We've already provided a brief description of the solar system as it relates to eclipses — in other words, the relationships of the Sun, Moon, and Earth.

However, there's much more to the solar system than that; in particular, its amazing scale is quite staggering. If you're interested in this, read on; in this page, we will attempt to explain the shape of the solar system in more detail, and even try to give an idea of its scale.

The Problem of Scale

This might seem like a simple thing to explain: let's just see a diagram of the solar system. OK, here you go!

This diagram shows the Sun and the eight planets we know of. But as you can see, it's not very satisfactory, for a few reasons.

First, Mercury, Venus, Earth, and Mars are missing! Well, actually, they're there, they're just far too small to be seen. In fact, even their orbits are too small to be seen on this scale: they're lost in the glare around the Sun. (If you look really closely, you can just see Mars' orbit.)

Second, even the big planets are too small to be seen. You can just about see a little dot representing Jupiter, which is the largest of all the planets, but that's it.

Then there are quite a lot of missing objects. You might notice that Pluto is missing, but if that's a concern, it's only because Pluto is very well known. In reality, Pluto is just one of many similar dwarf planets, and they're all missing from this diagram. If Pluto should be there, then so should ErisEris (dwarf planet)
An article on the dwarf planet Eris, which orbits the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/Eris_(dwarf_planet)
, HaumeaHaumea
An article on the dwarf planet Haumea, which orbits the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/Haumea
, 2007 OR10(225088) 2007 OR10
An article on the trans-Neptunian object 2007 OR10, which is probably a dwarf planet. [Wikipedia]
https://en.wikipedia.org/wiki/(225088)_2007_OR10
, MakemakeMakemake
An article on the dwarf planet Makemake, which orbits the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/Makemake
, Quaoar50000 Quaoar
An article on the trans-Neptunian object Quaoar, which orbits the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/50000_Quaoar
, Sedna90377 Sedna
An article on the trans-Neptunian object Sedna, which orbits the Sun far beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/90377_Sedna
, and CeresCeres (dwarf planet)
An article on the dwarf planet Ceres, the largest object in the asteroid belt between Mars and Jupiter. [Wikipedia]
https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)
, at least; but in reality, there are probably hundreds of dwarf planets lurking in the farther reaches of the solar system.

Which brings us to the biggest problem — this diagram only shows about half of a thousandth of the width of the solar system! The overwhelming majority of our solar system is completely unknown and unexplored, and its vast scale makes any realistic diagram of it basically impossible. If we made a map of the whole solar system which was a kilometre across, the Earth would be less than a thousandth of a millimetre — about the size of a small bacterium.

Given this outrageous scale, it's quite hard to explain exactly how all the pieces of our celestial home turf relate to each other. Nevertheless, we're going to have a go. The simplest and most basic part of the picture is the Sun and planets, so let's start there.

The Sun and Planets

The Sun absolutely dominates our solar system, containing over 99.8% of its total mass; hence, it is the immovable anchor around which everything else orbits.

The best-known things orbiting the Sun are, of course, the planets. There are eight planets we know of, and they are all well within the innermost reaches of the solar system. One reason for this is that we don't yet have the ability to see planets orbiting farther out, even in our own solar system; so there could be others out therePlanet Nine
An article on Planet Nine, a hypothetical large planet in the outer region of the Solar System. [Wikipedia]
https://en.wikipedia.org/wiki/Planet_Nine
. But we can at least list the ones we know about.

