Skip to main content
SearchLoginLogin or Signup

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 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 ME 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 rN2 ~ 0.65 au from an M star, rather than 15 AU for the Sun, and the typical collisional velocities there would be Vrel ~2 km/s rather than 1.5 km/s. The number of fragments produced scales as Mdisk (Mdisk / rN23) Vrel Vrel1.65 t, where the power of 1.65 comes from the Housen-Holsapple relations for ejected mass over impactor mass. If the disk mass Mdisk scales as Mstar, the mass of the star, and time of the dynamical instability also scales as t ~ (Mstar / rN23)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.

No comments here