Our Solar System is not really significant: it is a tiny fraction of one percent of the galaxy, which is one of untold billions. In a universe that extends for something like 45 billion light years across, it is too small to notice.
But to us, its physical immensity seems obvious – and what’s more, it is close enough to observe in detail.
Light, travelling at 186 000 miles per second, takes eight minutes to reach us from the Sun. That’s 93 million miles. The Sun itself is over a hundred times wider than Earth, yet we see it as a soccer ball in the sky. Neptune, now the outermost of the full planets, is thirty times as far away from the Sun, so light takes four hours to reach it. These distances are mind-boggling. At most, we work on thousands of miles, the distances between major cities. But even Neptune, at 2.8 billion miles from the Sun, is not the end of the solar system.
Before Neptune was the last recognised planet, that honour fell to Pluto. Poor, distant Pluto, was reclassified in 2006 on account of its small size. It is now known as a “dwarf planet”, similar to some large bodies in the asteroid belt. Pluto is often – though not always, because of overlapping orbits – further from the Sun than Neptune. Pluto’s orbit takes it, at furthest, 4.4 billion miles from the Sun. At this distance, the heat received from the Sun is negligible (the surface temperature of Pluto is around -230C) and the Sun appears as a very bright star in the sky, but much, much smaller than we see it.
Is this the end of the solar system? Sun, eight planets, asteroids? A diameter of around four and a half billion miles?
One of the reasons Pluto was reclassified was because of doubt as to where Pluto came from and belonged. It became clear in the early 1990s that there were lots of smaller objects beyond Pluto, and as the research developed, astronomers realised that there was a vast group of these that existed as a belt right around the Sun. The name for this area now is the Kuiper Belt, and it is home to thousands of icy objects. It is believed to be where many comets come from. This area, which Pluto is just at the front of, extends back to 50 astronomical units – nearly 5 billion miles from the Sun and 50 times as far from the Sun as Earth. Pluto is now seen as the largest of these objects rather than a planet in its own right.
So this is it, right?
Not quite. Beyond the Kuiper Belt is another region of mainly small objects called the scattered disk. Not all of these objects are small, and one of them, Eris, is larger than Pluto, with a diameter of 1600 miles. It is still only a dwarf planet, though. Eris can get up to 97 AU from the Sun during its long orbit- nearly a hundred times further than the Earth, and something like 9 billion miles from the Sun. Because the objects in this region are composed mostly of ice and other frozen gases, and because the region can be volatile, with bodies moving all over the place, most comets are believed to come from here.
The scattered disk is not quite it for the solar system, even though light from the sun takes 13 hours to reach the furthest edges of it.
Beyond it, however, astronomers aren’t quite sure what exactly is there. It seems clear that there are things further than the scattered disk, but their ideas are mainly speculative. Much, much further than the scattered disk is thought to lie the Oort Cloud, a vast region of comets some 50 000 AU from the Sun – that’s 50 000 times as far away from the Sun as the Earth is. Here, billions of objects, again mainly composed of ice and other frozen gases lie in a huge ring that surrounds the entire solar system. One of the reasons for this hypothesis is that there are objects known to exist beyond the scattered disk, such as 90377 Sedna, a dwarf planet candidate which orbits between 88 and 975 AU from the Sun (and takes something like twelve thousand years to orbit it).
This distance is truly vast. The Oort Cloud is thought to receive light from the Sun one year after it leaves the Sun. One whole-light year – and we are still in our solar system, and only a quarter of the distance to the nearest star, and nowhere near any other stars!
One final object may lie beyond the Oort Cloud, but there is no experimental evidence for it: it is simply an idea, rather than a theory. Some astronomers, looking at periods of extinction, thought they could see a 26 million year periodic pattern. What if, every 26 million years or so, something disturbed the regions at the edge of the solar system and caused more comets and meteorites to come close to earth (causing more impacts)? What if it were a large object, such as a companion star to the Sun? These astronomers suggest, tantalisingly, that the Sun may have a companion red dwarf star, which they call Nemesis – the bringer of destruction.
Nemesis would, if it existed, be far beyond the Oort Cloud, over one light-year away and as far as two.
The solar system is vast and mysterious – but it is, for all that, not even the tiniest fraction of the smallest percentage of what lies out there!