Object Diameter Distance from Sun
(km) (millions of km) (AU)
The SunSun
An article on the Sun. [Wikipedia]
https://en.wikipedia.org/wiki/Sun
1,392,000 0 0
MercuryMercury (planet)
An article on Mercury, the planet in our solar system closest to the Sun. [Wikipedia]
https://en.wikipedia.org/wiki/Mercury_(planet)
4,879 57.91 0.39
VenusVenus
An article on Venus, the second planet from the Sun in our solar system, and the planet closest to Earth in size. [Wikipedia]
https://en.wikipedia.org/wiki/Venus
12,104 108.2 0.72
EarthEarth
An article on our home planet, the Earth. [Wikipedia]
https://en.wikipedia.org/wiki/Earth
12,742 149.6 1.00
MarsMars
An article on Mars, the fourth planet from the Sun. [Wikipedia]
https://en.wikipedia.org/wiki/Mars
6,779 227.9 1.52
JupiterJupiter
An article on Jupiter, the fifth planet from the Sun, and the largest planet in our solar system. [Wikipedia]
https://en.wikipedia.org/wiki/Jupiter
139,822 778.6 5.20
SaturnSaturn
An article on Saturn, the giant ringed planet, and the sixth planet from the Sun. [Wikipedia]
https://en.wikipedia.org/wiki/Saturn
116,464 1,433.5 9.58
UranusUranus
An article on Uranus, the seventh planet from the Sun. [Wikipedia]
https://en.wikipedia.org/wiki/Uranus
50,724 2,875.0 19.2
NeptuneNeptune
An article on Neptune, the eighth planet from the Sun, and the outermost planet that we know of in our solar system. [Wikipedia]
https://en.wikipedia.org/wiki/Neptune
49,244 4,504.4 30.1

Looking at the numbers, you can see that the planets divide roughly into 2 groups: the 4 inner, terrestrial, planets, are all small (between 4,800 and 12,800 km in diameter) and quite close to the Sun (within 1.5 AU); the 4 outer, giant, planets, are all large (over 49,000 km), and quite far from the Sun, extending out to 30 AU, 20 times farther than the inner planets.

Of course, there's much more going on here than just the planets. Most notable is the asteroid belt between Mars and Jupiter, about 2–4 AU from the Sun; within this, CeresCeres (dwarf planet)
An article on the dwarf planet Ceres, the largest object in the asteroid belt between Mars and Jupiter. [Wikipedia]
https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)
, the largest asteroid and also a dwarf planet, orbits at about 2.8 AU.

And then there are the moons; the planets have numerous moons between them, and some of these are very significant. Jupiter's moon Ganymede, and Saturn's moon Titan, are both larger than Mercury; and there are seven moons — including Earth's — which are larger than Pluto.

But there's a lot more to discover farther out, as well. Even at the orbit of Neptune, we have 2,000 times farther to go to get to the edge of the solar system.

Beyond Neptune

There is an immense amount of space in our solar system beyond the known planets, and there is a lot happening there. However, we don't know a lot about it, because out there, the light from the Sun is so faint that any objects — even large planets — would be so dim that they would be extremely hard to detect. Still, there are some things we know, and some things about which we have strong circumstantial evidence.

The first of the things we know about, just beyond Neptune, is the Kuiper Belt. This is a belt of icy objects, similar to the asteroid belt, and extending from about 30 to 50 AU from the Sun. There is a wide range of sizes of objects in the Kuiper Belt, from grains of ice up to dwarf planets; some of the larger ones are PlutoPluto
An article on Pluto, formerly classified as a planet, but now known to be just one of many dwarf planets orbiting the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/Pluto
, MakemakeMakemake
An article on the dwarf planet Makemake, which orbits the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/Makemake
, HaumeaHaumea
An article on the dwarf planet Haumea, which orbits the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/Haumea
, Quaoar50000 Quaoar
An article on the trans-Neptunian object Quaoar, which orbits the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/50000_Quaoar
, and Varuna20000 Varuna
An article on the trans-Neptunian object Varuna, which orbits the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/20000_Varuna
.

The Scattered Disc is a wider belt of objects with more eccentric orbits, and may be the source of short-period comets. It overlaps the Kuiper Belt, but extends outwards possibly as far as 150 AU. The largest known Scattered Disc object is ErisEris (dwarf planet)
An article on the dwarf planet Eris, which orbits the Sun beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/Eris_(dwarf_planet)
, which orbits between 38 AU and 98 AU — it's currently about 96.5 AU away. Eris is around the same diameter as Pluto, but significantly more massive.

