General Structure of the Solar System

Lessons

Senior 4 · Physics

General Structure of the Solar System

4 lessons

Lesson 3 of 4

The Solar System: Planets, Dwarf Planets and Small Bodies

25 min

Wanderers and visitors

Look at the sky from Rwanda on a clear night and most points of light keep their places in fixed patterns. A few bright ones do not: over weeks they drift slowly against the stars. The ancient Greeks called them planētai, wanderers. Five are easy to see with the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. Now and then a visitor with a glowing tail appears for a few weeks, and on most nights, if you watch patiently, a "shooting star" streaks across the sky in a second. All of these belong to the Sun's family, the solar system. In this lesson you sort its members, compare the planets with real data from NASA, and find out what comets, asteroids, meteors and meteorites are.

The Sun and its eight planets

The Sun holds almost all the mass of the solar system, and its gravity holds everything else in orbit. Eight planets go round it in nearly circular orbits, almost in one plane and all in the same direction. Table 3.1 gives their main properties.

Planet	Diameter (km)	Mean distance from Sun ( $10^{6}$ km)	Distance (AU)	Orbital period	Density ( $\mathrm{kg\,m^{-3}}$ )	Mean temperature (°C)	Moons
Mercury	4879	57.9	0.39	88.0 days	5429	167	0
Venus	12 104	108.2	0.72	224.7 days	5243	464	0
Earth	12 756	149.6	1.00	365.2 days	5514	15	1
Mars	6792	228.0	1.52	687.0 days	3934	−65	2
Jupiter	142 984	778.5	5.20	11.9 years	1326	−110	95
Saturn	120 536	1432.0	9.57	29.4 years	687	−140	274
Uranus	51 118	2867.0	19.2	83.7 years	1270	−195	28
Neptune	49 528	4515.0	30.2	164 years	1638	−200	16

Table 3.1: The planets. Data: NASA Planetary Fact Sheet, read October 2026 (moon counts change as more are discovered).

The table splits naturally into two groups. The four inner (terrestrial) planets, Mercury, Venus, Earth and Mars, are small, dense and rocky, with solid surfaces and few or no moons. The four outer (giant) planets are huge, with low densities: Jupiter and Saturn are gas giants, mostly hydrogen and helium; Uranus and Neptune, the ice giants, contain more water, ammonia and methane. All four have rings and many moons. Between the two groups lies the asteroid belt, and beyond Neptune the Kuiper belt of icy bodies.

Figure 3.1: The planets' diameters to scale. Data: NASA Planetary Fact Sheet.

Definition

A planet, by the International Astronomical Union's 2006 definition, is a body that orbits the Sun, is massive enough for its own gravity to have made it nearly round, and has cleared the neighbourhood around its orbit of other objects of similar size. A dwarf planet orbits the Sun and is nearly round but has not cleared its orbit; the IAU recognises five: Ceres, Pluto, Haumea, Makemake and Eris (NASA). Example: Pluto is round and orbits the Sun, but it shares its region of space with many other Kuiper-belt objects, so since 2006 it has been classed as a dwarf planet.

Worked example 1: how much of the solar system is the Sun?

NASA gives the Sun's mass as $1\,988\,400 \times 10^{24}\,\mathrm{kg}$ and the planets' masses (in $10^{24}\,\mathrm{kg}$ ) as 0.330, 4.87, 5.97, 0.642, 1898, 568, 86.8 and 102. What fraction of the total mass is in the Sun?

m_{\text{planets}} = 0.330 + 4.87 + 5.97 + 0.642 + 1898 + 568 + 86.8 + 102 = 2667 \times 10^{24}\,\mathrm{kg}

\text{fraction} = \frac{1\,988\,400}{1\,988\,400 + 2667} = 0.9987 = 99.87\%

The Sun holds about 99.9% of the mass; Jupiter holds most of the rest.

Worked example 2: a pattern in the orbits

Use Table 3.1 to test whether $\frac{T^{2}}{a^{3}}$ is the same for all planets, with $T$ in years and $a$ in AU.

