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This is a little misleading because it presents the idea that there is a firm separation between "comets" and "asteroids", each of which have very different pro

This is a little misleading because it presents the idea that there is a firm separation between "comets" and "asteroids", each of which have very different properties. In reality we've discovered in recent decades that there is significant overlap. Pure comets were formed in the outer solar system beyond the frost line, and they are composed of a mixture of primarily chondritic grains and ices. As noted such bodies are not very structurally strong so if they approach the Sun too closely they will break up from the stress of gravity as well as from the intensity of off-gassing vaporizing gasses. On the other hand you have classic asteroids which are formed inside the frost line and are made up of either chondritic material, stone, or metal (depending on their histories). Such asteroids can be much stronger but they lack the ices which might off-gas during a close solar encounter. Except, that dichotomy is a simplified fiction. The reality is that there are objects in the middle, sometimes called "iceteroids" which we've discovered over the years. These could be the cores of comets which have lost most of their ices or these could be objects with more complicated histories, such as asteroids which have accreted surface layers of ices through various processes. So this is a mystery but it's not some enormous mystery it's a small mystery. The idea that an asteroid could have acquired a small amount of ices on its surface isn't exactly world shattering news but because we don't know the history of the object we don't know the process by which it ended up with its observed properties.

Balgair 8y ago
To me, the interesting thing is that this iceteroid (love the term!) is definitively extra-solar and that means that it is likely that other systems have both asteroids and comets. Though we've seen asteroids in other systems like Fomalhaut [0], we've not been up close and personal with the compositions of extra-solar material. Here in the Sol system, we have a pretty strange arrangement of planets [1], with big ones all in the outer system and smaller ones in the inner system. So looking at just our system is not very indicative of 'typical' system evolution, in terms of iceteroids. Once we figure out where Oumuamua came from, that'll give us some idea of the compositions and evolutions of other systems (data point of 1 is better than 0), like the frost line. Oumuamua should be viewed as awesome and exciting, but as the tip of the ... iceberg ... when it comes to the real questions of stellar evolution and all the awesome stuff happening there. [0]https://en.wikipedia.org/wiki/Fomalhaut https://en.wikipedia.org/wiki/Fomalhaut [1]https://en.wikipedia.org/wiki/Nice_model https://en.wikipedia.org/wiki/Nice_model
- InclinedPlane 8y ago
  Good point. One thing we learned from exoplanet hunting is that there is a much greater diversity out there than we had originally thought, implying that planet formation is a lot more of a dynamic process than our previous models indicated. Prior to the discovery of exoplanets our planet forming models were very much "over fit" to the example of our own Solar System, giving the impression that other stellar systems would end up looking similar to our own. The reality is that our Solar System is, as you say, probably unusual. One aspect of this is that it seems like planetary migrations are pretty common (both inward and outward). If that sort of thing happens early during planet formation then it could easily explain an "asteroid" (which we understand is almost certainly a planetessimal fragment, though many asteroids are just that) which has acquired additional material including ices. The obvious followup here is that we need way, way more closeup observations of small bodies in our own Solar System, we probably also need to develop better techniques for observing interstellar interlopers, and we need to improve our studies of extrasolar planetary systems. Fortunately the first part is already covered, between the Hayabusa 2, Osiris-REX, Lucy, Psyche, and New Horizons missions we should collect a lot more information on such bodies. However, it's probably going to be a long time before we have answers even to simple questions like the formation histories of most small bodies in stellar systems and what the "average" stellar system looks like.
  - Balgair 8y ago
    I have close friends working on Psyche! You are right that our understanding of small bodies is going up very quickly, it's quite exciting (in asteroid terms) to be alive now; a lot is being answered and re-questioned with actual evidence. Hopefully (knock on wood) JWST gets up there soon and a LOT more questions can be answered. One issue we have is that there is this 'gap' in planet formation modeling. We can figure out small bodies and large bodies, but things that are ~1m to ~1km in size don't work in the models; they should all just fly apart. Somehow planets do form, but getting there is a pickle, theoretically. Asteroids are likely THE key to understanding that gap, and that should help out with a LOT of system formation dynamics. Also, I'm still bullish on finding a LOT of platinum/gold on asteroids, because space mining. Like, eat your heart out Bruce Willis.

Balgair 8y ago

To me, the interesting thing is that this iceteroid (love the term!) is definitively extra-solar and that means that it is likely that other systems have both asteroids and comets. Though we've seen asteroids in other systems like Fomalhaut [0], we've not been up close and personal with the compositions of extra-solar material. Here in the Sol system, we have a pretty strange arrangement of planets [1], with big ones all in the outer system and smaller ones in the inner system. So looking at just our system is not very indicative of 'typical' system evolution, in terms of iceteroids. Once we figure out where Oumuamua came from, that'll give us some idea of the compositions and evolutions of other systems (data point of 1 is better than 0), like the frost line. Oumuamua should be viewed as awesome and exciting, but as the tip of the ... iceberg ... when it comes to the real questions of stellar evolution and all the awesome stuff happening there. [0]https://en.wikipedia.org/wiki/Fomalhaut https://en.wikipedia.org/wiki/Fomalhaut [1]https://en.wikipedia.org/wiki/Nice_model https://en.wikipedia.org/wiki/Nice_model

InclinedPlane 8y ago
Good point. One thing we learned from exoplanet hunting is that there is a much greater diversity out there than we had originally thought, implying that planet formation is a lot more of a dynamic process than our previous models indicated. Prior to the discovery of exoplanets our planet forming models were very much "over fit" to the example of our own Solar System, giving the impression that other stellar systems would end up looking similar to our own. The reality is that our Solar System is, as you say, probably unusual. One aspect of this is that it seems like planetary migrations are pretty common (both inward and outward). If that sort of thing happens early during planet formation then it could easily explain an "asteroid" (which we understand is almost certainly a planetessimal fragment, though many asteroids are just that) which has acquired additional material including ices. The obvious followup here is that we need way, way more closeup observations of small bodies in our own Solar System, we probably also need to develop better techniques for observing interstellar interlopers, and we need to improve our studies of extrasolar planetary systems. Fortunately the first part is already covered, between the Hayabusa 2, Osiris-REX, Lucy, Psyche, and New Horizons missions we should collect a lot more information on such bodies. However, it's probably going to be a long time before we have answers even to simple questions like the formation histories of most small bodies in stellar systems and what the "average" stellar system looks like.
- Balgair 8y ago
  I have close friends working on Psyche! You are right that our understanding of small bodies is going up very quickly, it's quite exciting (in asteroid terms) to be alive now; a lot is being answered and re-questioned with actual evidence. Hopefully (knock on wood) JWST gets up there soon and a LOT more questions can be answered. One issue we have is that there is this 'gap' in planet formation modeling. We can figure out small bodies and large bodies, but things that are ~1m to ~1km in size don't work in the models; they should all just fly apart. Somehow planets do form, but getting there is a pickle, theoretically. Asteroids are likely THE key to understanding that gap, and that should help out with a LOT of system formation dynamics. Also, I'm still bullish on finding a LOT of platinum/gold on asteroids, because space mining. Like, eat your heart out Bruce Willis.