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Submission + - First Extrasolar Object has Unusual Shape 2

RockDoctor writes: Following the fleeting appearance of Interstellar body 1I/2017 U1 ’Oumuamua in our skies over the last several months, the expected flood of technical papers are starting to appear. Not quite first out of the blocks, but delivering the most interesting results so far, are famous not-a-planet hunters David Jewitt and Jane Luu (whose 1992 discovery of the first Kuiper Belt Object (KBO) (15760) 1992 QB1 marked the acceleration of the study of the outer solar system).

The 1I/2017 U1 is sub-kilometre in size, with a slightly reddish colour (which has been reported previously, and is less red than is typical for outer solar system objects) and rotates in about 8.26 hr. So far, so unspectacular. But what is remarkable is that the brightness varies by 2.0 ± 0.2 magnitudes, which translates to a shape 6 times as long as it is wide (~35m x 230) if it's brightness does not change across it's surface. This is most remarkable but not quite unprecedented. Asteroid 4116 Elachi has a 1.6 magnitude brightness variation over a 38 hr period for an implied shape 4.3 times as long as it is wide.

To hold together under it's own gravity, a body this size and shape but with no strength would need a density of 5 times that of water (ice in interstellar space). This implies that the body has significant cohesion strength and is not a "rubble pile".

If the surface reflectance (albedo) does vary across the surface, the shape may not be as extreme as the 6:1 brightness ratio suggests, but there is not enough colour data available to adequately assess this question.

With only one example known at this time it is not known if this is typical of interstellar bodies, or unusual. However this single detection implies around 10000 such bodies in the "inner" solar system (within the orbit of Neptune), staying here for around 10 years each, and with future telescope plans, around one such discovery a year in the foreseeable future.

Some aspects of this body are unremarkable (it much resembles Jupiter's "Trojan" asteroids, which probably got to their current orbits by violent scattering in the early development of the Solar System. This is certainly not inconsistent with the object having been scattered out of it's origin system. But that shape is unusual among the Solar System's minor planets and detritus.

(PDF direct link.)
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First Extrasolar Object has Unusual Shape

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  • Another paper [arxiv.org] published on Arxiv since I wrote the submission describes 1I/'Oumuamua thus : "Assuming that 1Iâ(TM)s shape can be approximated by an ellipsoid, the amplitude constraint implies that 1I has an axial ratio of 4.1 to 6.9, which is strikingly elongated", which is compatible with the paper I describe in the submission. Clearly there will be more observations published in the near future, and that the astronomical community are continuing perplexed by the unusual shape of this object, and the
  • Another paper [arxiv.org] (yes, it's hard to keep up!) makes this suggestion : "A similar curve would be produced by a contact binary with equal sized, prolate components each with a/b (longest over shortest axis) â¼2.7, elongated along a line connecting their centers."

Natural laws have no pity.

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