Planet	$a$ (AU)	$T$ (years)	$T^{2}$	$a^{3}$
Earth	1.000	1.000	1.000	1.000
Mars	1.524	1.881	3.54	3.54
Jupiter	5.204	11.86	140.6	140.9

To the accuracy of the data, $T^{2} = a^{3}$ : the square of a planet's period is proportional to the cube of its distance from the Sun. This is Kepler's third law, which you will explain with Newton's law of gravitation in S5. It lets us find any period from the distance: Ceres, at 2.77 AU, takes $2.77^{1.5} = 4.6$ years to go round.

Small bodies: asteroids, comets and meteoroids

Asteroids are rocky or metallic bodies, from a few metres to hundreds of kilometres across, most of them orbiting in the asteroid belt between Mars and Jupiter. They are left-over building blocks from the time the planets formed, never gathered into a planet, probably because of Jupiter's strong pull.

Comets are "dirty snowballs": nuclei of ice and dust a few kilometres across, from the cold outer solar system. Many follow long, thin elliptical orbits (Figure 3.2). Far from the Sun a comet is just a dark nucleus; as it approaches the Sun, the ice turns to gas, forming a glowing cloud, the coma, and two tails, one of dust and one of gas, which always point away from the Sun, pushed by sunlight and the solar wind. Halley's comet returns about every 76 years; it was last seen in 1986 and is due back in 2061 (NASA).

Figure 3.2: A comet's elongated orbit: its tails appear near the Sun and always point away from it (not to scale).

Meteoroids are small fragments of asteroids or comets in space. When one enters the atmosphere at tens of kilometres per second, the air in front of it is squeezed and heated so strongly that the meteoroid glows and vaporises: the streak of light is a meteor, or shooting star. When the Earth passes through the trail of dust left by a comet, many meteors appear in one night: a meteor shower. A meteoroid large enough to survive the fall and reach the ground is a meteorite.

Definition

An asteroid is a small rocky or metallic body orbiting the Sun, mostly between Mars and Jupiter. A comet is a small icy body that develops a coma and tails when it nears the Sun. A meteoroid is a small fragment of rock or metal in space; a meteor is the streak of light it makes when it burns up in the atmosphere; a meteorite is a piece that survives and lands on the ground. Example: a grain of comet dust that flashes across the sky for a second is a meteor; it never reaches the ground, so it is not a meteorite.

Worked example 3: why a tiny grain makes a bright streak

A meteoroid of mass 1.0 g enters the atmosphere at $20\,\mathrm{km\,s^{-1}}$ . Find its kinetic energy, and compare it with a 1.0 kg stone thrown at $20\,\mathrm{m\,s^{-1}}$ .

E_k = \tfrac{1}{2}mv^{2} = \tfrac{1}{2} \times 1.0 \times 10^{-3} \times (2.0 \times 10^{4})^{2} = 2.0 \times 10^{5}\,\mathrm{J}, \qquad E_{\text{stone}} = \tfrac{1}{2} \times 1.0 \times 20^{2} = 200\,\mathrm{J}

The grain carries a thousand times the energy of the stone, and it gives it up to the air in about a second, heating the air and itself until they glow.

Warning

Common mistakes. (1) Calling a meteor a "falling star": it is a grain of dust or rock, not a star. (2) Thinking a comet's tail streams behind it along its path: the tails point away from the Sun, so a comet leaving the Sun travels tail first. (3) Confusing meteoroid (in space), meteor (the streak of light) and meteorite (on the ground). (4) Saying the outer planets have no solid parts: they may have rocky cores, but no solid surface to stand on.

Tip

Inside small and rocky, outside big and gassy. Close to the young Sun only rock and metal could stay solid, so the inner planets are small and rocky; far out, ices and gas could gather too, building the giants. Remember the order with "My Very Excellent Mother Just Served Us Nuts": Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Meteors: Crash Course Astronomy #23, presented by Phil Plait. Watch how a meteoroid becomes a meteor and sometimes a meteorite, and how meteor showers come from comet debris.

