A passing by star makes comets come towards us?

I’ve read this a few times. The Oort cloud (is it even proven to exist, BTW?) is home of the “baby” comets… and its really far out from the sun. Comets start getting pulled towards the sun when a “passing by star” disturbs them. Sounds plausible enough for my feeble mind.

Question I have though is this. Where are these stars that “passed-by” the Oort cloud? We should be able to see them still, even if it has been millions (or even tens/hundreds of millions) of years, correct? Don’t we have a good idea of how close and fast moving our “neighbor” stars are? Do we know which ones came close enough to disturb the cloud? Could we calculate back as to when a star came close enough to disturb them and figure out “roughly” how long it will take for the comets to come into the “inner” solar system? Or is that getting pretty far fetched?

This is really all just a SWAG:

Every [some unkown millions of years] the sun passes thru the galactic disk, which is much more densely populated than outside the disk. That is where the opportunities for the “passing stars” occurs. I think that the term “passing star” makes it sound like a drive-by tug, but I don’t think they have to be particularly close to be considered “passing.”

In a few million years, when we go thru this disk again, maybe we’ll find the culprits.

Okay, somebody is riding a merry-go-round. You drive by and lasso that person. Nothing is more obvious than that he will fall off the merry-go-round.

But in which direction? Depending on your route and relative to the direction the merry-go-round is turning, he may fall outward onto the ground or towards the center of the merry-go-round.

In short, if an orbit is disturbed due to gravitational pull, the results will not necessarily be the immediately reasonable supposition of a trajectory towards the passing star or one at some compromise between it and the sun. A star heading “west” past a comet going “east” in its orbit (throwing in familiar terms for ease of visualization) may simply effect a braking tendency.

True enough, Polycarp, but I guess the meat of my question is which stars cause(d) this? And where are they now?

Hard to say. Say that a star came along 10 million years ago. Even if it was only traveling 1/10,000 the speed of light relative to us (not all that fast by galactic standards) it would be 1,000 light years away from us by now. There are an enormous number of stars within that radius. Taking any individual one, calculating backwards, and taking into account all the other encounters it may have had in the interim, is a task that few astronomers would want to waste computer time on.

Makes sense if you look at it that way. I wouldn’t have thought that “nearby” stars would move anywhere near that fast, though… but then again I’m the dummy asking the questions! :slight_smile:

I was kind of thinking that any “death” star that would have been close enough to cause this would have been within, say, I dunno… 50 ly away (now that is a figure I just pulled out of my ass).

If the star was moving sufficiently fast enough though, I agree. If it was far enough away now it would be a lot of time to probably narrow it down to a specific one or two.

IANAn Astronomer, but I think the following explanation is soiund: The Oort Cloud is very far away, and the comets there are only tenuously held in solar orbit. Being so far away, their “year” is millions of years long, and their orbital speed is very slow, so it takes very little outside force to radically alter their orbit.

Along comes a star. If the star passes by, and is moving more or less in the same direction as the comet is at that particular point in its orbit, that will accelerate the comet, and its orbit will become even wider, or it may break away from the sun completely. But if the star passes the comet in a direction opposite to its direction of orbit, then that will have a retrograde effect on the comet’s orbital motion and slow it down relative to the sun. This will make the comet’s perigee lower.