End of the Sun's Realm

The Sun is constantly blasting out a stream of charged particles, called plasma, in all directions. The known planets in the inner solar system are bathed in this solar wind; but it doesn't extend forever. When it comes up against the interstellar medium (the thin gas between the stars), it comes to an abrupt end.

The region of space dominated by the solar wind is called the heliosphere. The point where it collides with the interstellar medium is called the termination shock; beyond this, the solar wind ends at the heliopause. This is roughly 100–120 AU from the Sun in the direction of the Sun's motion. Beyond this, the substance of space is the interstellar medium, the very thin gas between the stars.

Even this far out, though, we have barely scratched the surface of the real solar system. There is still a vast realm of space in which objects are gravitationally bound to, and orbiting, the Sun.

Explored Space

Given the immensity of the solar system, it's strange that the farthest known object in it should be human-made.

Actual human space exploration hasn't got very far. We've been to the Moon, which took the Apollo 13 astronauts 400,171 km from the Earth. This is the farthest that any human has ever travelled, but it's a tiny distance in solar system terms — just 0.0027 AU, a tenth of a thousandth of the distance to Neptune. And with the ending of the Apollo era, we've drawn back even farther. Since 1972 the farthest any human has been from Earth is 609 km (Hubble servicing mission 3A), and since 2009, just 406 km (the International Space Station). Given that the Earth is nearly 13,000 km across, this is really just skimming the atmosphere.

We've done a lot more with unmanned exploration. We've sent probes to the Sun — several are studying it right now — and all the planets from Mercury to Saturn; Uranus and Neptune were visited in flyby by Voyager 2, and Pluto was visited by New Horizons in 2015. For visually exploring objects in the solar system, New Horizons holds the record, having taken detailed pictures of Pluto and its moons while 32.9 AU from the Sun. (The famous Pale Blue DotA Pale Blue Dot
An article from the Planetary Society on the Pale Blue Dot picture, the most distant picture ever taken of the Earth. [Planetary Society]
http://www.planetary.org/explore/space-topics/earth/pale-blue-dot.html
image was taken by Voyager 1 when 40.5 AU away; but while this picture has immense cultural and historic importance, it doesn't reveal any new details.)

Having completed its primary mission at Pluto, New Horizons is heading to a new encounter with a small Kuiper belt object, 2014 MU69(486958) 2014 MU69
An article on the Kuiper belt object 2014 MU69, which is scheduled to be visited and photographed by the New Horizons probe in January, 2019. [Wikipedia]
https://en.wikipedia.org/wiki/(486958)_2014_MU69
, which it will fly by on the 1st of January 2019. Hopefully the pictures sent back from the flyby, at 43.4 AU from the Sun, will extend our view of the solar system still farther. However, New Horizons is far from being the winner in overall exploration.

The Pioneer 10 and 11 probes have achieved amazing distances from earth, at 121.2 AU and 98.6 AU respectively; but they haven't been doing real science for a long time. The clear winners are the Voyager 1 and 2 probes. Launched in 1977, both of these probes are still doing science, and they are speeding away from the Earth. Voyager 2 is currently 117.8 AU away, and Voyager 1 is 142.4 AU away; making it not only the farthest space probe, functioning or not, but in fact the farthest known object in the solar system.

Travelling at 17.0 km/s, Voyager 1 is also the fastest probe leaving the solar system. Its speed equates to about 17,600 years to travel one light year, so making the distance to Proxima Centauri (if it was heading in that direction; it's not) would have taken about 75,165 years. Of course it will have run out of power long before then; in fact, it's expected to shut down around the year 2025.

So we can claim to have explored our solar system — in the most minimal way — out to 142 AU. But this is still just a tiny drop in a very big ocean. Even though we don't know of any specific object farther out, there's a lot of space there, and it almost certainly isn't empty.

