Skip to main content# The Frequency of ’Oumuamua-Like Interstellar Objects

Presentation #304.05 in the session Comets and ISOs: Dynamics, Origins and Theory.

Published onOct 20, 2022

The Frequency of ’Oumuamua-Like Interstellar Objects

All observed features of the first interstellar object, ‘Oumuamua, are satisfactorily explained if it was a collisional fragment of an exo-Pluto made mostly of N_{2} ice [1]. This matches its compositional constraints (no detectable H_{2}O, CO, CO2, or dust), its albedo and color, its extreme axis ratios (due to > 95% mass loss as it rounded the Sun), and in particular exactly matches its non-gravitational acceleration. Our Solar System potentially ejected ~ 10^{-2} M_{E} of fragments ~100 m in size, roughly half of them N_{2} ice, from the surfaces of differentiated Pluto-like dwarf planets during the outer Solar System dynamical instability [2]. If all stellar systems ejected such fragments with equal efficiency, this would translate into 2 × 10^{14} objects per solar mass of star, or 5 × 10^{-4} au^{-3}, which compares not unfavorably to the statistical range allowed by the observation of ‘Oumuamua, 2 × 10^{-3} to 0.12 au^{-3} [3]. To better assess the match to observations and improve predictions for the Vera Rubin Observatory (VRO), we examine whether fragments might be ejected more efficiently from M star systems.

We find [3] that N_{2} ice can survive (T < 50 K) on planetary surfaces at *r*_{N2} ~ 0.65 au from an M star, rather than 15 AU for the Sun, and the typical collisional velocities there would be *V*_{rel} ~2 km/s rather than 1.5 km/s. The number of fragments produced scales as *M*_{disk} (*M*_{disk} / *r*_{N2}^{3}) *V*_{rel} *V*_{rel}^{1.65} *t*, where the power of 1.65 comes from the Housen-Holsapple relations for ejected mass over impactor mass. If the disk mass *M*_{disk} scales as *M*_{star}, the mass of the star, and time of the dynamical instability also scales as *t* ~ (*M*_{star} / *r*_{N2}^{3})^{1/2}, then M stars are roughly 60 times more efficient at ejecting N_{2} ice fragments than the Sun. This suggests the number density of fragments in the Galaxy should be closer to 0.03 au^{-3}, which is very close to the inferred value 0.04 au^{-3} [3]. Icy fragments from M stars should be common, and we expect VRO to expand the number of known interstellar objects this decade.

[1] Jackson, AP & Desch, SJ (2021) JGR 12606806D. [2] Desch, SJ & Jackson, AP (2021) JGR 12606807D. [3] Desch, SJ & Jackson, AP (2022) Astrobiology, in press.