Our Place in Space

Space Week


Our resident IntoScientist and passionate educator, Dave Canavan looks into “Our Place in Space” as we celebrate World Space Week.

How Much Do You Really Know About Your Solar System?

If I were to ask you to name the planets in order from the Sun, could you? It is a very common question that most high-school pupils know so go on and test yourself. If you said Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, then you’ve done very well. If you said Pluto on the end, you have also done very well but we’ll get to Pluto later.

The first thing we should clear up is the definition of what a planet actually is. For centuries there has not been a true definition so in 2006 the International Astronomical Union declared a definition of a planet as a celestial body that orbits the Sun, is nearly spherical due to its own gravity as a result of its size, and has cleared its orbit because it is the largest object in its orbit.

Poor Pluto

It is the final part of the definition as to why Pluto is no longer classified as a planet and is now merely a dwarf planet; one of over a known 40 but potentially hundreds. Pluto is part of a relatively unknown, massive belt of objects called the Kuiper Belt after the astronomer Gerard Kuiper. It consists of asteroids and frozen bodies and the dwarf planet Pluto.

Due to Pluto orbiting with this other debris in an uncleared orbit, it is therefore no longer classified as a planet. Other planets ‘sweep up’ asteroids and other space debris in its orbit whereas Pluto doesn’t. It also has a moon which is only half the size of Pluto, unlike other planets where their moons are much smaller. Poor Pluto!

The Structure of the Solar System

Space is not called ‘Space’ for no reason. The fact of the matter is that there is hardly any matter at all! Over 99% of all the matter in our solar system is the Sun but by far the majority of our solar system is space.

There are four inner planets and four outer planets. Earth is the largest of the four inner planets, with Venus being very similar in size to Earth. Mars is next biggest and tiny Mercury is the smallest planet in our solar system.

The four inner, terrestrial planets are relatively small (having diameters of less than 13,000km), and are composed of mainly rock and metal, making them dense. They also have atmospheres.

After Mars we have the four outer planets known as the gas giants comprising of Jupiter; the largest, Saturn and Uranus; famous for their rings, although all of the outer planets have rings, and Neptune; the furthest planet from the Sun. All of these gas giants are enormous with diameters over 48,000km, are less dense than the inner planets and are composed of mainly hydrogen and helium.

Our Place in Space

Indeed, Jupiter is so large that it is bigger than all the other planets put together. The famous ‘red-spot’ on Jupiter’s surface which is a continuous raging storm, can fit over 3 Earths in it! One of Jupiter’s moons, Ganymede, is bigger than Mercury.

To help visualize the sizes and distances, imagine the Earth is reduced to the size of a grape (about a billion times smaller than it is). The moon would be about 30cm away from the grape and the sun would be 150m away and be 1.5m in diameter! Jupiter would be the size of a large grapefruit and about 750m from the Sun. Saturn would be twice as far again and the size of an orange and Uranus and Neptune would both be lemons! As for us, we would be the size of an atom, and to get to the nearest star next to the Sun, Alpha Centurai, you would have to travel over 40,000km!

How Far Away?

Like I said before, there is a lot of space in-between the planets, and as a rule, the further away a planet is from the Sun, the larger the distance between it and the previous planet’s orbit.

When dealing in space units, we are literally dealing in Astronomical Units. This, fittingly, is the term used to describe distances in our solar system. One Astronomical Unit (AU) is the approximate distance of the Earth to the Sun which is about 149,500,000km!

When dealing with other stars or galaxies, we talk in terms of the distance ‘light years’. A light year is the distance light can travel in one year; a staggering 10 trillion kilometres!

Although one AU is a set figure, the values below are approximate values. This is primarily because a planet is not the same distance from the Sun all of the time due to the planets having elliptical (oval-shaped) orbits. The Earth varies from being about 147 million km away to over 152 million km away.

Mercury is between 0.31 and 0.47AU, Venus is about 0.7AU, and Mars is about 1.38AU. Jupiter varies between just under 5AU to under 5.5AU, with Saturn getting much further, between 9 and 10AU. Uranus ranges between 18 and 20AU and Neptune is about 30AU! For the record, Pluto can be up to 49AU: over 7,000,000,000 kilometers!

As you can see, the distances do seem to increase exponentially but as yet there is no exact formula to explain why.

Incidentally, Pluto can orbit as close as 29.6AU to the Sun, meaning that at some points in its orbit it is closer to the Sun than Neptune. But considering that one Pluto year (complete orbit around the Sun) is 277.7 Earth years, and then considering that it has to be at the time when Neptune is at its farthest from the Sun, it will never be the case in our lifetime.

Deep Impact!

In between the smaller inner planets and the gas giants is an asteroid belt known as the main belt. Asteroids are small, often irregularly-shaped rocky objects that orbit the Sun. They are probably the remains of an earlier planet in our solar system and are left-overs from when our solar system originated about 4.6 billion years ago.

There are hundreds of thousands of known asteroids in the main belt, but there are possibly millions more. When an asteroid ventures out of the belt for some reason such as a collision with another asteroid, the Earth can be in peril from impact. If the asteroid is similar in size to Mt. Everest, then we could see a similar impact to that which wiped out the dinosaurs. Worryingly, the chances of this happening at some point in the Earth’s future is guaranteed!

Comets are different from asteroids as they are chunks of ice and rock left over from the formation of the solar system, unlike solid, rocky asteroids. Either way, a collision would be most unwelcome! But as we are a mere speck in the vast Universe, let’s not worry about it!

Read more of Dave Canavan’s articles at www.thechingchokhunter.com or to learn more about world space week www.3plearning.com/lets-celebrate-world-space-week/