‘Oumuamua’s Many Cousins
The interstellar asteroid 1I/2017 U1 (otherwise known as 1I/’Oumuamua) is fast receding into the distance, towards the constellation of Pegasus (1). The existence of this rocky visitor from the stars was announced last October (1). Its trajectory was too fast for it to be a solar system comet – even one from the furthest reaches of the Oort Cloud. That was an exciting discovery, because that meant that 1I/2017 U1 was the first confirmed observation of an object arriving in the solar system from deep space.
Although 1I/2017 U1 was initially considered to be an interstellar comet, that thinking changed when it failed to emit any gases as it performed its perihelion transit around the Sun (3). This barren rock, confirmed as an interstellar asteroid (4), is now speeding away from the Sun. It spent a relatively short time in the observation zone of professional telescopes, thanks to its great speed, but this was enough to reveal more weirdness (5). It is an elongated object spinning head over tip, doing cartwheels through the solar system. Some wondered whether it might be artificial, given the lack of coma as it traversed past the Sun. But attempts to pick up signals from the object came up blank (6). Still, its shape is nothing like any known body in our Solar System. If solar system asteroids resemble rocky potatoes, then 1I/2017 U1 is more like an interstellar carrot, spinning haphazardly through our system. To remain intact under these conditions, its internal structure must be robust (7).
The colour of our interstellar carrot is neutral with a reddish hue. The colouration may be patchy across its surface. Solar system minor bodies (asteroids, Kuiper Belt Objects, Trojans) vary in colour, often dependent upon which population group any particular object belongs to. Continuing my daft vegetable analogy, solar system potatoes come in different varieties. Many are neutral in colour, some are reddish, others distinctly red. Like comparing a Maris Piper to a King Edward. If we compare 1I/’Oumuamua’s colouration to those of various classes of solar system objects, then it seems to most resemble those of the dynamically excited populations of Kuiper Belt Objects. However, it is less red than the scattered Trans-Neptunian objects whose orbits extend beyond the heliopause (7).
This creates a slight conundrum, because the red appearance of these more distant travellers has been attributed to their exposure to cosmic radiation beyond the protective ‘shield’ of the heliosphere (8). This causes the polymerisation of surface organic materials, creating ‘ultrared’ compounds whilst moving through interstellar space beyond the heliopause. The problem, then, is reconciling that idea with an asteroid from interstellar space. If such an object were to have surface organic materials present, as many outer solar system objects do, then its interstellar flight from its own original solar system should have turned these materials deep red. That is not what has been observed.
One possibility is that 1I/2017 U1 is a shard of material ejected from a star’s inner system, where volatile organic materials would already have been eradicated from the object’s surface by the action of the star. There are other possibilities, too:
“1I/’Oumuamua’s largely neutral color opens up a number of possibilities. It could imply that the correlation of ultra-redness with heliocentric distance has an alternative cause. It could suggest that 1I/’Oumuamua formed with an organics-poor surface within its star’s water ice line. 1I/’Oumuamua’s color being within the observed range for minor planets in the solar system could support that 1I/’Oumuamua originated from a star from the Sun’s birth cluster, which should have a similar chemistry. A possible additional complication could be resurfacing due to surface activity, which would affect surface color. This seems unlikely as no surface activity was detected during 1I/’Oumuamua’s perihelion passage, but 1I/’Oumuamua could have had past activity in its origin system or in another close encounter.” (7)
As 1I/2017 U1 is a solitary object in a class of its own, it’s impossible to tell whether its properties are typical of interstellar asteroids or not. However, if its solitary arrival on our cosmic shores presents a statistical norm over the time we have been actively seeking out such objects, then calculations suggest there is an absolutely massive population of interstellar asteroids across the galaxy (9).