Out Into the Dark

Sedna90377 Sedna
An article on the trans-Neptunian object Sedna, which orbits the Sun far beyond Neptune. [Wikipedia]
https://en.wikipedia.org/wiki/90377_Sedna
is currently about 86 AU from the Sun, but its 11,400-year orbit carries it from 76 AU to 936 AU. The only reason we know about it is because it is currently in a part of its orbit close to the Sun. An object like Sedna spends relatively little time in this part of its orbit, which means that there are very probably a lot more objects like Sedna; and some of them may be much larger.

A recent candidate is Planet 9Planet Nine
An article on Planet Nine, a hypothetical large planet in the outer region of the Solar System. [Wikipedia]
https://en.wikipedia.org/wiki/Planet_Nine
, a hypothetical planet which may orbit far beyond Neptune. The Planet 9 hypothesis was created to explain the eccentric orbits of some of the dwarf planets; so far, there is no direct evidence for Planet 9, and other hypotheses are possible. But it's certainly an exciting possibility.

If it exists, Planet 9 may have a highly elliptical orbit, possibly coming as close as 200 AU from the Sun (over 6 times farther than Neptune), and moving up to 1,200 AU out. It may be between 25,000 and 50,000 km in diameter, 2-4 times bigger than the Earth, and maybe 10 times heavier.

You can join in the search for Planet 9, and other outer solar system objects. The Backyard Worlds: Planet 9Planet 9
A citizen science project to find Planet 9, and other new objects at the edges of our solar system. You can join in to search for distant solar system objects in telescope images. [Backyard Worlds]
https://www.zooniverse.org/projects/marckuchner/backyard-worlds-planet-9
project is a citizen science project where you can search telescope images for objects like Planet 9. This is not only a great way to make a real contribution to astronomy — and potentially become the discoverer of Planet 9 — but looking at the images will give you an idea of why finding these objects is so hard, as you scour very noisy images for very faint moving specs.

If Planet 9 exists, and orbits out to 1,200 AU, this is still just over 2% of the way out to the edge of the solar system. There's no way to know how many other objects there may be orbiting beyond that distance. Given the vast amount of space out there, though, we can speculate that there may be many. But at present, this is an immense, and completely unexplored, volume of space.

The Final Belt

Although we can't detect anything in the outer reaches of the solar system, there is circumstantial evidence for certain things out there. Comets which drop into the inner solar system must come from somewhere, and analysis of their orbits suggests that there is a vast reservoir of icy objects at the outer edges of the solar system. This is called the Oort cloudOort cloud
An article on the Oort cloud, a hypothetical belt of icy objects orbiting at the edge of our solar system. [Wikipedia]
https://en.wikipedia.org/wiki/Oort_cloud
, after Jan Oort, one of the astronomers who worked on the hypothesis.

The Oort cloud is generally reckoned to be the final frontier of the solar system; beyond it, nothing can be in a stable orbit around the Sun.

The actual boundaries of the cloud are very hard to even estimate. The distance to the inner edge of the cloud is estimated at between 2,000 and 5,000 AU. The outer edge may be 50,000 AU (0.8 light years) away, or even as far as 100,000 AU (1.6 light years); sometimes estimates as high as 200,000 AU (3.2 light years) are given, but this runs into trouble with the Hill sphere, as explained below.

So the Oort cloud may cover 90% or more of the width of the solar system, and yet we really know nothing about it, or anything else in that region of space — we only have evidence of its existence from visiting comets. Still, it is fairly certain that the cloud exists, and that there are large objects in it whose gravity knocks comets down into the inner solar system once in a while. Beyond that, it's a mystery.

