View Full Version : Oort cloud and nearest neighbor

2010-Dec-30, 03:59 PM
I often see resources giving the extent of the Oort cloud as 2-4 light years.

However a triple star system is within 4.5 light years of the sun, and as a whole, is more massive than the sun.

So how is this reconciled? Wouldn't it make sense that if the sun has this cloud, then Alpha Centauri would also have one extending a similar distance? I've read that AC's Oort cloud got destroyed during formation, but this just seems like a way to remove a lot of inconveniences of a dual cloud existence. Even if it's not there, then AC must have more of an effect on the Oort cloud than say Jupiter.

2010-Dec-30, 06:20 PM
Remember that Proxima has a multi-thousand-AU orbit with a multi-Myr period. The idea that α Cen's Oort cloud (if it had one) would have been destroyed doesn't seem odd at all to me.

Furthermore, I haven't seen references saying the Sol Oort cloud is supposed to go out to 4 LY, but I may just as well be missing them.

As for Sol being in α Cen's Oort cloud... if it is, I don't know it could be guaranteed that we would actually notice. Trillions of comets, sure, but spread about a volume of space with a radius of 4.6 LY.

2010-Dec-30, 07:00 PM
See it said that it extends about a light year but is getting pretty tenuous by then. And is only loosely bound to the Sun AIUI. 4 LY is a bit too large, I think.

2010-Dec-30, 07:38 PM
Nasa once had a page that discussed this (it's not longer available from what I can see) where they suspected that the Centari Oort cloud (if it exits) and our systems Oort cloud might acctualy intersect. Thats about the only place where I can remember reading that Oort clouds might extend to 2-3 LY distance myself.

2010-Dec-30, 11:38 PM
I actually looked up the reference the website I saw 4 ly quoted, and the biggest thing I could find in that paper was 100,000 AU, which is under 2 light years by a bit... so I am not sure what they were smoking.

But it makes sense, that even if there are trillions of comets, a sphere that large is truly enormous. How could we differentiate from Oort cloud objects then, from just other space rocks? Undoubtedly there must be random space rocks outside any solar system just floating around, ejected from many places by numerous reasons.

2010-Dec-31, 12:05 AM
It mostly depends on how you define the outer limits of our Oort cloud = ten comets (mostly tiny) per cubic light year likely does extend beyond 4 light years and over lap with Centaurus's and Barnard Star's Oort cloud, if they have Oort clouds. If you define Oort cloud as more than a million comets per cubic light year, then our Oort likely reaches out less than one light year. This is mostly guess work as we have not directly determined that there is even one comet in any Oort cloud. Like common clouds, the outer limits likely are not spherical as they are perturbed by the near by stars and other near by mass, if any. Neil

2010-Dec-31, 03:54 AM
Really, anything outside the Sun's gravitational hill sphere isn't really ours. So it's not really reasonable to say that our Oort cloud can extend out to α Cen. If there's rocks that far, they most likely belong to α Cen anyway.

2010-Dec-31, 04:23 AM
There are occasionally updates appearing on the hunt for comets of extrasolar origin (i.e. flung out of another system by a close planetary approach, or maybe even us perturbing someone else's Oort cloud). Such things would be so rare that it would not be surprising that no comet clearly on an initial hyperbolic approach has been seen, but as detection improves, it would be surprising for that state of affairs to remain for several decades. (I did hear of a report once suggesting radar detections of a population of large dust grains entering the solar system from roughly the direction of Beta Pictoris, but that doesn't seem to have been strengthened by later data).