E. Earth and Space Science: Space Exploration · Space Exploration: The Sun, the Solar System and the Universe

Comparing the Members of the Solar System

E2.4 A1.1About 70 minutes

Learning goals

I can describe the major components of the solar system: the Sun, planets, dwarf planets, moons, asteroids, comets and the Kuiper belt and Oort cloud.
I can compare the inner and outer planets using data such as size, density, composition and orbital period.
I can explain the three rules an object must meet to be a planet.
I can find and explain patterns in solar system data.

Words to know

terrestrial planet
A small, rocky, dense planet with a solid surface: Mercury, Venus, Earth and Mars.

gas giant
A huge planet made mostly of hydrogen and helium, with no solid surface: Jupiter and Saturn.

ice giant
A giant planet with large amounts of water, ammonia and methane 'ices' under a hydrogen-helium atmosphere: Uranus and Neptune.

dwarf planet
An object that orbits the Sun and is nearly round but has not cleared its orbit of other objects, such as Pluto and Ceres.

density
Mass per unit of volume (density = mass ÷ volume). Water's density is 1 g/cm³.

orbital period
The time an object takes to travel once around what it orbits. Earth's is about 365 days.

Kuiper belt
A region of icy bodies beyond Neptune, about 30 to 50 AU from the Sun, home to Pluto and many comets.

Oort cloud
A huge, spherical cloud of icy bodies thought to surround the solar system far beyond the Kuiper belt; the source of long-period comets.

Remember from Grade 6

In Grade 6 you learned the order of the eight planets, that the inner planets are rocky and the outer planets are giants, and you met moons, asteroids, comets and meteoroids. Now you will compare them with real data, find patterns, and learn exactly what makes something a planet.

Why isn't Pluto a planet any more?

Pluto was called the ninth planet from its discovery in 1930 until 2006. Then astronomers kept finding more icy worlds beyond Neptune, including Eris, which is about the same size as Pluto and has more mass. Should every one of them be a planet? The International Astronomical Union (IAU) decided to write a clear definition. Pluto did not meet it.

What makes a planet?

In 2006 the IAU decided that a planet must:

orbit the Sun;
have enough mass for its own gravity to pull it into a nearly round shape;
have cleared the neighbourhood around its orbit (it is by far the dominant object in its orbit).

An object that meets rules 1 and 2 but not rule 3 is a dwarf planet. Pluto shares its region with many other Kuiper belt objects, so it is a dwarf planet. The IAU recognizes five: Ceres (in the asteroid belt), Pluto, Eris, Haumea and Makemake (beyond Neptune). A moon orbits a planet, not the Sun, so it is not a planet even if it is large and round.

The main zones of the solar system, in order from the Sun (not to scale). Rocky planets are close to the Sun; the giants are farther out; icy bodies fill the Kuiper belt and, much farther away, the Oort cloud.

Planet	Average distance from Sun (AU)	Diameter (km)	Density (g/cm³)	Orbital period	Type
Mercury	0.39	4 879	5.43	88 days	terrestrial
Venus	0.72	12 104	5.24	225 days	terrestrial
Earth	1.00	12 756	5.51	365 days	terrestrial
Mars	1.52	6 792	3.93	687 days	terrestrial
Jupiter	5.20	142 984	1.33	11.9 years	gas giant
Saturn	9.54	120 536	0.69	29.5 years	gas giant
Uranus	19.2	51 118	1.27	84 years	ice giant
Neptune	30.1	49 528	1.64	165 years	ice giant

Planet data (rounded, from NASA fact sheets). 1 AU (astronomical unit) is the average Earth–Sun distance, about 150 million km.

Patterns in the data

Size: the four inner planets are small; the four outer planets are giants. Jupiter's diameter is about 11 times Earth's.
Density: inner planets are dense (made of rock and metal). Outer planets have low density (mostly hydrogen and helium, or ices). Saturn's density is less than water's!
Orbital period: the farther a planet is from the Sun, the longer its year. It has farther to travel and moves more slowly.
Moons and rings: inner planets have few or no moons (Mercury and Venus have none, Earth has one, Mars has two small ones). All four giants have rings and many moons.
Atmospheres: Mercury has almost none; Venus has a crushing carbon dioxide atmosphere that makes it the hottest planet (about 465 °C); Mars has a thin carbon dioxide atmosphere; the giants are almost all atmosphere.

Density of each planet. The inner (rocky) planets are all more than 3.9 g/cm³; the giants are all below 2 g/cm³, and Saturn is below the dashed line for water.

The smaller members

Moons: some moons are worlds of their own. Jupiter's Ganymede is the largest moon in the solar system and is bigger than Mercury. Saturn's Titan has a thick nitrogen atmosphere. Jupiter's Europa has an icy crust with evidence of a salty liquid ocean below it, which makes it a target in the search for life.
Asteroids: rocky or metallic bodies, most of them in the asteroid belt between Mars and Jupiter. Ceres, the largest object there, is a dwarf planet.
Comets: mixtures of ice, dust and rock. When a comet gets close to the Sun, ice turns into gas and forms a coma and tails that point away from the Sun. Comets with short orbits mostly come from the Kuiper belt region and just beyond it; comets with very long orbits are thought to come from the distant Oort cloud. Halley's Comet, which returns about every 76 years, is the most famous short-period comet.
Meteoroids: small pieces of rock or metal. A meteoroid that burns up in our atmosphere makes a meteor; one that reaches the ground is a meteorite.
Kuiper belt: a doughnut-shaped region of icy bodies about 30 to 50 AU from the Sun.
Oort cloud: a huge, spherical shell of icy bodies thought to lie thousands of AU away. It is too far and too dark to see directly; scientists infer it from the orbits of long-period comets.

