Interstellar Interlopers, Coming in from the great beyond Options

[dudley]

The length of Oumuamua was first given as 180, and later as 400 meters. Recognizing that it is a point source, can this wide range of values be solely attributed to interpretation of like data? Could the object have changed its rotational orientation with respect to Earth? We're told that the less directly its ends align with Earth, as it spins, the less its brightness will vary, and so the less elongated it will seem. Could its ends align with Earth better now, than formerly, causing the longer length estimate?

[nprev]

I haven't seen error bars for any of these measurements, but they have to be getting larger by the hour as the object recedes. It's important to note that what we're likely seeing in the press are best-guess median numbers instead of what is really known, which would be a set of constrained ranges plus or minus for each dimension, and the resolution range given this thing's small size and large distance is likely at best several tens of meters...and decreasing. To my knowledge there haven't been any truly high-precision measurements to date such as stellar occultation observations.

In the end, all we can really say that we know with high confidence is the rotation period, that it's probably markedly non-spherical (although large surface albedo variations are not ruled out and may therefore modulate how far out of round it is), and that it has spectral attributes that resemble those of some groups of asteroids. And, of course, that it's of extrasolar origin.

[fredk]

Adding to nprev's comments, the lightcurve only pins down the ratios of the axes (and only roughly). To estimate actual dimensions, all they can do is compare the observed brightness (since it's only a point source) with assumptions about how dark the surface is (albedo). A different assumption for the albedo gives different dimensions (eg with darker albedo it must be larger to appear as bright from earth). So that may explain part of the variation out there.

[JRehling]

In real estate, the three most important things are location, location, location.

With fragments from another place, the three most important things are selection bias, selection bias, selection bias.

Meteorites found on Earth are disproportionately irons because they (1) Survive the trip and (2) Are easily distinguished from terrestrial rocks.

Meteorites from Mars appear to be disproportionately from volcanic surfaces because, it's been hypothesized, they (1) Survive the trip and (2) Begin their departure from Mars at high elevation, already above some of the martian atmosphere.

So if this thing from outside the solar system looks very odd, we have to consider that it has been through several rounds of exceptional selection, and therefore selection bias. Its inbound velocity of 26 km/s relative to the Sun would indicate, if it escaped from earthlike distance from a sunlike star an initial velocity of about 70 km/s. Not a lot of structures would survive that. So it may be less surprising to see that it's remarkably rigid: Snowballs aren't going to survive the trip that it survived.

[Floyd]

Excellent point on multiple rounds of exceptional selection.

[Paolo]

I remember reading an article in Sky & Telescope (but it may have been a different magazine) in 1997 or 98 explaining that expulsion from the solar system is a common fate for planet-crossing asteroid such as near Earth objects.

turns out it was a paper in Nature in 1994: Asteroids falling into the Sun where it is shown that expulsion from the solar system is the second most likely fate of NEOs. Even Toutatis was computed to be injected in a hyperbolic trajectory in the paper. Since it was published, we have better understanding of how the Yarkovsy effect is more efficient in moving object to the resonant areas and replenish the NEO population than collisions, mentioned in the paper.

[HSchirmer]

A/2017 U1 had no observed coma, so it had little in the way of volatiles, at least near the surface. It could have had volatiles below a darker "crust" that didn't break through, but a dark object passing 0.2 AU from the Sun should have heated up quite a bit. It might be a chip off some exosystem's version of Mars. The surface color might be a thin coating or go all the way down. It's certainly spent a lot of time in interstellar space, which may have caused slow but thorough transformations that nobody's yet imagined.

I'd guess that in future decades we'll be able to spot much smaller versions of this thing which visit the solar system with considerable frequency and we'll start to explore them. There's a lot of untapped potential for huge light-bucket telescopes to monitor the skies and then things that are dimmer and dimmer will be detected earlier. If the size distribution of interstellar objects is like that of the asteroids in our solar system, then at some size threshold, there seemingly must be interstellar interlopers every year. But the process of interstellar travel might introduce some unimagined selection effects. For one, reaching the escape velocity of a star may be extremely violent.

https://www.nature.com/articles/s41550-017-0361-4.epdf?referrer_access_token=Tv9RQYVZTfUOErHaUga4wtRgN0jAjWel9jnR3ZoTv0PcZvQWL5Z-R4Pni3X6og8peG3Rn-XTaAylcyVMjW1tWwY3l34gQKTwe31gWMPPHhEgp-C7DNSLtS7xKRF0ZepVCvzXiiOND3IZLksD176yGOWWBk0HdDnSqmjrLfFWwvHLSg1yjnizORiTdn0S3u
2h83v2EKfs-pNVvrFczQjveJ9dd2V6Dr4iSmtcmg959UlmyNoK6006UlQ8oAg5dUQOjAJxzCCP14FpFFLiuj2OiQ8qW
kEzTkVaR45J9J-7RpVD6nn2jbr-sdgEOCoUsdUE&tracking_referrer=www.spiegel.de://https://www.nature.com/articles/s41...=www.spiegel.de


"We've discovered that this is a planetesimal with a well-baked crust that looks a lot like the tiniest worlds
in the outer regions of our solar system," Bannister said in a statement.
"It has a greyish/red surface and is highly elongated, probably about the size and shape of the Gherkin skyscraper in London."

