FWIW if I live long enough for someone to confirm that Planet X is a black hole I’ll strip naked and dance in Times Square because golly is that such an out of the blue hypothesis.

The highest energies of UHEC protons are probably not caused by "instant" accelerations as in a supernova (which certainly produces high energy protons that fly across the cosmos). Instead, they're more likely drawn up magnetically around the supermassive black holes powering active galactic nuclei (quasars, blazars and so on) and into jets, where they spend possibly weeks or accelerating through a natural counterpart to a particle accelerator which benefits from things like inverse Compton scattering (a hard X-ray from the black hole hits our proton boosting the proton's momentum and lengthening the X-ray's wavelength (i.e., reducing its momentum)).

But because we don't know OMG!'s origin, we can't say if OMG! tests any acceleration limit originating in uncertainty relations. Also, the Caianiello limit is unhealthy for protons (which OMG! likely was) staying protons: that they are composite structures makes them liable to be blown apart by extreme acceleration. Assuming it's a proton and had a blazar origin, OMG! is (really rough back-of-envelope work) likely to have fallen between 7 and 40 orders of magnitude short of the maximum (proper) acceleration. The intuition here is that OMG! is well short of Planck energy (by some 7 orders of magnitude) and super-high acceleration should take a proton from thermal energies to Planck energies. Of course a proton much closer to Planck energy could lose proportionally more along the way than a less-energetic proton, so who knows? There could be a doctoral dissertation waiting to be written on this!

Also, sorta disappointingly, OMG! could have had an electric charge greater than +1 (i.e., some ion with more than one proton) and in that case could have originated as close as our own sun <https://en.wikipedia.org/wiki/Solar_energetic_particles> <https://en.wikipedia.org/wiki/HZE_ions>.

In this paper he considers the possibility that the universe is almost dominated by tiny (~ micrograms) black holes formed in the very early universe, persisting to the 1970s and leaving tracks in sufficiently large and sufficiently numerous cloud chamber detectors given sufficient time. (This was also decades before optical detectors like at SNOLAB, but incongruously after the Nobel prize for bubble chambers, although it was on the cusp of the activation of <https://en.wikipedia.org/wiki/Gargamelle> and the Big European Bubble Chamber -- he would have been aware of how close they were to activation, and results from smaller predecessors). Fascinatingly, several years after publication, conceptually similar papers were put forward for hunts for dark matter, even though it is impossible to extract from this Hawking 1971 paper that he was thinking of galactic rotation curves.

The paper also pre-dates evidence for the acceleration of the expansion of the universe, and contains the curious sentence: "An upper bound on the number of these objects can be set from the measurments by Sandage (7) of the deceleration of the expansion of the Universe." However, the excellent Allan Sandage paper of 1961 <https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_que...> does not really let one take sides ("It suffices to say that no uncontested evidence is presently available [to choose between steady-state and expanding universe]") like Hawking 1971 seems to. Nevertheless, Hawking raises an interesting and slightly spooky premonition of dark matter at super-galactic scale, keeping clusters of galaxies looking like clusters over long periods of time, rather than separating into individual isolated galaxies: "This extra density could stabilize clusters of galaxies which, otherwise, appear mostly not to be gravitationally bound." In context Hawking was clearly thinking about a universe that grows into some very long term non-expanding/non-contracting configuration which includes clumps of galaxies.

Hawking 1971 tl;dr is "good luck actually observing these small weakly-interacting things, if they exist". Which is appropriate for 21st century particle dark matter experimentalists too. :^)

Penultimately, some patience with google scholar has found that a small handful of the many articles citing Hawking 1971 explore the idea of primordial black holes as dark matter. (but see below).

And finally, sometimes asking questions like yours (how does one build a very small black hole?) and taking them seriously leads to some very early, vague, ideas about how the universe might work in bulk, which is probably very very similar to what provoked Hawking's letter about 52 years ago.

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[1] https://sci-hub.se/https://doi.org/10.1038/248030a0

The relevant part of his evaporation letter: "There might, however, be much smaller black holes which were formed by fluctuations in the early Universe². Any such black hole of mass less than 10^15 g would have evaporated by now." In that context, I think a very low motivation for someone not terribly interested in the history of primordial black hole research to have previously hunted down a 1971 note whose idea was shortly thrown under the bus by its author is fairly understandable. :^)

Hawking 1971 is however an early note on primordial black holes (the only obvious earlier one in the literature is from Zel'dovich and Novikov in 1966), and that seems to be behind many of the citations I briefly sampled. Practically all also cite the more extensive follow-up Carr & Hawking 1974 "Black holes in the early Universe" <https://ui.adsabs.harvard.edu/abs/1974MNRAS.168..399C/abstra...> which is very close in submission date to the publication date of Hawking's "Black hole explosions?" yet does not appear to consider evaporation.

