Just how big is the Solar System?

Our near neighbours

Everyone knows about the eight planets of the Solar System, of which Earth is the "third rock from the Sun". What is often not appreciated is just how far apart the orbits of several of the planets are.

Jupiter's orbit is about 600 million kilometres from that of Earth, but the next planet out, Saturn, is another 700 million kilometres away. To get to Uranus you need to nearly double that gap again, as it is 1300 million kilometres further away than Saturn. And to get to Neptune the gap is even bigger, at another 1720 million kilometres. Jupiter is practically in our back garden by comparison!

(It should be pointed out that the accompanying image is for artistic impression only – it is a very long way from being to scale!)

Further down the street

But that is not all - not by any means. Beyond the orbit of Neptune lies the Kuiper Belt of minor planets and objects that were formerly thought to supply the raw material for short-period comets. At these distances, it makes sense to use the measure of the "Astronomical Unit" or AU. One AU represents the distance from the Sun to the orbit of Planet Earth, so Neptune's orbit is given as 30 AU. The Kuiper Belt extends to 55 AU - getting on for twice as far again.

The geometry of the Kuiper Belt is complex, and astronomers talk about the "scattered disc" of objects that have eccentric orbits that take them inside the Kuiper Belt but also way beyond it - at up to 100 AU, which is more than three times further away than Neptune. The scattered disc is now believed to be the source of most short-period comets.

Much further still!

And then there is the Oort Cloud of objects that are even further away - possibly to as much as 100,000 AU. It seems incredible that anything this far away can still be within the gravitational pull of the Sun, but it seems to be so. However, the pull is very much weaker out there and other influences come into play, namely the gravitational influences of other stars, given that the furthest edge of one side of the Oort Cloud is half way to the next star, namely Proxima Centauri.

The Oort Cloud, which appears to encase the Solar System in all directions, is therefore thought to be the source of long-period comets that are shaken out of their orbits as the Sun moves through the Milky Way and passes relatively close to other stars. These are comets that can take many thousands of years to complete their journey from the outer reaches of the Solar System towards the Sun and back again.

So how big is the Solar System? It is absolutely vast, and most of it consists of empty space!

© John Welford