1-2 Parts of the Solar System

What makes a planet?

In 2006, the International Astronomical Union (IAU) faced a question: what, exactly, is Pluto?

From its discovery in 1930, Pluto had been known as the ninth planet of the Solar System, the collection of objects that orbits around the Sun. However, it didn't really seem to fit with the other planets, for a few reasons.

And so, at their annual meeting in 2006, the IAU settled on a three-part definition of a planet. An object needs to satisfy all three of these tests:

1. Does it orbit the Sun, like Earth and Jupiter?

2. Is it large enough to be round?

3. Is its mass big enough to have cleared other objects out of its neighbourhood?

Pluto fails the third test, as it crosses over another planet's orbit in an unusual way. So... sorry, Pluto, you have now been demoted to the status of a dwarf planet, an object that passes the first two tests but not the third. There are many dwarf planets in the Solar System, as we will see.

If an object isn't large enough to make itself round -- its gravity isn't big enough to squish the rocks into a spherical shape -- it is then called a small Solar System body.

Regions of the Solar System

Yes, there are eight planets in the Solar System, and you may have memorized their names and order already; that's a useful thing to commit to memory.

However, what is more useful is to realize this:

Objects in the same part of the Solar System will have similar properties.

If we look down on the Solar System from above -- that is, if we're up in space above our North Pole, looking down -- we can divide the Solar System into different parts.

This diagram is nowhere near proper scale. If it was, the Sun would be a tiny dot, some circles would be so small they'd barely be visible, and the Kuiper Belt would dominate the picture.

Here are some basics about what you'll find in each of these regions. For a lot more great information on the planets (and other objects), go to NASA's Solar System website here.

Inner Planets (Mercury, Venus, Earth, Mars)

• relatively small in size

• thin atmosphere, although Mercury doesn't really have much of one at all

• few moons, if any

• all have solid surfaces

• relatively warm, although Venus is outrageously hot

• also called terrestrial planets (meaning "Earth-like") or rocky planets

Asteroid Belt

• very small in size (the largest is about 500 km across)

• no atmosphere

• they can have tiny "moonlets"

• made of rock and some small amount of metal such as iron

• all of them put together would have less mass than our Moon

• go here for an interactive 3-D view of asteroids and the inner solar system

Outer Planets (Jupiter, Saturn, Uranus, Neptune)

• large to extremely large in size

• very thick atmosphere

• dozens of moons

• no visible solid surface; you're only seeing the tops of clouds

• cold to very cold

• also called jovian planets (meaning "Jupiter-like") or gas giants

• if you want to get lost in amazing pictures of these planets, go here

Kuiper Belt

• objects smaller than our Moon

• mostly made of rock and ice

• extremely far from the Sun

• hard to see because they are small and distant

• we have detected some moons around them so far

• Pluto has now officially been classified as a Kuiper Belt Object (KBO)

Distances in the Solar System

If you wanted to measure the width of a piece of paper, you would probably use the unit of centimetres (cm).

If you wanted to describe the distance between Toronto and Montreal, you would probably use the unit of kilometres (km).

But, if we are talking about distances in the Solar System, neither of those units of measurement are large enough so that the number wouldn't be gigantic. This means we need a better unit of measurement to describe these distances.

Scientists have determined that the Earth is, on average, about 150 000 000 km (or 1.5 × 108 km) from the Sun. If we define a new unit of measurement based on this, it turns out to be pretty handy for the Solar System. We call this one astronomical unit, or AU for short. This means:

1 AU = 150 000 000 km

(In reality it's a tiny bit smaller than this, but for our purposes this will do just fine.)

Below is a chart showing how much nicer it is to use AU than km for distances from the Sun to each of the planets. It's also much easier to judge these distances, in terms of the Earth: for example, looking at the number of kilometres, it wouldn't be easy to see that Jupiter is about five times farther from the Sun than we are. But if you use AU, it's extremely easy to see this.

Here's a clip to show you how this works:

Moons

You've seen this before:

A moon is a natural object which orbits around a planet. They can be big or small, they can have some really strange properties, they can be very interesting or... a little boring.

As mentioned above, the inner planets tend to have fewer moons: Mercury and Venus have none, Earth has one, Mars has two. But, the outer planets have dozens of moons -- really, like their own little Solar System on their own!

Jupiter's Moons and the Solar System's Structure

Jupiter's four large moons, often called the Galilean moons because they were first seen by Italian astronomer Galileo Galilei in the early 1600s, are worth a little more examination. They have some amazing properties, including active volcanoes and sub-surface water oceans; you can read more about them here.

When Galileo first saw them, there was still debate in the scientific community in Europe about what was at the centre of the universe. The traditional idea was that the Earth was in the middle, and everything revolved around us. This is called a geocentric universe, as the prefix "geo-" means "Earth" (such as "geography," literally "drawing the Earth," and "geometry," literally "measuring the Earth").

But some scientists, including Nicolaus Copernicus a few decades earlier, were starting to think, based on observations of the planets, that the Sun was in the centre. This is called a geocentric universe, as the prefix "helio-" means "Sun."

At that time in Europe the Roman Catholic Church had a lot of political power, and the Church's official position was that the Earth had to be in the centre. So, scientists all over Europe had to say that the Earth was in the centre... even if some were starting to have their doubts.

Galileo first observed these four large moons in January of 1610 over a couple of weeks. He noted where Jupiter was, and where different points of light surrounding Jupiter were; a part of his original sketches is shown below.

To Galileo, it looked like those points of light came from objects going around Jupiter. But remember, at that time, the "offical position" of the powers-that-be was that objects only went around the Earth, not anything else. So, for Galileo to make a claim that what he had discovered was a group of four objects revolving around something other than Earth, that was a very controversial and dangerous thing to say.

Eventually the greater scientific community came to accept the idea that the Earth could not be in the centre of everything. Galileo's discovery helped to open the door to that idea a little wider, and eventually even the Roman Catholic Church came around to that idea.

However, Galileo faced (and had to endure) the ultimate price for his outspoken views: he was tried in a Church-sanctioned court and sentenced to house arrest for the rest of his life in 1633. The Roman Catholic Church only truly backtracked on their sentence in 1992, over three hundred years after his sentencing. For more information, you can read about "The Galileo Affair" here.

Practice

The Basics

1. Place the following parts of the Solar System in order, from closest to farthest from the Sun: asteroid belt, inner planets, Kuiper belt, outer planets.

2. What is the difference between a geocentric and a heliocentric model of the Solar System?

3. Pluto is not a planet. What is it?

Extensions

1. Name three key differences between inner and outer planets.

2. Do some research to find out what makes Saturn's largest moon, Titan, very unique.