How Big is the Solar System?

Unfortunately there isn’t an easy answer to the question “How big is the solar system?” in fact there is no scientific definition. It all depends on what factors are used to specify what the solar system is, let’s look at some of the possible answers.

If you think of the solar system as the sun and the eight major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) then we have to look at the last of those planets, Neptune. It takes 165 years to orbit the sun at an average distance of 2.8 billion miles (4.5 billion km). This simple definition which would make the solar system around 5.6 billion miles (9 billion km) across.

But what about poor old Pluto, the former planet which is now classified as a dwarf? It orbits in a region of the solar system beyond Neptune known as the Kuiper Belt. The Kuiper Belt is populated with bodies composed of rock and ice which extends to around 5 billion miles (8 billion km) from the sun. Using the outer edge of the Kuiper Belt as a marker would make the span of the solar system around 10 billion miles (16 billion km).

The sun is constantly emitting a stream of charged particles known as the solar wind. Travelling at speeds of around 250 miles per second these particles reach far beyond the Kuiper Belt, creating a bubble around the sun known as the heliosphere. At around 11 billion miles (17.5 billion km) these charged particles begin to slow down as they meet interstellar space, it’s believed the influence of solar wind ends completely at around 12 billion miles (19.5 billion km). If we consider the edge of the heliosphere bubble to be the end of the solar system then that makes it around 24 billion miles (39 billion km) across.

But what about another of the sun’s great influences, gravity, how far does that reach? The answer lies in those fascinating celestial objects, comets. When objects from the Kuiper Belt enter the inner solar system they become comets, more definitively they’re known as short-orbital comets. This is because comets originating from the Kuiper Belt orbit around the sun in less than 200 years. But there are comets which take thousands of years and possibly millions of years to orbit the sun. Where do they come from?

It is believed long-orbital comets originate from an area of the solar system known as the Oort Cloud. A hypothetical cloud populated by icy bodies which surrounds the solar system. It’s impossible for us to observe any objects in the Oort Cloud but it’s believed it extends 1 light year from the sun, which is a quarter way to the nearest star, Proxima Centauri! Using the Oort Cloud as a boundary would make the solar system 2 light years in diameter.

It should be pointed out that this wouldn’t mark the end of the sun’s gravitational influence. It could be many more light years before the gravity of other stars came into play and there may be more significant objects beyond the Oort Cloud which could be considered part of the solar system. So if someone asks you the question “How big is the solar system?” you can choose one of these answers or maybe more correctly you could say “we don’t really know”.

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