Of course, it is, in no way, physically impossible that there are interstellar interlopers out there (there are, of course), only that such things are unusual.  Again, the burden of proof would be on those who would suggest that the object was artificial in nature, as opposed to a natural object that simply got ejected from its parent star.  If the latter, however, then there must, statistically, be a lot of these things out there.

I just find it to be suspicious, but suspicion is not evidence.

(December 13, 2017 at 9:41 pm)Jehanne Wrote:

(December 13, 2017 at 9:12 pm)Gawdzilla Sama Wrote: Why?

If it formed around its parent star, then it was in an elliptical orbit to begin with.  To go from an elliptical orbit (E < 0, negative energy, where K < U, "kinetic energy is less than potential") to a hyperbolic orbit (E > 0, where K > U) would require some interaction within its home system.  And, then, it would need to be expelled in such a way as to intersect our Sun in its galactic orbit around the Milky Way at around 250 km/s.  Given the "local frame of rest", it seems unusual that any natural object could gain enough energy to, 1) escape its parent star, and 2) to have an intersecting orbit with a neighboring stellar system.

But, "extraordinary claims demand extraordinary evidence" (unless, of course, one is a Christian or Muslim).

P.S.  Local standard of rest:

https://en.wikipedia.org/wiki/Local_standard_of_rest

My point was that there's no reason to believe such objects form exclusively around stars.

The reason to believe this kind of object would be difficult to form in any environment other than around stars, or at least progenitors of stars, is random motion excerts a pressure on any collections of particles In interstellar space just as if they were a gas.  Small assembligaes of particles are therefore difficult to form because particle pressure prevent any over concentration from forming.  Any particles that touch and stick together are more likely to be blasted apart by the next extremely rare collision through random motion than to stick to form larger assemblagess.

The only time this condition change is if gravitational instability causes gas cloud to collapse.  This becomes a self reinforcing process and will eventually lead to formation of one or more collapse gas cores in the cloud that become progenitors of stars.   Only when the core is fairly massive and well on its way to becoming an infant star, would it excert enough gravity on the surrounding to create such an overbdensity of particles and gas can be sustained, and particles are likely to stick and grow rather than collide and then be blasted apart.

I've a calculation that suggest without gravitational help from a star or star progenitor,  the average time when particles in interstellar space can randomly accrete to asteriodal size is many orders of magnitude greater than the age of universe.

(December 14, 2017 at 8:17 am)Anomalocaris Wrote: The reason to believe this kind of object would be difficult to form in any environment other than around stars, or at least progenitors of stars, is random motion excerts a pressure on any collections of particles In interstellar space just as if they were a gas.  Small assembligaes of particles are therefore difficult to form because particle pressure prevent any over concentration from forming.  Any particles that touch and stick together are more likely to be blasted apart by the next extremely rare collision through random motion than to stick to form larger assemblagess.

The only time this condition change is if gravitational instability causes gas cloud to collapse.  This becomes a self reinforcing process and will eventually lead to formation of one or more collapse gas cores in the cloud that become progenitors of stars.   Only when the core is fairly massive and well on its way to becoming an infant star, would it excert enough gravity on the surrounding to create such an overbdensity of particles and gas can be sustained, and particles are likely to stick and grow rather than collide and then be blasted apart.

I've a calculation that suggest without gravitational help from a star or star progenitor,  the average time when particles in interstellar space can randomly accrete to asteriodal size is many orders of magnitude greater than the age of universe.

So stars can accrete by collecting smaller bodies but those smaller bodies can't accrete?

That is right.

Think of it this way.  Gas and dust Cloud experience  temporary local over-densities due to random motion, passage of pressure wave, and other causes.  If the mass of overdensities are not enough,  natural pressure of the gas and dust will cause over-density to simply disperse, and nothing will happen.  For something to happen, Some thing has to not only hold overdensity together, but add to it.

The only way this can initiate is if some large scale event, like try he shockwave from a supernova, create overdensity in large volume of space, so that total mass of the over density is enough right at the beginning to create its own gravity strong enough to arrest the natural tendency to disperse under its own pressure.  When this happens, the overdensity become self perpetuating, and will start to pull in more gas from surrounding, get bigger and bigger, until eventually the region runs out of more gas, or the overdensity comes so massive it ignites and becomes a star, and photo pressure from the light it emits arrest further in fall of surrounding material.

The threshold mass for an initial overdensity in interstellar gas and dust to be able to arrest its own dispersal with its own gravity is calculated to be several Jupiter masses.   No initial overdensity the mass of a mere asteroid in interstellar medium is going to coalesce into an asteroid rather than disperse.  Unless some initial overdensity is over several Jupiter masses, it would disperse and leave no remnant or residual object.  

Only way for smaller objects to accrete is if some large nearby object can excert so much gravity that it pulls in material from around itself, concentrate them into an accretion disk or cloud, and confine them with its own gravity.   If this happens the density of dust and gas around the large object goes up to many orders of magnitude above interstellar medium.  Then the vastly increased chance of collision between small particles as well as reduced relative velocities between particles due to cohesive motion in the accretion disk lets particles stick and not disperse.   This is how small objects can accrete.

So the upshot is in interstellar space, objects more than several times the mass of Jupiter can form by themselves and then gain further mass.  Objects less than several Jupiter masses can't form by themselves , and require An accretion disks or accretion cloud confined by the gravity of nearby stars or protostars.

This is why we think any interstellar asteroid or comet we find almost certainly formed as an circumstellar asteroid or comet around another star, and then somehow got loose.

.

I don't believe your explanation.

(December 14, 2017 at 10:53 am)Gawdzilla Sama Wrote: I don't believe your explanation.

Suit yourself

(December 14, 2017 at 10:53 am)Gawdzilla Sama Wrote: I don't believe your explanation.

The conservation of angular momentum comes into play, which results in an accretion disk, and from there, planets, asteroids (failed planets), comets, etc.

(December 14, 2017 at 4:35 pm)Jehanne Wrote:

(December 14, 2017 at 10:53 am)Gawdzilla Sama Wrote: I don't believe your explanation.

The conservation of angular momentum comes into play, which results in an accretion disk, and from there, planets, asteroids (failed planets), comets, etc.

That that makes it impossible for such an object to form elsewhere?

(December 14, 2017 at 5:56 pm)Gawdzilla Sama Wrote:

(December 14, 2017 at 4:35 pm)Jehanne Wrote: The conservation of angular momentum comes into play, which results in an accretion disk, and from there, planets, asteroids (failed planets), comets, etc.

That that makes it impossible for such an object to form elsewhere?

No. It's just unusual given the immense volume of space for there to be interstellar interlopers. Clearly, much more scientific study is needed.