Playing tennis with ʻOumuamua Balls

Diagram of primary production channel for “semi-relativistic” hyper-velocity stars. (The Fastest Unbound Starts in the Universe)

Panspermia is viable on galactic scales, and the entire Milky Way could potentially be exchanging biotic components across vast distances. If bacteria and other possible extremophiles have sufficiently long survival lifetimes, the Galactic Centre can act as an engine for panspermia and seed the entire galaxy(1).

We present an analytic model to estimate the total number of rocky or icy objects that could be captured by planetary systems within the Milky Way galaxy and result in panspermia should they harbor life. We estimate the capture rate of objects ejected from planetary systems over the entire phase space as well as time. Our final expression for the capture rate depends upon the velocity dispersion as well as the characteristic biological survival time and the size of the captured object. We further take into account the number of stars that an interstellar object traverses, as well as the scale height and length of the Milky Way’s disk. The likelihood of Galactic panspermia is strongly dependent upon the survival lifetime of the putative organisms as well as the velocity of the transporter. Velocities between 10 – 100 km/s result in the highest probabilities. However, given large enough survival lifetimes, even hypervelocity objects traveling at over 1000 km/s have a significant chance of capture, thereby increasing the likelihood of panspermia. Thus, we show that panspermia is not exclusively relegated to solar-system sized scales, and the entire Milky Way could potentially be exchanging biotic components across vast distances.

The inspiration for this study came from the first-known interstellar visitor to our Solar System – the asteroid ‘Oumuamua, discovered in October 2017(2). The authors of the paper think that:

ʻOumuamua could be captured through their gravitational interaction with Jupiter and the Sun. The Solar System acts as a gravitational “fishing net” that contains thousands of bound interstellar objects of this size at any given time.

Number of captured `Oumuamua-size objects versus their velocity dispersion. (Galactic Panspermia)

Typical stellar velocities throughout the Milky Way are a few hundred km/s. However, the discovery of hyper-velocity stars leaving our galaxy with speeds of nearly 10^3 km/s has suggested a mechanism by which pairs of massive black holes might act as stellar slingshots. Dynamical interactions with black holes can eject asteroids and comets at extreme velocities, and thus they may traverse the entire radius of the Milky Way in ~ 10^6 years. Such hyper-velocity objects can become intergalactic(3).

In this paper we describe a mechanism by which binary massive black hole (BMBH) mergers can liberate these tightly bound stars from their host black holes, resulting in semirelativistic hypervelocity stars (SHS) that are capable of crossing large swaths of the observable universe and hence can serve as a new cosmological messenger.

Who knows. Maybe life is but a by-product of a sort of galactic tennis or badminton played by professional Black Hole Players.


(1) Ginsburg, Idan, et al. ‘Galactic Panspermia’. ArXiv:1810.04307 [Astro-Ph], Oct. 2018.,
(2) Meech, Karen J., et al. ‘Discovery and Characterisation of the First Known Interstellar Object’. Nature, November, 2017.
(3) Guillochon, James, and Abraham Loeb. ‘The Fastest Unbound Stars in the Universe’. ArXiv:1411.5022 [Astro-Ph], Nov. 2014.,

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