# Do all stars have an Oort cloud or is it a rare occurence?

Do all stars have an Oort cloud like ours that will be filled with comets and other objects? If not, why are they not around every star?

Awesome question, especially since we know so little of the answer.

Nobody knows for sure how the Oort Cloud formed - I'll put that out there right now - but the current hypothesis is that it was originally part of the Sun's protoplanetary disk. All of the ice and rock coalesced into small bodies - proto-comets, if you will. While these bodies were much closer in to the Sun than they are today, they were tossed far out by gravitational interactions with the gas giants. Other interstellar comets could also have been captured by the Sun, adding to the population.

So why is the Oort Cloud spherical? After all, the protoplanetary disk was just a flat disk. Why were the orbits of the objects perturbed? Well, the Oort Cloud objects are only loosely bound to the Sun - relatively, that is. They can be influenced by passing stars or other objects. It appears that galactic-scale tidal forces, combined with the influence of passing stars, molded the Cloud into its current spherical shape.

So what does this all tell us? Well, we know other stars have protoplanetary disks, right? Some also have exoplanets - gas giants like Jupiter. They are also subject to tidal forces and the passing of nearby stars. So, theoretically, there's no reason why other stars shouldn't have Oort Clouds.

So can we find them? The answer is, most likely, no. Here's why. According to Wikipedia,

The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi), and billions with solar system absolute magnitudes brighter than 11

An absolute magnitude of a solar system object of 11 is very dim. Now, the object's apparent magnitude is how it would look from a given distance; the absolute magnitude is how it looks from a distance of 1 AU (in the case of Solar System objects, this quantity is denoted $H$). Oort-cloud objects are 2,000 - 50,000 (or more) AU away-- so these objects, to us in the same solar system, have an apparent magnitude much fainter than 11.

The point of that poorly-explained interlude is that these objects are faint. Very faint. And objects in Oort Clouds around other stars would appear even fainter. Using the distance modulus, we can calculate the apparent magnitude of an object if the distance to that object and its absolute magnitude are known:

$$m-M=5(\log_{10}d-1)$$

(from here)

where $m$ is apparent magnitude, $M$ is a scaling of $H$ normally used for stars, and $d$ is the distance in AU.

Given an Oort Cloud object $x$ light-years away, you can figure out how bright (or dim) it would appear, given that 1 light-year is 63241 AU. Try this with the distances of nearby stars, and you'll realize how dim objects in these stars' Oort Clouds would be.

As a final note: We don't know for sure if other Oort Clouds exist. From what I've been able to find, we don't have sufficiently powerful telescopes to observe these hypothetical Clouds, and so we don't (and may never) know if they exist.

I hope this helps.

This paper was instrumental in this answer. Start at page 38 for the relevant information. This page, too, has some good information.

As I found from a link from an answer to this question on Physics, we've found Kuiper-Belt-like disks around other stars. This means it is certainly plausible for these stars to have Oort Clouds, too. And exocomets have been detected, which is another good sign.

• Concerning the assertion that passing stars may disturb Oort cloud objects, this question is in need of elucidation! – dotancohen Oct 19 '14 at 12:28
• I am pretty sure that absolute magnitude for solar system bodies is calculated at 1 AU (rather than 10 pc; it also says so in the note on Wikipedia; and it's like a factor of 2000000, which in magnitude terms is about 16 magnitudes dimmer)! Because while the 11 absolute magnitude you mentioned is low, there are STARS with that brightness, and there cannot be Oort cloud objects brighter than stars. This is especially important because we cannot even properly 'see' our own Oort cloud. So seeing other stars' Oort cloud (and the need to explain it in detail) is out of question. – Takku Oct 20 '14 at 16:17
• Also, I get a feeling that the dominant balancing forces at the distance are (a) radiation pressure and (b) gravity. Since both are radial, balanced and weak, the Oort cloud has spherical symmetry. And it's also true that the galactic tidal forces do play a role in making it spherical and also in imparting angular momentum to it. – Takku Oct 20 '14 at 16:23
• @Takku Where's the note on the Wikipedia page? (I'll change my answer if it's wrong) – HDE 226868 Oct 20 '14 at 16:40
• Note #14. Right next to the term absolute magnitudes. – Takku Oct 20 '14 at 18:00

All stars could have their own Oort cloud, but all stars don't. As HDE says the Oort cloud was formed by material in the sun's protoplanetary disk and interstellar comets that were caught by the sun. Some theories say that almost all comets were formed around the Sun, and this wouldn't allow us to say much about comets around other stars. However there are others such as Levison et al. in Capture of the Sun's Oort Cloud from Stars in Its Birth Cluster that contend that a majority of the comets have to have their origin from other stars' protoplanetary disks. This must be so because the current models fail to explain the amount of comets in the Oort cloud.

The sun was born in a star cluster in close proximity to other stars. These stars would have to be the source of any significant contribution to the Oort cloud by stars other than the Sun. That is why Levison et al. tested their hypothesis by constructing a young star cluster and simulating its dynamics with an n-body simulator. They found that their hypothesis was confirmed by the simulation. If every star in the star cluster starts with some comets in their respective protoplanetary disks, then some stars collect many more comets from their neighbors before they leave the star cluster, as the Sun did long ago.

I recently wrote an N-body simulator to replicate their paper. The result is not available in English, but the experiment is described briefly here. There is an animation there that will help you understand what I am getting at. My simulator ended up being a lot slower than theirs so I couldn't have nearly the same number of objects in my simulations, but I observed the same tendencies as they did.

To answer the question, I did see that some stars lost all their comets and I also saw some stars that gained comets. It all depended on the dynamics in the star cluster. Some left it very early with just the comets that formed in their own protoplanetary disks, some left it with more comets than that and some left it with no comets at all. Based on these simulations I would say that, as I started by saying, all stars could have their own Oort cloud, but all stars don't.

• Great answer; +1. The simulation part was excellent. – HDE 226868 Oct 18 '14 at 23:19
• @HDE226868 Thanks :) – user2631 Oct 18 '14 at 23:29