[X] My Assistant

[Jyril]

Although the Oort Cloud has most likely far larger number of objects than the Kuiper Belt, let's not forget how insanely large it is. It has been calculated (can't confirm, too lazy to check) that the average distance between Oort Cloud objects is about the same as the distance between the Earth and Saturn. So colliding with one would be really bad luck...

[David]

Although the Oort Cloud has most likely far larger number of objects than the Kuiper Belt, let's not forget how insanely large it is. It has been calculated (can't confirm, too lazy to check) that the average distance between Oort Cloud objects is about the same as the distance between the Earth and Saturn. So colliding with one would be really bad luck...

If the average distance between Oort Cloud objects is 10 AU, and if the Oort Cloud is about 50,000 AU distant, then by my (doubtless very fallible) calculations, a patch of sky measuring 1x1 degree of arc ought to contain c. 7500 of them. They ought to be occulting stars like crazy. Why haven't we detected them already?

[helvick]

If the average distance between Oort Cloud objects is 10 AU, and if the Oort Cloud is about 50,000 AU distant, then by my (doubtless very fallible) calculations, a patch of sky measuring 1x1 degree of arc ought to contain c. 7500 of them. They ought to be occulting stars like crazy. Why haven't we detected them already?

Just taking a quick stab at this.

If we took a 100km diameter Oort cloud object that was at 50K AU then it would be ~3e-6 arc seconds across. A sun type star at an average distance away in the galaxy (50K light years) is about 7e-7arc seconds across. So the Oort object could occult the star in theory.

Assuming stars are fairly uniformly distributed to make things simple then that 1x1 degree box will cover about 750000 stars assuming there are a 100 billion stars in the galaxy.

The 1x1 degree block has about ~1.4e18 spots the size of a kuiper belt object so if your 7500 number is right then the chance of any one of those spots (and a star behind it) being occluded by any one of those objects is around 1 in 2e14. The chance of any one of those having a star behind it at any point in time is about 1 in 2.5e8.

Assume it takes about a second for a typical object to move sufficiently to be covering a different spot then it will take on average 7.9 years before one occultation occurs in that particular spot. Of course there are many spots and if it was all uniform then I reckon there might be 1 occultation every 30 minutes or so somewhere in the sky. The problem is that you'd have to watch the whole sky very carefully indeed to catch it.

[alan]

The Taiwanese-American Occultation Survey will look for occultations causeed by Kuiper belt objects and Oort cloud objects. They expect the occultations to last ~ 1/2 second and produce diffraction patterns. Discription here
http://widefield.lbl.gov/2004/posters/occultation_lehner.pdf

[ngunn]

Messenger, I had an intuition of something like that from long ago. I was not convinced by the theory as what Ort cloud objects were in orbit around the Sun. If so, these orbits would be more or less circular, but not so excentric. If all the comets of the Oort cloud where in the very elliptic orbits we see, they would all be burned by the sun for long. If they needed to be disturbed from a more or less circular orbit, They would come in groups.

The idea you propose is much more appealing: the comets are not in orbit, they fall back on the Sun. In the process, they all more or less gather some lateral motion which makes them appear on very elliptic orbit (as a free fall trajectory is none else than a very elliptic orbit).

If the theory of the disturbance was true, only very few comets would have their orbital speed just canceled, and most would be on far orbits. There are too much comets falling right on the sun for this.

I thought the Oort cloud objects were supposed to be early ejecta from the giant planet zone, not distant circularly-orbiting bodies that only occasionally get perturbed inwards. If in fact nearly all of them have perihelia less than 30 AU then there don't have to be so many of them to account for what we see. After initial scattering, history would have selected them in two ways. The shorter period ones visit the sun too often and sublime away, while those with hyperbolic velocities never return at all. After 4,500 million years we are left with mainly the ones that have aphelia at 'Oort cloud' distances. Since they travel most slowly at aphelion that's where most of them are at any given time, hence a spherically symmetric 'shell' of objects with relatively low angular momenta. For once(!) I can't see any problem with this conventional view.

[Guest_Richard Trigaux_*]

I thought the Oort cloud objects were supposed to be early ejecta from the giant planet zone...

Yes, this is the conventional theory. Or at least one of the conventional theories, as another would be that the Kuyper Belt and Oort clouds would rather be remnants of the accretion disk which never condensed into planets. In facts none of these theories is supported by firm evidences.

... For once(!) I can't see any problem with this conventional view.

... except that comet dust was recently found to contain olivine and pyroxene, which are not meant to form at the distance of the Oort cloud, not into a accretion disk (in the hypothesis the Oort cloud is a remnant of such a disk) and even not at the distance of the giant planets (in the hypothesis it is the giant planets which scattered the oort cloud objects). This makes the formation and structure of the Oort cloud more difficult to understand.

If this olivine dust arrived at the time of the accretion disk, arises que question of how it was raised in this place. If it was formed more recently from the solar wind, it is difficult too to explain how it did. Perhaps into the shock wave with the interplanetary medium??

[ngunn]

... except that comet dust was recently found to contain olivine and pyroxene, which are not meant to form at the distance of the Oort cloud, not into a accretion disk (in the hypothesis the Oort cloud is a remnant of such a disk) and even not at the distance of the giant planets

I don't think this is too serious a problem. In the early solar system there would have been scattering by the inner planets too, throwing some rocky material outward. Also there could have been quite large protoplanets in the giant zone capable of forming rocky minerals that were later blasted apart by collisions. Furthermore most of the comets we observe have made numerous previous visits to the inner solar system where they could have picked up rock dust. With orbits as eccentric as the long period comet orbits are they could have picked up material from almost anywhere, and their individual histories could be radically different. I think we'll have to study quite a large sample at close range before we can significantly improve on current guesses.