What was the interstellar asteroid (‘Oumuamua)?

One of the most remarkable discoveries in 2017 was the interstellar ‘asteroid’, now named ‘Oumuamua, which was discovered on Oct.19 having passed closest to Earth on Oct.14.  It was the first object ever discovered that had definitely come from outside our solar system, and it was like nothing in our solar system.  Instead, it was uncannily like the alien starship Rama in Arthur C. Clarke’s memorable novel ‘Rendezvous with Rama’ (1973).  So was it a novel kind of natural object, or was it an alien artefact?  The following information is from the paper in Nature (2017 Dec.21), and informative pages on a NASA web site and Wikipedia:

Meech et al., Nature 552, 378-381 (2017 Dec.21)



Readers who are not familiar with the physical and astronomical background could refer to those pages; here I will take the physics as given and just give my own discussion.  (Last night’s ‘The Sky at Night’ on BBC4 also gave plenty of interesting discussion, although there did seem to be much jumping to conclusions.)


‘Oumuamua was discovered on Oct.19, when 0.22 AU (33 M km) from Earth, having passed only 0.16 AU (24 M km) from Earth on Oct.14: quite a close passage, but an object so small would probably not have been discovered otherwise.  However, by this time it was receding from the solar system, having rounded the Sun unseen with a perihelion of only 0.255 AU (38 M km) on Sep.9. 

The trajectory was clearly hyperbolic (e = 1.19 or 1.20).  It had approached the solar system with considerable speed:  26 km/s.  So it appeared to be *aimed* close to the Sun, on a trajectory which would swing round the Sun then pass not far from Earth!  Of all its strange properties, this was the one that I initially felt was most suggestive of intelligent origin.  However, closer consideration suggests otherwise.

First, the incoming speed and direction (from RA 18h 42m, dec.+34.3 deg, not far from Vega) were very close to that of the ‘local standard of rest’ –i.e., Oumuamua was almost stationary relative to the average motion of stars in this part of the Galaxy, and especially of old red dwarfs.  So it may actually have been an ancient drifting object, and the solar system happened to be heading toward it at 26 km/s.

Secondly, is it really unlikely that it swung closely round the Sun then passed not far from Earth?  As it would not have been detected if it had not passed this close to Earth, the question is simply: how unlikely is it that Oumuamua rounded the Sun first?  I have made some rough simulations of possible trajectories (families of hyperbolae with e = 1.20), for an object incoming from the direction that it did and passing within 0.22 AU of Earth.  I found that the chance of it swinging round the Sun first is about half the chance of it flying past Earth directly (which one might expect of a random event). So the actual trajectory was not at all improbable, and there seems to be no reason to think it was deliberately aimed.

(Of course, we are all assuming that it travelled on an unperturbed trajectory, but we do not know what it did before it passed Earth…)

Thirdly, there is no obvious source for it.  Although it came from a direction 5 deg from the present position of Vega (a bright star with a ‘debris disk’ around it, only 25 light-years away, which is only ~300,000 years travel time for Oumuamua), Vega was nowhere near that position 300,000 years ago.  Oumuamua may have been drifting for millions or billions of years.

Size and shape:

Another extraordinary property was its shape.  Its brightness and light curve implied that it was ~400 metres long (+/- 300 m) but only 1/10 as wide – so it was cigar-shaped!  Although plenty of elongated comets and asteroids are known in our solar system, none are as elongated as this.  


Everyone expected interstellar visitors to be comets: after all, there are thought to be millions of deep-frozen comets floating around the periphery of our solar system (the Oort cloud), some of which get perturbed to fall into the inner solar system, and others of which could get perturbed to be ejected.  But Oumuamua had no coma nor tail at all, despite having just passed inside the orbit of Mercury.  So it was not a comet, but an asteroid or something novel.

(There is theory that it was actually a comet whose surface had been baked into a rigid dark crust by aeons of exposure to interstellar space, but I am very doubtful about this, because the same should apply to all those comets in our Oort cloud – whereas in fact, they are covered in ices which are especially volatile when they approach the Sun.)

Its surface was reddish, but that could be a coating deposited during long exposure in space.

Was it a rigid object?  We don’t know; its rotation period was quite slow so it need not have been mechanically strong. But it seems likely that such an elongated object would be rigid.

Two months after its flyby, astronomers listened very sensitively for radio emissions from it with the Green Bank radio telescope.  None were detected.  (But even if it was an alien artefact, I see no reason why it should have been beaming radio waves to Earth at that time.)


Oumuamua’s initial rotation period (7.3 hours) soon changed (to 8.1 hours) and seems to have been irregular (as just shown on ‘Sky at Night’) – i.e., it was tumbling.  When we send probes into space, they only tumble if control has been lost.  So this also casts doubt on the alien probe theory.


Oumuamua was an extremely elongated solid body, not a comet, which came from interstellar space.  No such object exists in our solar system, nor could be ejected from it, so far as we know.  We cannot rule out the idea that it was an alien artefact, but it seems more likely to have been an ancient object that had been tumbling in space for a very long time. But what was it?  One idea proposed [by Matija Ćuk: arXiv:1712.01823] is that it could be a shard of a shattered planet.  Such objects could exist: when solar systems form, there are probably many young planets swinging on unstable orbits, and a close encounter could tidally disrupt a planet, possibly producing vast numbers of such elongated, solid rock fragments, which could be ejected from the system at the same time. 

(Presumably no such event occurred when our own solar system was forming, but instead there were some giant collisions – which produced rubble and molten droplets, apparently not solid shards. The best-known of these collisions produced a debris disk around the Earth that then formed the Moon.)

According to this theory, there could be many such objects drifting in interstellar space, even though none reside in our solar system.  In that case, more objects like Oumuamua may be seen in future.  However, they cannot be very common, or they would already have been picked up by the ongoing searches for near-Earth asteroids, just as Oumuamua was.

 –John Rogers (2018 Feb.12)

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