NewScientist (23 March 2019, paywall) has some estimates to pass on about the siblings of ‘Oumuamua:
Parts of Earth may originally be from another part of the galaxy, having crossed light years to form the ground beneath our feet. That is the conclusion of a study suggesting that the Milky Way should be full of free-floating rocks like ‘Oumuamua, the interstellar asteroid that visited our solar system in October 2017, and they may act as seeds to form planets in nascent planetary systems.
Our traditional picture is that planets form out of discs of gas and fine dust around a star, but some observations seem to show them being born much faster than that model predicts. Interstellar objects like ‘Oumuamua may be the solution to this discrepancy.
Researchers have estimated that there should be about 29 trillion ‘Oumuamua-like objects per cubic light year in our galaxy, floating free after having been thrown out of orbit around their home stars. They are likely to be relatively small, dark, and fast-moving, which is why we have only seen one so far.
My question, as you may have guessed, is how are they distributed in that cubic light year? Is ‘Oumuamua just a random traveler, or are we about to see a horde of its siblings descend on the Solar System?
An artist’s concept of interstellar asteroid 1I/2017 U1 (‘Oumuamua) as it passed through the solar system after its discovery in October 2017. Observations of ‘Oumuamua indicate that it must be very elongated because of its dramatic variations in brightness as it tumbled through space. Image credit: European Southern Observatory / M. Kornmesser
