 2 , N 2 , and CH 4 ), or a combination of both.…”

Synthetic Population of Interstellar Objects in the Solar System

“…This volume is almost 5 orders of magnitude larger than the volume of the model sphere, resulting in a similar ratio of the numbers of initial objects and final objects that make up the synthetic population of ISOs. Number-density of ISOs is very unconstrained, with estimations ranging from 10 −9 𝑎𝑢 −3 ∼ 10 7 𝑝𝑐 −3 (Moro- Martín et al, 2009) to ∼ 10 −1 𝑎𝑢 −3 ∼ 10 15 𝑝𝑐 −3 (Laughlin and Batygin, 2017;Trilling et al, 2017b;Jewitt et al, 2017;Do et al, 2018;Levine et al, 2021). However, even for the most conservative assumptions about number-density and reasonable size-frequency distributions, it is quite easy to achieve numberdensity of 1 object per 𝑎𝑢 3 for 10 meter-sized objects already, which could for instance be detected by the Vera Rubin Observatory's Legacy Survey of Space and Time (Schwamb et al, 2021).…”

Sculpting the circumbinary planet size distribution through resonant interactions with companion planets

“…Given that the abundance of gas giants is roughly similar around single and binary stars (Armstrong et al 2014;Martin & Triaud 2014;Martin et al 2019), and it is believed they migrated to their current location near the stability limit, this resonant sweeping may be an efficient production line for interstellar interlopers. Levine et al (2021) note that the non-ballistic trajectory of 'Oumuamua calls into question a circumbinary origin. An alternative may be ejection from an evolving single star system (Katz 2018).…”

“…The escape velocities from binary and single star systems are expected to be different (Levine et al 2021). As a future study it would be interesting to quantify the expected differences on populations of free-floating planets, which may be testable by Roman.…”