'Oumuamua: First extrasolar object spotted?

[Brian G Turner]

The BBC reports on latest research that suggests 'Oumuamua may be a comet rather than asteroid.: Interstellar visitor's identity solved

However, it seems a little bit at grasping at straws to define it - it has previously been suggested that it might be a planetary fragment: [1712.01823] 1I/`Oumuamua as a Tidal Disruption Fragment From a Binary Star System

In which case, wouldn't it's definition depend on its original - and still unknown - origin, rather than its current situation?

 

[Venusian Broon]

However, it seems a little bit at grasping at straws to define it

I disagree.

A comet is 'a celestial object consisting of a nucleus of ice and dust and, when near the sun, a ‘tail’ of gas and dust particles pointing away from the sun.'

It may be a very old 'comet husk' as the evidence they have found of the extra force and how to explain it seems to suggest that there is still outgassing of dust and gas (water vapour?) from the object. And hence a better classification

A planetary fragment or asteroid? Surely that would be mostly rocky and devoid of ice? If it was then it surely wouldn't have had this mysterious course correction.

It means perhaps that rather than search for it's creation in 'tidal disruption from a binary star' that perhaps there are other objects in the Kuiper belts/Oort clouds that look much like it and we should be looking for mechanisms for its formation in how those regions formed instead? (p.s. I'm saying it was still 'flung' out of an Oort cloud/ of another star, not our Oort cloud btw...)

Just my initial thoughts on the article

 

[Brian G Turner]

A comet is 'a celestial object consisting of a nucleus of ice and dust and, when near the sun, a ‘tail’ of gas and dust particles pointing away from the sun.'

The problem to me is our definition of "celestrial object" invariably assumes "formed within our solar system", which isn't the case here. Also, we have no evidence of a nucleus of ice and rock, merely the suggestion of ice and rock.

Our definition of a comet seems dependent solely on bodies believed to have formed on the outskirts of our solar system - I think 'Oumaumau is showing the limitations of that, hence why it has been variably asserted to be comet or asteroid by different research groups.

 

[Venusian Broon]

The problem to me is our definition of "celestrial object" invariably assumes "formed within our solar system",

I, in all my years of science education, have never assumed this. Celestial means anything and everything beyond the Earth's atmosphere. Stars and Galaxies, for example, have always been part of the definition for me!

As I stated, there is nothing wrong with it being an old comet kicked out of it's original star system and arriving, for a moment, amongst us and the sun.

Also, we have no evidence of a nucleus of ice and rock, merely the suggestion of ice and rock.

The evidence suggests that there is outgassing. Outgassing requires something for it to outgas. Ice & dust is a good bet. One would have to get a bit closer to do more experiments as to exactly what this is, alas I think it's on its way out for good.

Otherwise what's your explanation?

Note that:
"Asteroids are differentiated from comets...in the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are composed of dust and ice." Hence this suggestion by the researchers and it's classification.

Note also that these objects tend to form, according to our limited understanding, in different parts of a star system, so trying to figure out what it is, helps our understanding of its origins.

Our definition of a comet seems dependent solely on bodies believed to have formed on the outskirts of our solar system

No, I don't think there are any astronomers who would have said that comets can't exist in other star systems. Oort clouds are, I believe, expected to form around all stars as they are born, so why wouldn't comets be absolutely everywhere, orbiting every star system?

Plus as there are a great many of them, given that they are cosmically tiny by mass, (for example, I believe 5000+ are known in our system), so with what we know of the gravitational 'nudging' and 'deflection' one could imagine that there are great many Oort cloud objects, (which we would define as comets), being flung all over the universe and escaping their original systems.

Anyway, seems like a good start for trying to understand it. Maybe, if fresh evidence turns up, this theory will be wrong, but we have to go with the evidence that we have

 

[J Riff]

It gots to be a planatery fragment, since that's the only thing ever blowed up round these heah parts.

 

[Brian G Turner]

Otherwise what's your explanation?

I just think we're better off keeping an open mind as to 'Oumaumau's actual nature, rather than rushing to pigeon-hole it as something familiar - especially when it doesn't show classic comet traits. As we already knew there was ice on the surface it would be more surprising if there wasn't any form of gas ejection.

Celestial means anything and everything beyond the Earth's atmosphere.

I know, I just meant in the instance of defining a comet as celestial. I suspect we're going to have to start inventing new catagories to account for observations such as 'Oumaumau and other extra-solar bodies. It's been a sleep-deprived week so I'm probably not explaining myself very well, though.

 

[Brian G Turner]

And now an attempt to backtrack where 'Oumaumau may have originated from: Tracking the interstellar object 'Oumuamua to its home

However, I suggest we'll need more data - and that may involve waiting until the next update from the Gaia mission, in 2021.

 

[Brian G Turner]

Gosh, I come across as a little grumpy and incoherent in the posts above - sorry about that.

In the meantime, interesting comments about observations from the Spitzer telescope, which suggest it wasn't giving off any dust or gas:
'Oumuamua one year later

After 30 hours of staring – a relatively long time – the object was not detected, and subsequent orbital analyses confirmed that the camera was pointed correctly toward it. The limit to its emission, however, was so low that it enabled the team to constrain some of its physical properties. The lack of an infrared signal, for example, suggests it has no gas or dust, species that would be expected if it were a cometary-like body.

Presuming they were actually looking in the right place.

 

[Brian G Turner]

Gosh, I come across as a little grumpy and incoherent in the posts above - sorry about that.

And that's especially @Venusian Broon:

Celestial means anything and everything beyond the Earth's atmosphere. Stars and Galaxies, for example, have always been part of the definition for me!

