An Interstellar
Oddity – Oumuamua 1I/2017 U1
A story on the BBC website highlights the
fact that the interstellar visitor, now known as Oumuamua (meaning “messenger”
in the Hawaiian language, the technical identifier is 1I/2017 U1) has a very
odd shape - it is extremely elongated, at least 10 times longer than it is
wide. By the standards of asteroid
demographics, that is definitely an outlier, a veritable needle in a haystack
of much rounder objects.
Oumuamua’s General Characteristics
First, a bit about the object itself:
- It is thought to be interstellar, due to its orbital characteristics. Basically, it is going so fast (about 25 km per second) that it will escape the pull of our sun, so it therefore almost certainly must have originated from outside the solar system. Essentially, that speed is too high to be from our solar system, even if it fell towards the sun from the far reaches of the Oort cloud and then got a gravitational kick or two from a planetary close encounter.
- It is also in an orbit that is highly inclined to the plane of the solar system, so that also adds evidence that it is probably extra-solar.
- Based on optical colors, it appears to be similar to asteroids of the D-type Jovian Trojans, and not like the Kuiper Belt objects that are much farther out and redder in appearance (though it is a bit red).
- There appears to be no coma and little dust scattering, so it likely doesn’t have a partially icy surface, like comets have (“dirty snowballs”). If it has ice or other volatiles, they must be buried under a non-volatile surface layer (possibly one that has been irradiated and hardened by cosmic rays during its interstellar voyage). So, it is more asteroid-like than comet-like.
- This leads researchers to think that it was an object ejected fairly early in the formation of another solar system.
- It looks to be about 230 meters by 35 meters, according to the Astrophysical Journal paper. That’s based on how its light curve changes, most easily interpreted as an elongated, tumbling object, as seen from Earth. It’s elongation (long axis divided by short axis) seems to be at least 6 to 1.
- Given the likelihood that its rotation axis is not head on, that could be much higher, 10 to 1 or more. It depends on the angle that we are seeing it from.
Oumuamua’s Unusual Elongation
First off – I acknowledge that there is
plenty of scope for double entendre humor in this subject. Let’s try our best to ignore that. I know it's hard, or perhaps I should say difficult. :)
To appreciate how odd Oumuamua’s elongation
is, one can look at the statistics for the elongation of asteroids, as in the Icarus
paper referenced below and noted in the BBC article. Essentially, there have been no objects in
the solar system with such an elongation, out of a survey of about 20
thousand. In fact, there have only been
about half a dozen that even reached elongations of 7 or 8 to 1.
As the above graph shows, even elongations
of greater than 3 are highly unusual.
However, one should bear in mind that the technique used to create this
distribution was based on statistical modelling of brightness changes of
asteroids, rather than direct observation of shapes. That might have an effect on the accuracy of
numbers in the far tail of the distribution, and thus the modelling could be
somewhat of an under-estimate for very long elongations.
Nonetheless, this is a pretty strange
object when you think about it. The odds
against the first visitor object to the solar system being such an extreme
outlier are high - at least 20 thousand to one, possibly much more, given that
the longest natural solar system space objects ever discovered are still less
elongated than 10 to 1. That’s pretty
amazing, and a bet that no smart gambler would ever make.
Assuming that this is a natural object, it
would seem to have implications for the sorts of bodies that are found in the
early stages of a solar system’s formation.
There must be a lot of splintery objects out there (though it is always
dangerous to extrapolate from a sample size of one). So, the question becomes, why would this first
interstellar object be so unusual in terms of such a basic measure as
elongation?
- Perhaps it is a selection effect. For some reason, elongated interstellar objects are easier to see than rounded objects, once they come into our solar system. But there seems to be no good reason for this to be true, since “rounded” objects are in the vast, vast majority of asteroids and other native solar system objects that we have catalogued.
- Perhaps elongated objects are far more likely to be ejected from their originating solar systems than rounder objects, the way a spiral football pass travels a lot farther than an end over end pass. But this seems hard to imagine, as a more aerodynamic shape wouldn’t be particularly advantageous in space.
- Maybe elongated objects are quite common in the early stages of a solar system’s evolution, but become rounded off and shortened over time, via collisions or some other erosional process (collisions with dust?). Perhaps supernova explosions throw out shards of heavy materials, and new solar systems that form from that debris have a high proportion of such objects early in their history, which are then ejected during the chaotic early stages of formation (lots of gravitational interactions and collisions).
- That would lead us to believe that shard-like objects are more uncommon than they really are, based on our observations of our solar system in its mid-life. The Icarus asteroid shape paper does indicate that families of asteroids in our solar system that are younger tend to have a higher proportion of more elongated members, so it is possible that interstellar wanderers that are ejected early in the history of their original solar systems, hold their shape for a long time, since there isn’t really anything to collide with, in interstellar space.
- The Icarus asteroid shape paper also indicates that solar system asteroid families that are farther from the sun tend to have more elongated members, which also supports the idea that a good way for an asteroid to maintain its initial shape is to stay away from the more crowded, higher velocity space lanes of the solar system. Again, that would help to explain how an interstellar interloper could remain very elongated.
- Perhaps objects become elongated during their journeys through deep space. Again, it is hard to imagine an underlying process that would produce this effect, unless tiny effects add up over hundreds of millions of years (some sort of dust accumulation along preferred directions during the long journey, some sort of polarization shaping by the galactic magnetic field?).
Some Other Explanations for Oumuamua’s Shape
What other space objects do we know that
have this kind of elongation? Granted,
this is going out on a limb, so to speak.
Here are a few, real and imagined:
Voyager 1(actual space craft)
XMM Newton Space Telescope
Discovery One (200: A Space Odyssey movie)
Batttlestar Gallactica
Could it be a non-natural object? Granted, it is a huge object, but who knows what an advanced alien civilization might be capable of. It’s a long shot, but worth keeping in
mind, if only because it's fun to speculate about such matters.
Anyway, we will have to keep watching our solar system and see how elongated the next few interstellar visitors are. As was said in a James Bond movie: Once is happenstance, twice is coincidence, the third time is enemy action. :)
Anyway, we will have to keep watching our solar system and see how elongated the next few interstellar visitors are. As was said in a James Bond movie: Once is happenstance, twice is coincidence, the third time is enemy action. :)
It is also an interesting coincidence that the
speed of Voyager 1 is about 17 km per second, on par with our interstellar
interloper’s speed of 25 km per second.
These are things that make you think. :)
I might also note that at 25 km per second,
this object would take about 50,000 years to cross the distance between our sun
and the nearest star. Odds are that it
came from a lot farther away than that, though, so it might well have been
travelling through deep space for tens or hundreds of millions of years.
Sources:
Icarus: The shape distribution of asteroid families: Evidence for evolution driven by small impacts, Gyula M. Szabó a,b,∗, László L. Kiss c
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Now that you have read about a real interstellar interloper (natural
or not), you should consider reading some Science Fiction. How about a short story, also about
interstellar interlopers. It also
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we know, anyway). Only 99 cents on
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Oh, and it has dogs. Everyone
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The Zoo Hypothesis or The News of the World: A Science Fiction Story
In the field known as Astrobiology, there
is a research program called SETI, The Search for Extraterrestrial
Intelligence. At the heart of SETI,
there is a mystery known as The Great Silence, or The Fermi Paradox, named
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Essentially, he asked “If they exist, where are they?”.
Some quite cogent arguments maintain that
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The short story is about 6300 words, or
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