Ok, so we've been visited by an interstellar version of a state-fair battered and deep-fried pickle?
Chrunchy crust on the outside, moist on the inside?

[Gerald]

This seems to be within the possible scenarios on the basis of the noisy data.
But lack of evidence regarding the interior appears to be a pretty thin basis to conclude "moist" in the interior.
I'd be pretty sure, that we'll also see different interpretations in the future.

[JRehling]

It seems like the number of possible hypotheses is hard to bound, but one that comes to mind would be that it is a body that was once part of another planetary system that was ejected via gravity assists and has taken a long time to cross interstellar distances.

A gravity assist adds a velocity less than the orbital velocity of the assisting (large) planet to the assisted (small) object.

For this object to get such a large velocity before arriving at our system would require some very exceptional circumstances – compared to anything in our system. It seems that the velocity of the assisting object must have been very high. So one scenario would be a double star system or a system with a hot Jupiter that accelerated a small object that got in the right place at the right time. This would necessarily imply a passage close to at least one star, although that passage could have been arbitrarily brief and the star need not necessarily be as hot as Sol.

By no means does this discussion exhaust the possibilities, but it seems to be copacetic with our knowledge of planetary systems around dwarf stars. I don't know how one could quantify the possibility that it might come from a system very atypical of ones we currently know of.

[nprev]

We've got evidence that many systems have planets extremely close to their stars, and not all are hot Jupiters. In fact, entire systems have been found that would fit within Earth's orbit. This may increase the probability of ejection due not only to greater velocities imparted during encounters but a significantly higher probability of multiple encounters over time.

For example, I can easily see an object like this in a close-packed system whipped into a cometary orbit by one encounter and then experiencing another at some later time that either resulted in outright ejection or caused it to pass so close to its host star that ejection resulted. This would be even more likely if the initial perturbation kept its orbit within the local ecliptic plane since such systems are likely bound very close to that due to tidal effects.

A fast, close stellar passage might even be expected to produce the hypothesized 'baked Alaska' composition here; burnt on the outside, raw on the inside.

[Gerald]

...
For this object to get such a large velocity before arriving at our system would require some very exceptional circumstances – compared to anything in our system
...

Bernard's star has a radial velocity with respect to the Sun of about 110 km/s. 'Oumuamua can have easily left its parent system with a very small excess velocity, and still arrive at our solar system with a high velocity, if there has been a high velocity between the other star and our sun. Various more or less close encounters with other stars over potentially a few billions of years could have changed the initial trajectory and velocity almost arbitrarily.

[fredk]

Just noticed this preprint from a few weeks ago that adds a new twist: they claim the object is tumbling, rather than spinning about a principle axis. This explains discrepancies between various period determinations, and may have something to say about a possibly violent past.

The preprint also points out that partial shadowing effects can exagerate the rotational brightness variation, and by modeling such effects they can get away with an axial ratio of less than 5:1. So even though they also observe a brightness variation of close to 10:1, this doesn't imply that the dimensions are 10:1. (Of course there's also the possibility of albedo variations as well.)

[JRehling]

Great point, Gerald, and certainly correct. The only thing we know for sure about an interloper's origin is that it left its system. There must be some interesting things to say about selection effects, but those will be biases that shift the probabilities one way or another rather than provide an absolute constraint for any specific interloper like this one.

And, also correct nprev! Regardless of how much of the arrival velocity is due to the system-to-system velocity, the largest boosts leading to escape will happen close to the star and could explain a baked-Alaska outcome.

[HSchirmer]

It seems like the number of possible hypotheses is hard to bound, but one that comes to mind would be that it is a body that was once part of another planetary system that was ejected via gravity assists and has taken a long time to cross interstellar distances.

If this started as a binary KBO on a comet-like path, could our sun eject it?
We know the outbound path, but until there are precovery images,
we're extrapolating the inbound path, correct?

Given that lots of solar KBOs are binary, (mostly the cold population, but some hot as well)
and binary encouters can result in one "captured" the other "ejected",
could a close binary KBO result in one body being accellerated out of our solar system?

[JRehling]

The velocity of 'Oumuamua is far beyond the escape velocity of the Sun. There is no circumstance of orbital mechanics in our solar system that could accelerate an object that was initially in solar orbit to this velocity.