“Gregory Laughlin of Yale University and Konstantin Batygin of Caltech…argue that [1I/’Oumuamua’s] current passage, if it’s not a fluke, suggests the presence of an enormous number [2 x 10 to the power 26 (8)] of such objects in our galaxy alone — enough to account for two Earth-masses of material for every star in the galaxy.”(5)
That’s an awful lot of ejected material being flung about out there. The currently accepted model of solar system formation, the Nice model, predicts this effect, whilst simultaneously recognising that it’s not that easy to eject material from the gravitational field of a star. The ‘throw line’ of each star system varies with the mass of the star. According to the Nice model, stars would need to have substantial planets placed in the right orbital locations to facilitate these ejections. It is thought that each star system would need the presence of gas giant world/s and, in particular, its own equivalent of a distant Neptune to make this happen:
“Our own solar system has contributed many volatile-rich planetesimals to the galaxy. Within the framework of the so-called Nice model of early solar system evolution, a transient period of dynamical instability is triggered in response to interactions between the giant planets and a primordial disk. In simulations, nearly all of this material is expelled into interstellar space as the instability unfolds, leaving behind today’s mass-depleted Kuiper Belt… Efficient ejection of planetesimals requires a massive secondary body with specific orbital conditions.” (9)
However, our solar system seems to have a different layout of planets from other planetary systems which have now been observed and studied. In other systems, small planets seem to team up with other small planets, like regularly-placed peas in a pod. Conversely, large planets pair up in other systems (10). So, the Nice model’s application to our own, unusually mixed-up system may not work so well with other planetary systems – at least, if you’re trying to explain mass expulsions of rocky materials across the galaxy.
The lack of volatiles on 1I/2017 U1 confuses matters further. It restricts the conditions of ejection from a star system still further. Laughlin and Batygin seem to imply that 1I/2017 U1 does contain volatiles, despite its distinct lack of cometary activity during perihelion. This would fly in the face of what was observed during the transit. They argue that, otherwise, each planet must be ejecting a phenomenal amount of material:
“If A/2017 U1 points to a volatile-bereft population, each such planet must eject >100 (Earth masses). In all likelihood, the comparatively low occurrence rate of these planets allows them only minor contributions, implying that A/2017 U1 is volatile-rich despite its lack of coma. Interstellar asteroids are likely ejected by as-yet unobserved long-period sub-Jovian planets.” (9)
If anything about this is clear at all, it’s that 1I/2017 U1 is creating a great many challenges! It seems reasonable to conclude that a large population of interstellar asteroids implies a large population of sizeable planets in long-period orbits around stars. If interstellar objects are largely volatile-free, as seems to be the case here, then the amount of material being ejected during the early life of a planetary system is truly astounding. But then the Nice model calls for early solar system catastrophism on a truly awesome scale, so a galaxy littered by similar debris from other stars seems in keeping with this. The sticking point is that pesky lack of volatiles.
None of this explains the curious shard-like shape of 1I/2017 U1, though. This seems to me, from a purely common sense point of view, to indicate that this object resulted from a collision, rather than having been ejected through mere gravitational interaction between bodies. It is, metaphorically, a piece of flying glass.
Written by Andy Lloyd, 21st January 2018
1) The Sky Live “1I/2017 U1 (Oumuamua)” https://theskylive.com/oumuamua-tracker
2) Andy Lloyd “Alien Red Asteroid Crashes Through Solar System” 28th October 2017 http://www.andylloyd.org/darkstarblog55.htm
3) Andy Lloyd “Interstellar Shard Puzzles Astronomers” 13-29th December 2017 http://www.andylloyd.org/darkstarblog57.htm
4) K. Meech et al “A brief visit from a red and extremely elongated interstellar asteroid” Nature, 21st December 2017, 552, pp378–381
5) Susanna Kohler “Update on an Interstellar Asteroid” 19th January 2018 http://aasnova.org/2018/01/19/update-on-an-interstellar-asteroid/
6) PhysOrg “No alien ‘signals’ from cigar-shaped asteroid: researchers” 14th December 2017 https://phys.org/news/2017-12-alien-cigar-shaped-asteroid.html
7) Michele T. Bannister et al “Col-OSSOS: Colors of the Interstellar Planetesimal 1I/’Oumuamua” The Astrophysical Journal Letters, 18th December 2017, Vol 851, 2,http://iopscience.iop.org/article/10.3847/2041-8213/aaa07c/meta
8) Dave Jewitt “From Kuiper Belt Object to Cometary Nucleus: The Missing Ultrared Matter” The Astronomical Journal, 2002, Vol 123, 2, pp1039-1049,https://pdfs.semanticscholar.org/64d7/7c6d3b97a64bf6653557c030f8d919bc5f4a.pdf
9) Gregory Laughlin & Konstantin Batygin “On the Consequences of the Detection of an Interstellar Asteroid”. 2017 Res. Notes AAS 1 43 http://iopscience.iop.org/article/10.3847/2515-5172/aaa02b/meta
10) Elizabeth Howell “Earth Resides in Oddball Solar System, Alien Worlds Show” 16th January 2018 https://www.space.com/39390-alien-planets-reveal-our-strange-solar-system.html with thanks to Shad