Oort Cloud

After the formation of the planets the leftovers were scattered, mainly by Neptune, onto orbits that took them towards the stars. Some of them kept right on going and flew off into the galactic disc, and others dived into the inner solar system. Still many more formed a huge spherical cloud called “the Oort Cloud”. This is where the gravitational pull of the sun was just enough to marshal them into a giant swarm of cometary nuclei. This at least is the theory. But I have my doubts. But let’s for the moment stay with the received wisdom.

Nasa's graphic of the Oort Cloud
A graphical representation of the Oort Cloud, prepared by Nasa

The Oort Cloud is the last bastion of Trans-Neptunian space—a poorly-understood region that contains the earliest, most diffuse components of the solar system. This spherical shell of dark, cold objects is believed to be the last fragments of the Sun’s protoplanetary disc, pushed ever farther from the Sun as the inner planets, particularly Jupiter, formed and grew. Propelled by Jupiter’s massive gravitational effects, these small objects travelled to the fringes of the solar system where they now fall under the influence of galactic perturbations and the movement of passing stars.

The Oort Cloud is spherical, an outer shell of the Solar System at a distance of roughly 1.6 light years from the sun. In other words, it is like we are all living inside of a bubble some 3.2 light years in diameter.

The Oort Cloud is a theory proposed by Jan Hendrik Oort (1900 – 1992). The materials are concentrated in two regions in this theoretical sphere: the Inner Oort Cloud and the Outer Oort Cloud. And this is where things start to get a bit confusing because no one seems to agree on the dimensions:

  • The Oort Cloud is a spherical shell that occupies space between 748 billion kilometres and 14 960 billion kilometres from the sun.
  • Icy planetesimals surround the sun at the outer edge of the solar system at a distance of between 299.2 billion kilometres and 30 000 billion kilometres.
  • The Oort Cloud sits roughly between 145 billion kilometres to 145 trillion kilometres from the sun.
  • The Oort Cloud is 50 000 AU from the sun.

Another clown wants to be even more specific: the distance from the sun of the inner cloud is from 300 billion kilometres to 3 000 billion kilometres; and the distance of the outer cloud is from 3 000 billion kilometres to 7 500 billion kilometres [That last figure is the same as 50 000 AU in the last point above].

Is there really any point in all this?

I am going to go with 50 000 AU, not just because it is a simple figure, but because it sets a reasonably “mean distance” for purposes of discussion. As you will remember an AU is an “astronomical unit” being the mean distance of the earth from the sun. That’s 149 597 900 kilometres. [50 000 AU therefore is equivalent to 7 479 billion kilometres]. So the Oort Cloud is about 7 500 billion kilometres away.

But even that could be wrong. This is what Nasa has to say on the subject and perhaps they have the best aggregated idea: “The inner edge of the Oort Cloud is thought to be located between 2 000 and 5 000 AU from the Sun (299-748 billion kilometres), with the outer edge being located somewhere between 10 000 and 100 000 AU from the Sun (1 496 – 14 960 billion kilometres).” If those distances are difficult to visualize, you can instead use time as your ruler. At its current speed of about 1.6 million kilometres a day, NASA’s Voyager 1 spacecraft will not enter the Oort Cloud for about 300 years. And it won’t exit the outer edge for maybe 30 000 years.

Taking all the figures above we end up with a “rough” idea that the Oort Cloud puts the solar system in some sort of bubble that has a thick shell, its innermost surface being 299 billion kilometres from the sun, and the outer edge being some 14 960 billion kilometres from the sun.

But, whatever the case, this cloud or whatever it is, is a fantastic distance away.

As for what it contains, here again must be speculation. It is thought to contain billions of icy bodies that are at least 15 kilometres wide. Another source says it  comprises several trillion objects of more than one kilometre across; and many billions that are over 20 kilometres across. Surely this is a wild guess?

It is proposed as the source of most of the long-period comets. And no doubt those sent out on their fantastically long orbital journey through the solar system return there.

It is also possible that if the Sun formed in a cluster of stars, as most stars do, then it might have exchanged comets with the growing Oort clouds of those nearby stars. That could be a significant contributor to the Oort cloud population.

I’m not sure if I even agree with the theory behind it, let alone all these phantasmagorical distances. If I understand it correctly the main “proof” behind this theory is that comets from this region dive into the solar systems from all directions. In other words, long-period cometary activity cannot come from a disc; it must come from a sphere. And as the theory goes (from our opening paragraph), after the formation of the planets the leftovers were scattered, mainly by Neptune, onto orbits that took them towards the stars…and many formed this huge spherical cloud.

But why? And how? Our galaxy is called the Milky Way because it is shown as a dense band in our night sky. What we are seeing is the disc of the galaxy. It is like turning a plate upside down, putting it on top of another plate and looking at this from the side. This represents the disc of material of the Milky Way galaxy. The solar system is also, by and large, a disc. The orbital inclination of the planets to the sun’s equator ranges from between 3.4° to roughly to 7°, so they form a fairly flat disc. The same can be said of the asteroid belt, the Kuiper belt and the Scattered Disc. All the building blocks of the solar system, and indeed of our own galaxy, are basically spinning round in a bulging disc of soup. So why is the Oort Cloud a sphere? The only explanation I have is that a sphere as proposed can only form from a radial distribution of materials. And that, to my way of thinking, is an explosion. Was the sun once a binary member with say a brown dwarf? Did the little dwarf explode? It’s as good a theory as any other!

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By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last updated: Wednesday, 1st April 2020