Worked example

Example 1: Is it a planet?

Astronomers find a new object 70 AU from the Sun. It orbits the Sun, it is round, and it moves among many other icy bodies of similar size. Classify it.

Rule 1: it orbits the Sun. ✓
Rule 2: it is round, so its gravity is strong enough. ✓
Rule 3: it shares its orbital region with many similar bodies, so it has not cleared its neighbourhood. ✗

It is a dwarf planet (it would be a trans-Neptunian object, beyond Neptune).

Worked example

Example 2: Calculating a planet's density

Earth's mass is about 5.97 × 10²⁴ kg and its volume is about 1.08 × 10²¹ m³. Calculate its average density.

density = mass ÷ volume
= (5.97 × 10²⁴ kg) ÷ (1.08 × 10²¹ m³)
= (5.97 ÷ 1.08) × 10²⁴⁻²¹ ≈ 5.53 × 10³ kg/m³ = about 5530 kg/m³
Since 1000 kg/m³ = 1 g/cm³, that is about 5.5 g/cm³.

Earth's average density is about 5.5 g/cm³, more than 5 times water. That tells us Earth must have very dense material, such as iron, in its core.

Hands-on activity

Find the pattern: graph the planets

You need: graph paper (or a spreadsheet), a pencil and ruler, and the planet table above.

Make a scatter plot with average distance from the Sun (AU) on the horizontal axis and orbital period (years) on the vertical axis. (Convert days to years: Mercury 0.24, Venus 0.62, Earth 1.00, Mars 1.88.)
Describe the pattern in one sentence.
Make a second graph: distance (horizontal) and density (vertical). Where is there a big jump?
Research (A1.1): choose one moon, one dwarf planet and one comet. Use a reliable source (such as NASA or the Canadian Space Agency) to find two facts about each, and record the source.

What to look for: graph 1 is a smooth curve that rises faster and faster: farther planets have much longer years. Graph 2 shows a big drop in density between Mars and Jupiter, where rocky planets end and giant planets begin. Good research notes name the website and date.

Just for fun

Saturn's average density is lower than water's, so people like to say it would float in a big enough bathtub. Good luck finding a tub about 120 000 km wide, and please don't pull the plug: the 'bubble bath' would be hydrogen gas.

Your turn

List the three rules for being a planet.

Why is Ceres a dwarf planet and not an asteroid or a planet?

Compare the inner and outer planets in terms of size and density.

Which planet has the longest year? Why?

Name the largest moon in the solar system and one interesting fact about it.

Where do long-period comets come from?

Why is Venus hotter than Mercury even though Mercury is closer to the Sun?

More practice: warm-up

Name the four terrestrial planets.

Where is the asteroid belt?

Which two planets are called ice giants?

What is 1 AU?

In what year was Pluto reclassified as a dwarf planet?

More practice: on your own

A planet has a density of 1.3 g/cm³. Is it more likely rocky or a giant? Explain.

How many times wider is Jupiter than Earth? (Use the table.)

Explain why we know so little about the Oort cloud.

Why do a comet's tails always point away from the Sun?

Europa is small and far from the Sun. Why do scientists think it might be a place to look for life?

Challenge

Kepler found that for planets orbiting the Sun, T² = a³, where T is the period in years and a is the distance in AU. Check this for Jupiter (a = 5.20 AU). Then predict the period of an asteroid at a = 4 AU.

Ganymede is bigger than Mercury. Why is it not a planet?

Use the density pattern to explain where most of the solar system's rock and metal ended up, and where its light gases ended up.

Check your understanding

Quiz

Which rule does Pluto fail to meet to be called a planet?

Which planet has a lower density than water?

Where is the dwarf planet Ceres found?

What happens to a planet's orbital period as its distance from the Sun increases?

Which describes a terrestrial planet?

Long-period comets are thought to come from:

For parents and teachers

Big idea: SNC1W expectation E2.4 asks students to describe the major components of the solar system and the universe and compare their characteristics. This lesson covers the solar system with real data; later lessons cover stars and galaxies. Graphing and checking sources builds research skills (A1.1).

Common difficulties:

Thinking 'dwarf planet' just means 'small planet'. The key difference is clearing the orbit, not size alone.
Mixing up the Kuiper belt and the Oort cloud. Kuiper belt: a disk, 30 to 50 AU. Oort cloud: a far-away sphere, thousands of AU.
Scientific notation in the density example. Remind your teen that when dividing powers of 10, you subtract the exponents.

How to help: ask your teen to explain the Pluto decision to you. Many adults learned nine planets, so it is a fun conversation.