You're totally right - I must have been exhausted to have made such a fundamental mistake. I blame getting a puppy around that time.

What I was meaning to say is that I wonder if objects like 'Oumaumau will force us to reconsider how we classify objects like that?

For example, do we define asteroids and comets only by their behaviour, or will other details count? For example, Eon Musk apparently launched one of his cars into space - if we presume that over millions of years it might accumulate a coating of dust so as to no longer look like a car, would we still call it a car, or even man-made debris, especially if it were to behave like an asteroid or comet?

If we don't define it as an asteroid or comet, then why not?

Hence with 'Oumaumau - should we really try to only define it in terms of familiar near objects, or should we allow us room to reserve a different name or term, if we - hypothetically - were ever able to ascertain its origins? For example, if it were found to be a fragment of a planet, traveling through interstellar space, would the definition of comet really apply?

In fact, if its not bound to the Sun then can it ever really be defined as a comet at all?

I'm not pushing for answers - I'm simply thinking aloud. Because I suspect that the more we learn about the universe around us, the more difficult it may become to define some objects we will observe in familiar terms, simply by fact of their unfamiliarity.

2c.

 

[Venusian Broon]

Grumpy? I didn't notice

Use a definition if it fits, if whatever your defining strays far from the definition then build a new definition.

My point was really that Oumaumau didn't require any new definition as of now. It is fascinating and unique because of it's trajectory and shape (other objects are now being proposed to have come in from outside the solar system, and have actually been captured, but there's always the probability there is another mechanism that means they didn't originate from outside!) but it's properties were familiar enough, after the observations we've made. Sure it must have come from another solar system, but why shouldn't another solar system produce objects similar to the ones that our does? Another tick for the Copernican Principle .

Re: Tesla car. I know your trying to find something different to make your point. But I'd really call it junk or man-made/artifical debris, another well used definition . Especially as I believe it is likely to fall apart and litter the universe with bits of car! However if it tends to 'comes to rest' in the asteroid field and behave like an asteroid (I don't think it's got much outgassing in it!), then it's also an artificial asteroid. What's wrong with that? No need to tie yourself in knots about what to call it.

Take satellites - I'd bet most people instinctively think that means a man-made object orbiting the Earth, but actually the word also applies to any natural object that orbits another, so really it should be that all those weather and spy platforms we've put up into orbit around us are really artificial satellites. But the term artificial tends to be dropped in everyday conservation...



Regarding comets that are not bound by the Sun or any other star, we actually do have a model (essentially a model of where long-period comets come from) and therefore a definition for them. We currently call such things Oort cloud objects.

Oh, (1) I'm sure there may be Oort cloud objects that are asteroids by definition (a quick look states that the current estimate is ~1-2%) although through the process of solar system formation, the majority are generally made of ice - hence will outgas if pushed into an orbit around a star.

and Oh (2), an actual asteroid pushed from this Oort cloud into a trajectory that looks like a long-period comet is...an asteroid. The definition is pretty clear - outgassing!

 

[Brian G Turner]

Take satellites

Ok, that's a pretty good example.

Regarding comets that are not bound by the Sun or any other star, we actually do have a model (essentially a model of where long-period comets come from) and therefore a definition for them. We currently call such things Oort cloud objects.

I had presumed that Oort Cloud objects were weakly bound to the sun's gravity, though I have seen suggestions that it could extend over 1.5 light years from the sun. If that were the case, it might seem that the Oort Cloud is not really something that follows our star, as much as interstellar debris that we happen to be passing through. Hmm, I like that idea.

 

[Brian G Turner]

And keeping with 'Oumaumau, a short piece on the outgassing today includes a nice animated image showing how that might have provided acceleration:

NASA Learns More About Interstellar Visitor 'Oumuamua

 

[Brian G Turner]

And now a new theory - if out-gassing wasn't observed, maybe that's because radiation pressure could account for the change in acceleration? In which case, could 'Oumaumau have been foamy? Phil Plait gives good coverage to this latest idea: No, 'Oumuamua is not an alien spaceship. It might be even weirder.

 

[Serendipity]

Interesting article on how Oumuamua may have formed... Interstellar object ‘Oumuamua believed to be ‘active asteroid’

 

[Ursa major]

You could imagine them as sandcastles floating in space

This analogy might work...


...if anyone had had a recent experience of seeing a sandcastle....

​

 

[Brian G Turner]

I'm wondering if the supposed out gassing might have been a layer of ice that could have formed on the outside of 'Oumaumau, which was evaporated from the heat of the sun as it neared. IMO that's a simpler and more practical explanation.

 

[Brian G Turner]

Oh, wait, they have said that in this latest research:

New formation theory explains the mysterious interstellar object 'Oumuamua

Since its discovery in 2017, an air of mystery has surrounded the first known interstellar object to visit our solar system, an elongated, cigar-shaped body named 'Oumuamua (Hawaiian for "a messenger from afar arriving first").

phys.org

"If 'Oumuamua was produced and ejected by the scenario of Zhang and Lin, plenty of residual water ice could be activated during its passage through the solar system. The resulting outgassing would cause accelerations that match 'Oumuamua's comet-like trajectory. "

 

[Paul_C]

"Likely made of hydrogen ice"

Hydrogen ice? Unheard-of composition could explain 'Oumuamua's weirdness

 

[Serendipity]

Interesting article about Oumuamua here - Interstellar visitor 'Oumuamua could still be alien technology, new study hints

 

[Brian G Turner]

I knew without clicking that it must be Harvard trolling the press again.