Secondary Science: Solar System, Part 2

The Outer Solar System

In our last blog, Solar System Part 1, we looked at the Inner Planets out to Mars.

We’ll complete our tour by looking at the Outer Planets and small members of our Solar System.


Let’s start beyond Mars with the asteroids.


The asteroid belt lies between Mars and Jupiter.

They are between 2 AU and 4 AU from the Sun.

Illustration of the location of the asteroid belt between Mars and Jupiter.

Asteroid Belt: NASA


Asteroids are small rocky and metal objects.

Rocky asteroids are made of silicate and clay.

Metal asteroids are made of iron and nickel.

Galileo image of 243 Ida (the dot to the right is its moon Dactyl)

Asteroid Ida and moon Dactyl.     NASA / Galileo


They range in size from less than a kilometre up to hundreds of kilometres.

The belt holds one million known asteroids.

That’s a lot but added together they would only make an object 3% the size of our Moon.


Asteroids are irregular in shape with one exception.

Ceres, the largest asteroid, is spherical.

Dawn image of the dwarf planet Ceres

Ceres: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / Justin Cowart


At 952 km across, Ceres is the largest asteroid and has now been classified as a dwarf planet.

Now we’ll move out to the gas giant planets.



NASA Jupiter Wallpapers - Top Free NASA Jupiter Backgrounds ...

Planet Jupiter: NASA


Diameter: 143,000 km

Distance from Sun: 5.2 AU

Year: 11.9 Earth years

Day: 9 hours 55 minutes


Jupiter is the largest planet, a gas giant.

It is made of hydrogen and helium topped by clouds.

It’s clouds are spun into stripes by its fast rotation.

The white clouds contain methane and ammonia, the red clouds contain sulphur compounds.

The great red spot is a storm which has blown for hundreds of years.


Jupiter has 95 moons.



NASA snapped a new image of Saturn, and it's a real stunner

Planet Saturn: NASA


Diameter: 120,000 km

Distance from Sun: 9.6 AU

Year: 29.5 Earth years

Day: 10 hours  39 minutes


Saturn is the second biggest planet, a gas giant like Jupiter.

The yellow colour of Saturn comes from a mist of ammonia ice at the surface.

Saturn’s  magnificent rings are made of millions of chunks of ice.

The rings are 270,000 km across but less than one kilometere deep.


Saturn has 146 moons.



Japanese astronomers explain the origins of Uranus' weirdness - Tech ...

Uranus: NASA


Diameter: 51,000 km

Distance from Sun: 19.2 AU

Year: 84 Earth years

Day: 17 hours  14 minutes


An outer layer of methane gives Uranus its blue colour.

It has thin dusty rings which show the planet is on its side.

Perhaps a collision with another planet knocked it over billions of years ago.


Uranus has 28 moons.



Neptune Full Disk View | NASA Solar System Exploration

Neptune: NASA


Diameter: 49,500 km

Distance from Sun: 30 AU

Year: 165 Earth years

Day: 16 hours


Neptune is the smallest of the gas giants.

Like Uranus it looks blue because of methane gas.

Storms see winds reaching over 2,000 km / hour.


Neptune has 16 moons.


Smaller than planets

As well as asteroids, the Solar System contains other smaller objects.

We’ll look at dwarf planets and comets.


Dwarf Planets

Dwarf planets are so called because they are small – smaller than our Moon.

They have eccentric, oval-shaped orbits unlike the eight regular planets.


Dwarf Planets are also known as Trans-Neptunian Objects (TNOs) because they are further from the Sun than Neptune.

They lie in a region beyond the regular planets called the Kuiper Belt.

This region is 30 to 55 AU from the Sun.

The best known dwarf planet is Pluto.



Pluto in High Resolution | NASA Solar System Exploration

Pluto: NASA / New Horizons


Diameter: 2,372 km

Distance from Sun: 39 AU

Year: 248 Earth years

Day: 6.4 Earth days


Pluto is the largest of the dwarf planets.

It is covered in ice with a surface temperature of minus 230C.


Pluto’s eccentric orbit sometimes carries it closer to the Sun than Neptune.

This oval orbit, together with its size, lead to Pluto being reclassified as a Dwarf Planet.


Pluto has 5 moons, the largest of which is Charon.


Other Dwarf Planets

Most dwarf planets are found in the Kuiper Belt, some 40 AU from the Sun.

They are named for gods of different countries.

Among them are Eris, Haumea, Makemake, Quaoar, Sedna, Gonggong, Orcus and Salacia.

File:10 Largest Trans-Neptunian objects (TNOS).png

Dwarf Planets, TNOs:   Wikipedia



Comets are found in the outer regions of the Solar System.

The Oort Cloud of comets lies far from the Sun, from 5,000 AU to 100,000 AU.

Inside that is the doughnut-shaped Kuiper Belt, home to more comets and the dwarf planets.


Comets are space icebergs, made of water ice and ices of methane and nitrogen.

In the ice are small rocks and dust covers the surface.


10 Things: What's That Space Rock? – NASA Solar System Exploration

Comet 67P/C-G:     ESA / Rosetta


There are millions of comets in the Oort Cloud.

Most are just a few kilometres across.


Occasionally, comets are dislodged from their distant orbits.

When they come close to the Sun, a remarkable change occurs.

Cometa Hale-Bopp

Comet Hale-Bopp, 1997


The ices sublime, turning from solid to gas.

The Sun blows the gas into space as a comet tail.

Dust blown from the comet surface makes a separate dust tail.


The most famous comet is Halley’s Comet.

I saw it in 1986. It will return to our skies in 2061.

I hope that you see it then!


The real Solar System


The Solar System is much larger than most people realise.

It’s edge is the Oort Cloud of comets, half way to the nearest star.


The doughnut shaped region, the Kuiper Belt, lies inside the cloud.

It is home to dwarf planets and more comets.


It is only at the centre we find the Sun and eight main planets.


Solar System in Wonderdome


Wonderdome planetarium can take you into space in your own school!

Our expert presenters will take you on a tour of the planets.

To see how to book a Wonderdome visit, please go to our home page.


Dennis Ashton, blog author

The author: Dennis Ashton is a Fellow of the Royal Astronomical Society and a Wonderdome presenter.

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