There are hundreds of billions of regular stars in our galaxy (and in similar galaxies), some much more massive than neutron stars, and all of them have a much larger cross-sectional area than neutron stars. (NSes are tiny -- compare with New York City -- <https://commons.wikimedia.org/wiki/File:Neutron_Star_Manhatt...>) So small black holes are much more likely to collide with ordinary stars, and even Long Island NY and other comparable features on Earth, other solar system bodies, and exoplanets (we've counted five thousand of those now, and a lot of them are considerably closer than the nearest neutron star). But we see no evidence of that.

If they're moving gently along with all the stars in their neighbourhood, collisions between these asteroid-mass black holes and anything appreciable is unlikely (just as star-star collisions are very rare) that we wouldn't expect to see that ever happen.

Wandering pretty violently in comparison to nearby bright objects, an asteroid-sized black hole is likely to zip by a star and its satellites on a (strongly) hyperbolic orbit, like <https://en.wikipedia.org/wiki/%CA%BBOumuamua>, just dimmer. It's even less likely to hit a neutron star because of the NS's compactness, and because the geodesics it generates produces greater hyperbolicity near its surface (because that's quite near the centre of mass) than near the surface of a regular star. And hitting a sun-like star is already hard for an asteroid: <http://curious.astro.cornell.edu/our-solar-system/comets-met...> (likewise, we have never seen a small black hole flare up near our sun).

It is more likely that -- very occasionally -- an asteroid mass object (by strong equivalence principle, it does not matter if it is rocky, icy, or a black hole) is captured into a weakly hyperbolic orbit, or even an elliptical one, by a ~stellar mass. There are reasons to think an NS might be more efficient at this than a fatter star. But then you get a (possibly wide) binary star-BH that soon becomes hard to distinguish from a binary that formed together and "grew up" as stars together, with one happening to get too fat and collapse. The missing nebula in the captured-BH case would raise questions.

I don't buy the idea of a small black hole that somehow manages to slice through a neutron star doing much to the neutron star. Typically many many solar masses of matter fall back onto neutron stars after they violently form, and almost certainly there are asteroid-sized chunks of heavy atomic nuclei landing on them often. If you had some cosmic pool shark aim a small BH so precisely that it not only enters the (very very very very very small) NS core, but stays there, coming to a stop just so, the NS would collapse into the BH [1], because no matter can rest just outside the event horizon. The inner hard "platform" matter would fall in first, and in turn the freshly unsupported upper layers which had been "standing" on that platform would fall in, and so on down the widening drain. But if you shoot an asteroid-mass through an NS, it's hard to think there'd be much consequence. Sufficently slowly, there could be a nuclear explosion but probably not one that's catastrophic to the NS's integrity: I think it'd be hard to distinguish from the nuclear explosions from the rain of supernova-remnant material falling back down onto it, or random interstellar matter an isolated NS might encounter, or crustal quakes.

(I could be convinced by an academic paper equipped with an NS equation of state though (see [1] again), in principle, but very likely not a youtube video, much less a second-hand summary of one :-) ).

To a zeroth order approximation, the effect on a planet of a collision with an asteroid-mass black hole is not going to too different from the effect of a pair of collisions with ordinary asteroids (the "pair" is from the ingress and egress sides of the "tunnel": actual asteroids will tend hit a surface and stay there, or break up in a thick planetary atmosphere if there is one, etc., while small black holes will sail through). The asteroid-mass black hole will certainly create lots of electromagnetic interaction on contact; you can think of how black holes tend to have bright accretion structures, so you expect a big flashy kaboom as with a rock (but with more X-rays and gammas).

While we can resolve exoplanets, we can't quite see them being hit by asteroids, assuming the Jupiter-type exoplanets are hit with some frequency, we could blame limits in our telescopes. (JWST will help very soon!). Rockier exoplanets are no more likely to be hit by asteroids and lit up brightly than Earth or Mars. Asteroids are just tiny masses compared to these bodies.

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[1] fate of a neutron star with an "endoparasitic" small (and possibly primordial) black hole, that "just so" is already inside the NS: https://arxiv.org/abs/2101.12220