The Limit of the Sun's Grasp

The absolute limit of the solar system is the distance at which the Sun's gravity can no longer hold orbiting objects against the gravity of other, nearby stars. This distance is called the Sun's Hill sphereHill sphere
An article on the Hill sphere, the limit of an object's gravitational attraction for orbiting objects. [Wikipedia]
https://en.wikipedia.org/wiki/Hill_sphere
. Beyond this, nothing can be in a stable orbit around the Sun.

Unfortunately, computing this distance is not simple, as it depends on the characteristics of the "other nearby stars", and they're all moving. However, a practical estimate for it is probably somewhere around two light years, or 125,000 AU.

Even this is only a rough guide, though, as orbits near the edge of the Hill Sphere become increasingly fragile; the likelihood is that nothing can really be in a long-term stable orbit anywhere close to the edge of the Hill sphere. So there's no realistic way to set an exact limit on the reach of the Sun's gravitational dominance — but there certainly is a limit.

The Hill sphere is the final outer limit on the Oort cloud. Once past the Oort cloud, we have finally left the solar system, and have ventured out into interstellar space. The next significant thing to see will be another solar system.

And Beyond...

The nearest star to our own solar system is Alpha Centauri; however, it's not quite that simple, as Alpha Centauri is now known to be a 3-star system.

The two main members of the system, Alpha Centauri A and B, orbit each other at a distance varying between 11.2 and 35.6 AU, around the distance of Uranus from the Sun — quite close in the grand scheme of things. These two stars together weigh about twice as much as the Sun, so their gravitational influence is quite strong. They are around 4.37 light years from the Sun.

The third star, Alpha Centauri C — also known as Proxima Centauri — is a small red dwarf, about an eighth of the Sun's mass, orbiting A and B every 547,000 years, in an elliptical orbit at a distance which varies from around 4,300 AU to 13,000 AU. Since it is currently on the side of its orbit towards us, Proxima is currently the closest star to us, at a distance of 4.25 light years. This is a very wide and eccentric orbit, and in fact it was only determined in 2017 that Proxima is actually part of the Alpha Centauri star system.

And beyond this are billions more stars. Barnard's Star, a very small red dwarf, is about 6.0 light years away; Wolf 359, another red dwarf, is about 7.9 light years; Lalande 21185, yet another red dwarf, is about 8.3 light years; and Sirius, a massive binary star and the brightest star in our sky (after the Sun), is about 8.6 light years.

All of these, of course, are just a handful among the hundreds of billions of stars in the Milky Way galaxy. This is a disc-like aggregation of stars, about 100,000 light years across. The Milky Way itself is just part of the Local Group of galaxies, which in turn is part of the Virgo Supercluster, a collection of galaxies 110 million light years across. This in turn is just one of about 10 million superclusters in the observable universeObservable universe
An article on the observable universe, the region of spacetime which it is possible for us to observe. [Wikipedia]
https://en.wikipedia.org/wiki/Observable_universe
, which is around 93 billion light years across, and contains at least 2 trillion galaxies.

And Beyond... ?

It's impossible to say what lies beyond the observable universe (by definition), but it's not likely that the physical universe stops there. It probably goes on for quite a distance; but whether infinitely far, or for a finite distance, is unknown. There may even be other universes beyond our own, but as of now, that's basically speculation.

Solar System Summary

This table lists some of the main landmarks described above. Bear in mind that a lot of these distances are highly speculative; but still, this should give you an overall idea of the scale of things.

Landmark Distance from Sun
(AU)
Mercury (the innermost planet) 0.39
Earth 1
Neptune (the outermost known planet) 30.1
Pluto (now) 32.6
Kuiper Belt outer limit 50
Eris (now) 96.5
Heliopause (roughly) 120
Voyager 1 142
Scattered Disc outer limit 150
Sedna (orbit limit) 936
Planet 9 (possible orbit limit) 1,200
Oort cloud start 2,000 - 5,000?

... the vast majority of the solar system
is completely unknown space ...

Edge of the Solar System 50,000? 100,000?
Proxima Centauri 268,500