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Practice

Use Table 3.1 to list the inner and outer planets, and give two differences between the groups, with numbers.
Saturn's density is $687\,\mathrm{kg\,m^{-3}}$ . What would happen to Saturn in an ocean big enough to hold it? What does its density tell you about what it is made of?
Show that the Earth's density is about $5.5 \times 10^{3}\,\mathrm{kg\,m^{-3}}$ , using a mass of $5.97 \times 10^{24}\,\mathrm{kg}$ and a mean radius of 6371 km.
Halley's comet has a period of about 76 years. Use $T^{2} = a^{3}$ (T in years, a in AU) to find the average radius of its orbit.
Using Table 3.1, how many times wider than the Earth is Jupiter? How many Earths would fit inside it, by volume?
Explain why a comet's tail points away from the Sun, and draw the comet's tail at three points of its orbit.
Explain the difference between a meteoroid, a meteor and a meteorite.
Pluto orbits the Sun and is round. Evaluate the decision to call it a dwarf planet rather than a planet.

Show answersHide answers

Inner: Mercury, Venus, Earth, Mars; outer: Jupiter, Saturn, Uranus, Neptune. Inner planets are small (diameters 4879–12 756 km) and dense (3934–5514 kg m⁻³) with 0–2 moons; outer planets are large (49 528–142 984 km), low-density (687–1638 kg m⁻³) and have many moons and rings.
Its average density is less than that of water ( $1000\,\mathrm{kg\,m^{-3}}$ ), so it would float. It must be made mostly of the lightest elements, hydrogen and helium.
$V = \tfrac{4}{3}\pi r^{3} = \tfrac{4}{3}\pi(6.371 \times 10^{6})^{3} = 1.083 \times 10^{21}\,\mathrm{m^{3}}$ ; $\rho = \frac{5.97 \times 10^{24}}{1.083 \times 10^{21}} = 5.5 \times 10^{3}\,\mathrm{kg\,m^{-3}}$ .
$a = T^{\frac{2}{3}} = 76^{\frac{2}{3}} = 18\,\mathrm{AU}$ : its average distance is a little beyond Uranus.
$\frac{142\,984}{12\,756} = 11.2$ times wider; by volume $11.2^{3} \approx 1400$ Earths.
Sunlight and the solar wind push the gas and dust released from the nucleus directly away from the Sun. So near the Sun the tails point outwards; approaching the Sun the tail trails behind; leaving the Sun the tail goes in front.
A meteoroid is a small body in space; a meteor is the light it makes burning up in the atmosphere; a meteorite is any part that survives to reach the ground.
Pluto meets two of the IAU's three tests: it orbits the Sun and is round. But it has not cleared its orbit: it is one of many similar bodies in the Kuiper belt, including Eris, which is almost as large. Calling Pluto a planet would mean calling many such bodies planets too. The decision groups Pluto with its true neighbours, though some argue the "cleared orbit" test is hard to apply.

Quick check

Match each body with its description.

Tap one on the left, then its partner on the right.

Key points

The Sun holds about 99.9% of the solar system's mass. Inner planets (Mercury, Venus, Earth, Mars): small, dense, rocky. Outer planets (Jupiter, Saturn, Uranus, Neptune): giant, low-density, ringed, with many moons.
IAU (2006): a planet orbits the Sun, is nearly round and has cleared its orbit; dwarf planets (Ceres, Pluto, Haumea, Makemake, Eris) have not.
For the planets $T^{2} = a^{3}$ (years, AU): Kepler's third law.
Asteroids (rocky, mainly between Mars and Jupiter), comets (icy, tails pointing away from the Sun), meteoroids, meteors (streaks) and meteorites (on the ground).

Sources and credits

Data: NASA, Planetary Fact Sheet and Sun Fact Sheet; NASA Science, Dwarf Planets and 1P/Halley, read October 2026.
Photo: Comet NEOWISE and meteor by Igorzinchenko, Wikimedia Commons, licensed under CC BY 4.0.
Photo: Laguna Manantiales iron meteorite by Daderot, Wikimedia Commons, released under CC0 1.0.
Video: Meteors: Crash Course Astronomy #23, CrashCourse, embedded from YouTube.

Written by Ganzaa: the lesson text, tables, diagrams, worked examples and practice exercises.

1Wanderers and visitors

2The Sun and its eight planets

3Small bodies: asteroids, comets and meteoroids

Wanderers and visitors

The Sun and its eight planets

Small bodies: asteroids, comets and meteoroids