‘Oumuamua’s encounter with the inner solar system is dying down on Twitter, yet still it bristles with consequence and the uneasiness of unanswered questions. Why no coma?

Occam’s razor is a dull instrument that points almost unerringly to the mundane (as opposed to pointing to interstellar probes). One thus draws several conclusions. (1) ‘Oumuamua’s aspect ratio is substantially less than 10:1. (2) Billions of years in the interstellar environment lead to the buildup of a tarry crust that resists temporary heating, and this process is enhanced for comet-like planetesimals that form in systems with supersolar C/O ratios. (3) Most stars have true-Neptune analogs.

The resulting prediction is that slightly tweaked ongoing surveys, and soon LSST, should start turning up interstellar asteroids and perhaps interstellar comets with some frequency. If another one is found in the near-term, it would be interesting to look at the optimal mission designs that could accomplish an opportunistic sample-return.

From ‘Oumuamua’s perspective, the close encounter with the Sun was a near-indescribable stroke of luck. To scale, the stars of the galactic disk are like grains of sand separated by miles and crawling through space at a few feet per year. The Galaxy is the archetypal collisionless fluid. Vaulting from ‘Oumuamua’s current encounter to its next connects the all too human interval of waking-up-at-3AM anxieties — the scale of days and months — to the frigid waste of a quadrillion years.

Why cold? When fusion has ended, dark matter annihilation and proton decay take over, and both (while uncertain) are certainly slow processes. Grand Unified Theories predict that proton decay should occur, but so far, there is no experimental evidence. The lower bounds on the proton half-life are ~10^34 years via the sluggishly competing processes of positron and muon decay.

If the proton were completely stable, the end states of stars present a curious state of affairs. Black holes of stellar mass, which are much more tightly bound than degenerate stars, will evaporate through the Hawking effect with a lifetime of “only” 10^66 years Although this time scale is aggressively long compared to the current 13.8-billion year age of the universe, it would be odd if black holes are ephemeral while white dwarfs and neutron stars are forever.

While jarring, this possible divergence of lifetimes is not exactly a matter of pressing concern. Two decades, ago, however, Fred Adams and I had priorities that were definitely skewed toward the really long term. Along with Manasse Mbonye and Malcom Perry, we looked into how quantum tunneling into black holes can erode white dwarfs. In Freeman Dyson’s 1979 article, Time Without End, it is pointed out that an otherwise stable white dwarf will spontaneously tunnel into a black hole on a time scale of order 10^10^76 (!) years. In our article, we argued that the whole star need not make the plunge at once, and that a 10^45 year half-life is a plausible value for black-hole induced proton decay. This has the added benefit of enabling a Hertzsprung-Russell diagram that traces stellar evolution to its absolute bitter end.

‘Oumuamua. Up close and alongside, in the vastness of interstellar space, its hurtling bulk imparts no sense of motion as it turns imperceptibly on its axis, blotting out the stars.

For a hundred years, the point-like Sun grew steadily brighter against its frigid airless horizons. First came light, then warmth, and finally searing illumination of the tarry reddish expanse, blistering sluggishly beneath a September Noon far more intense than any summer of Earth.

`Oumuamua is departing the solar system as rapidly as it arrived, heading outward at a current rate of 2.5 million miles a day. Our tiny chance of sending a probe to catch it diminishes with each lagging tick of inactivity. Nonetheless, world-wide interest is mounting, in part as a consequence of two new articles reporting detailed observations. The first, by Jewitt et al. was posted to arXiv last week, while the second, by Meech et al. (which independently comes largely to the same overall conclusions), appeared in Nature earlier this week. Nature being Nature, the Meech et al. article was accompanied by a media push, spearheaded by an extraordinary piece of space art.

Maybe it’s press release fatigue from one “habitable” world after another — a monotony of warm suns glinting off imaginary oceans — that makes this image so arresting.

The observational facts remain stark and limited. `Oumuamua’s double-peaked light curve suggests that it has a large aspect ratio, perhaps as high as 10:1. Assuming that it’s a poor reflector, it’s several hundred meters on its long axis. Its overall color is reddish. It has to have physical strength, or its 7-hour rotation period would be enough to overcome its negligible self-gravity and tear it apart. Most alarmingly, it shows no sign of a coma. At most, less than a sugar cube’s worth of cometary dust per second was emanating from it as it tore through the inner solar system. (As a matter of fact, ‘Oumuamua as observed is entirely consistent with Tintin’s rocket.)

For more on ‘Oumuamua, I have a blog post up at Scientific American.

This was no fruit of such worlds and suns as shine on the telescopes and photographic plates of our observatories. This was no breath from the skies whose motions and dimensions our astronomers measure or deem to vast to measure. It was just a colour out of space — a frightful messenger from unformed realms of infinity…

Aww, come off it.

Wild-eyed extravagances aside, A/2017 U1 — the asteroid-like visitor from interstellar space — is an extraordinary object. In traversing the gulfs, its next encounter with a star that is as close as last month’s encounter with the Sun likely won’t occur for another quadrillion years, and so the mere fact that it zipped through suggests that quite a few interstellar asteroids are out there. And this, in turn, has some remarkable consequences. A straightforward cross-section based estimate suggests that the galaxy contains of order a hundred billion earth masses of A/2017 U1-like planetesimals. Hot Jupiters, terrestrial planets, and super-Earths are all incapable of using gravity-assist to eject bodies out of their parent systems, leaving the strong hint that as-yet undetected Neptune-like planets must be extremely common.

In general, extrapolations from a sample size of one don’t have a good track record. Exhibit A would be our own Solar System — hot Jupiters were discovered at better than 100-sigma significance because solar-system expectations had been projected throughout the galaxy; proper planetary systems should have terrestrial bodies near 1 AU and gas giants at 10 AU.

The arrival of A/2017 U1 seems nicely timed to revival of the AAS’ new low-maintenance communication channel, the “Research Note“:

The purpose of the Research Notes is to provide a home for short submissions that are not suitable for publication as a journal article, but are likely to be interesting or useful to members of our community. Appropriate submissions would include brief summaries of work in progress, comments and clarifications, null results, and timely reports of observations (such as the spectrum of a supernova), as well as results that would not traditionally merit a full paper (e.g., the discovery of a single unremarkable exoplanet, a spectrum of a meteor, or contributions to the monitoring of variable sources).

I especially like the part about “single unremarkable exoplanets” being equivalenced to the “spectrum of a meteor”. In any event, Prof. K. Batygin and I have just submitted a research note that gives our take on the implications of A/2017 U1. Here’s a link to a draft of the note, which we’ll also post on the arXiv within the next several days.