# Why is the Oort cloud spherical?

Having read up a bit about the Oort cloud hypothesis, I find I don't quite understand this. Why is the Oort cloud spherical while the rest of the solar system is disk-shaped?

• From Wikipedia: Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud.[3] On the other hand, the Hills cloud, which is bound more strongly to the Sun, has yet to acquire a spherical shape. Oct 20 '14 at 19:29
• Objects inside Neptune's orbit have had the opportunity to interact with other objects many, many times. Orbits that are not circular or in-plane have a greater chance of interactions that modify the orbit. Over time, such orbits are eliminated. The Oort cloud is so sparse and speeds slow that interactions between objects are minimal and other orbits can persist for long periods. Oct 20 '14 at 21:43
• The better question may be: Is the Oort could spherical? It assumes less and is less likely to be proven to be a poor question if it turns out that the Oort cloud is actually quite distorted by interactions much of the time. Having said that, I don't know if that is the case or not... just trying to give food for thought for what may be a really exciting scientific question that may take a lifetime of work to answer. Oct 21 '14 at 2:37

I actually answered a related question a couple of days ago on Astronomy. Small world!

One of the important properties of the Oort Cloud is that objects in it are not strongly influenced by the Sun. After all, its inner edge is roughly 2,000 AU away - 300 billion kilometers from the Sun. The Sun's gravitational influence in that region is rather weak, so the Cloud can be easily subjected to other effects.

Galactic Tides

The first big effect (which stackErr mentioned) is the galactic tide. Tidal forces can stretch an object into different shapes. As Wikipedia puts it,

Galactic tides may then deform an otherwise spherical Oort cloud, stretching the cloud in the direction of the galactic centre and compressing it along the other two axes, just as the Earth distends in response to the gravity of the Moon.

So any disk-shaped object this size this far away from the Sun would inevitably become stretched out into an oblong or spherical shape.

Nearby Stars

The outer limits of the Oort Cloud may be over 1 light-year away. By comparison, the distance to the nearest star system (Alpha Centauri A/B and Proxima Centauri) is roughly 4 light-years. Once again, the weakness of the Sun's gravity means it is easier for the Cloud to be deformed. Passing stars long ago could have contributed to its shape.

As this picture shows,

Stars can and will come closer than that - just within the next 80 millennia! Within the past 4 billion years, odds are that we've had a lot more closer encounters. So any stars within a light-year or so would have had a pretty big impact on the Cloud.

Also, some of the bodies in the Oort Cloud may have originated in other star systems, or from somewhere completely different. Interstellar comets and Exocomets could have been captured by the Sun. This would mean that they wouldn't have to have orbits in the plane of the rest of the solar system - they could travel in orbits tilted at odd angles.

• The graph really helps. I didn't have a sense of the distances involved, I had no idea that there were points in time where the Oort cloud's radius could be nearly a third of the distance to the nearest star. That makes the tidal forces explanation much clearer.
– Jack
Oct 20 '14 at 23:40
• It is also highly probable that the Sun was once part of a much denser cluster of stars. Close encounters would have been much more common. Oct 21 '14 at 9:55