What causes Kuiper/Ort cloud objects to become comets?

[Albertgauss]

TL;DR Summary

Comet origins

I was reading about comets and know they come from the Oort or Kuiper Cloud. However, being composed of materials, comets that continually orbit the sun shed the substance that makes them up and, from what I've read, will only thus last for thousands, maybe tens of thousands of years. I know there are comets with longer orbits than this, but they seem rare to the solar system comets.

That would mean comets would have to be continually replenished somehow if the solar system is several billion years old.

From what I understand, new comets do come from Oort or Kuiper cloud. But what could nudge 10^13 kilogram rocks like this into orbits at all? Our nearest star is light years away and that seems unlikely. I have also read about a red star (maybe a dwarf star of some kind) that came "close" to our solar system 70,000 years ago, but it doesn't seem like that would nudge many bodies out there into comets around the sun.

Are there any theories about this or is this really still a mystery?

Just looking for general answers or links to articles that might have more informaion.

 

[mathman]

Guess: There are enough objects in these clouds so that gravitational interactions take place frequently, altering orbits.

 

[phyzguy]

On the first question, they only have a short life when they plunge into the inner solar system and come close enough to the sun that the volatile ices evaporate. Out in the frigid depths of the outer solar system, they last basically forever.

On the second question, as @mathman said, it is gravitational interactions that alter the orbits and send them into the inner solar system. Not just of the small comet-like object with each other, but there are also larger objects like Arrokoth. If a potential comet comes close to an object like this, it can easily have its orbit perturbed so that it heads in toward the sun

 

[Drakkith]

Note that objects as far out as the Oort cloud are moving very slowly (less than 1 km/s?), meaning that even small perturbations can induce large orbital changes. Even a few hundred m/s change can change a huge, nearly circular orbit into one that comes screeching into hug the Sun.

 

[Albertgauss]

I guess so. I suppose there are calculations to prove the above. It seems that Kuiper or Oort objects are far less dense than our asteroids belt. I know the average spacing of an asteroid in the asteroid belt is 500,000 to 1 million km, but the objects out in Kuiper or Oort would presumably be spread out even farther, much farther than that. Wiki says below:

"Despite its vast extent, the collective mass of the Kuiper belt is relatively low. The total mass of the dynamically hot population is estimated to be 1% the mass of the Earth. The dynamically cold population is estimated to be much smaller with only 0.03% the mass of the Earth."

The collisions alluded to above would be very rare it seems, and I feel like there are far more comets than such collisions above could produce. But this is all speculation.

Anyway, just curious. From what I can see, it is still very mysterious the origin of comets. I'm satisfied with this post and feel like it is answered.

 

[phyzguy]

The collisions are very rare. There are ~10^12 objects in the Oort cloud, and only a handful of new comets each year.