Secondary Science: Solar System, Part 1

The Solar System

The Solar System consists of a star, the Sun, and 8 planets.

There are also smaller objects like dwarf planets, asteroids and comets.


Our place in the Universe

The Universe is huge.

Some estimates put it at 93 billion light years across.

Galaxies: Webb Space Telescope


The Universe is mostly empty space.

In that space are galaxies. Each galaxy is a city of stars.

There are around two trillion galaxies in the Universe – that’s 2,000,000,000,000!

Typical galaxies contain around three hundred billion stars.


The Milky Way

Our own star city is the Milky Way, a spiral galaxy.

It contains at least two hundred billion stars – that’s 200,000,000,000 stars!


Our Solar System is located in the spiral arms, about two-thirds of the way out from the centre.

Milky Way:   NASA

Our galaxy is 100,000 light years across.

The Solar System is about 25,000 light years from the centre.


Solar System

The Sun

The Sun is a yellow star with a surface temperature of 5,800 C.

It is 1,400,000 km across, about 109 times the width of Earth.

Antares Launch Postponed Due to Weather – in Space – Christian Ready

The Sun.   Credit: NASA / Tom Wolf


Our Sun contains 99.8% of the mass of the Solar System.

The planets, asteroids and comets make up only 0.2% !


The immense gravitational pull of the Sun holds all the planets in orbit around it.

Solar System: NASA


The Planets

In order from the Sun, the eight planets are:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Astronomers Have Been Predicting New Planets in our Solar System for ...

They divide into two types of planet.

Mercury, Venus, Earth and Mars are small rocky worlds.

Jupiter, Saturn, Uranus and Neptune are huge planets made of gas, gas giants.

Planet Sizes

Solar System Sizes | NASA Solar System Exploration

Planet sizes: NASA Lunar and Planetary Institute

These images were all taken from robotic space probes.

They show the different sizes of the rocky planets and gas giants.


Planet Distances

The distances between planets are huge and varied.

It is hard to put them to scale in a diagram.

But this animation from NASA gives some impression of the distance scale.

See the source image

Solar System distance to scale:  NASA/JPL/Edu


The four small, rocky planets are relatively close together and close to the Sun.

The four gas giant planets are distant from the Sun and each other.


If you would like to explore the true scale of the Solar System, click here.

It’s a lovely animation called ‘If the Moon were only 1 pixel’

You need great patience to make it to Pluto!


Planet Orbits

Planets are held in orbit by the Sun’s gravitational pull.


Planets closer to the Sun move faster in their shorter orbits.

For example, Mercury moves at 173,000 km/ hour as it goes round the Sun in 88 days.

Earth is orbiting at 107,000 km / hour to complete its orbit in 365.25 days..


Jupiter, with a much larger orbit, moves around the Sun in 11.9 years at 46,800 km / hour.

Neptune, the outermost planet, strolls at a mere 19,500 km / hour in its 165 year orbit of the Sun.


This Youtube video gives an idea of the relative speeds of planets in orbit.


Planet Facts

We’ll take a look at each planet in turn.


For distance from the Sun, we’ll us the Astronomical Unit, AU.

One AU is the distance of earth from the Sun, 1.5 million km.

So the AU shows how far a planet is from the Sun compared to Earth.


Each planet’s ‘day’ is the time for it to spin round once on its axis.

The ‘year’ is a planet’s time to orbit the Sun.



Planet Mercury: NASA

Diameter: 4,879 km

Distance from Sun: 0.4 AU

Year: 88 Earth days

Day: 58 Earth days

Mercury is the smallest regular planet.

It is covered in meteorite craters.

Close to the Sun, the daytime temperature reaches 430 C.

But because Mercury has virtually no atmosphere, the night temperature drops to minus 180 C.



NASA Has Recreated The Deadly Surface of Venus in Ohio

Planet Venus: NASA

Diameter: 12,100 km

Distance from Sun: 0.7 AU

Year: 225 Earth days

Day: 243 Earth days

Venus is covered in clouds, all over, all the time.

Radar images have shown the surface.

It is covered in volcanoes.

APOD: 2005 September 3 - Venus Unveiled

Surface of Venus: NASA


Venus has a thick atmosphere, mainly carbon dioxide.

This greenhouse gas retains heat from the Sun so Venus has become the hottest planet.

The surface temperature is over 460 C.

Venus spins round in the opposite direction to all the other planets.

Its day is longer than it’s year!



Planet Earth From Space Nasa | Amazing Wallpapers

Earth from space: NASA


Diameter: 12,750 km

Distance from Sun: 1.0 AU

Year: 365.25 days

Day: 24 hours

Our home planet Earth has a mean temperature of around 20 C.

This means that we have liquid water. That water supports life.

As far as we know at the moment, Earth is the only planet with life.



Pictures of Planet Mars - Universe Today

Planet Mars: NASA

Diameter: 6,790 km

Distance from Sun: 1.4 AU

Year: 687 Earth days

Day: 24 hours 40 minutes

Mars has seen more visits by robotic spacecraft than any other planet.

Long ago there were seas, lakes and rivers on Mars.

Life may have begun there.


But now Mars is cold, any water frozen into the ground.

The thin air is mostly carbon dioxide.


The red colour of Mars comes from iron oxide in the sandy surface.

Mars has the largest volcano in the Solar System and the longest, deepest valley.

Mars has two small moons, Phobos and Deimos.


Coming Next


In our next blog we’ll continue our review of the Solar System.

We’ll explore the gas giant planets along with asteroids and comets.


Dennis Ashton, blog author

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

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