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 N₂ ice [1]. This matches its compositional constraints (no detectable H₂O, 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 N₂ 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 × 10¹⁴ 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 N₂ 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_N2³) 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_N2³)^1/2, then M stars are roughly 60 times more efficient at ejecting N₂ 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.