There are 9 planets in the solar system.

Other than that the solar system is practically empty.

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

In order of distance from the Sun.

Orbits are almost in a plane (within a few degrees) and close to circular. Mercury and Pluto are most eccentric and have highest inclination to the ecliptic.

Overview - sizes to scale

Solar System Planets

Name

a(AU)

Sidereal Period (yr)

e

i

Diameter

 Mass

Mercury

0.4

0.24

0.2

7

0.38

0.055

Venus

0.7

0.6

0.007

3.4

 0.95 0.8

Earth

1.0

1.0

0.017

0

1

1

Mars

1.5

1.9

0.093

1.8

0.5

0.1

Jupiter

5.2

12

0.048

1.3

11

320

Saturn

9.5

29

0.056

2.5

9

95

Uranus

19

84

0.046

0.8

4

15

Neptune

30

164

0.009

1.8

4.9

17

Pluto

40

248

0.25

17.14

0.18

0.0025
Earth Diameter (=1) == 12,800 km
Earth Mass (=1) == 6*1024 kg

Solar system live - Solar system overview - more overview

Family Portrait - NASA overview

Planet orbit Java

Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto have moons.

The inner 4 planets are the Terrestrial planets. They are small, high density, low mass, rocky and have little or no atmosphere.

Terrestrial planets

The next 4 planets are the Jovian planets. They are massive, low density, gaseous and probably have no solid surfaces.

Pluto is weird.



Saturn has rings. So do all the Jovian planets, just less spectacularly so!







We'll deal with the moons later...

There are two ways to find out what the planets are made of.

Go there. We've done that with Mars and Venus.

Or do spectroscopy - either from Earth, or, better still, from a spacecraft passing by or in orbit around the planet.

Spacecraft have been sent to all the planets except Pluto.

Hence, we know the Terrestrial planets are rocky - they're made of silicon and iron compounds, predominantly.

The Jovian planets are gaseous - they are made from hydrogen, helium, methane, ammonia, carbon monoxide and carbon dioxide, water vapour etc. The Jovian planets probably don't have any rock or metal surfaces, though the pressure at the centre of them may be high enough to force the gas into a solid.

We can add up the various stuff we see and infer and find out what elements the solar system is made of.

In addition to the planets, the solar system has some debris.

HOU - find an asteroid

There is a bunch of asteroids - rocks ranging from few hundred km across to dust size. Most of the asteroids are between Mars and Jupiter, where the asteroid belt is located. There are millions of asteroids, mostly the smaller ones, and the asteroid belt is still very very empty. Asteroids are far apart. The typical distance between asteroids in the belt is about 100,000 km!

Asteroids

There are other asteroid families. The Trojan asteroids share orbit with Jupiter, but always either lead or trail the planet.

The Earth crossers are asteroids that have been nudged out of the asteroid belt, either by collisions with other asteroids, or by Jupiter's gravity. The dive into the inner solar system, crossing Earth's orbit. Those will all either collide with the Earth, or be ejected from the solar system.

Another small belt of asteroids seems to be out beyond Uranus. Those are poorly known.

Finally there is the Kuiper belt of asteroids/comets. This is an extended wedge of asteroids beyond the orbit of Pluto - it was only recently discovered.

The solar system also contains comets.

Comets

Comets come in two types - short period, low inclination comets, and long period, high inclination comets.

The short period comets are probably from the Kuiper belt, nudged into the inner solar system by collisions with each other, or gravitational perturbations of the planets. Some are then trapped in short period orbits by Jupiters gravity, some crash into the Sun and others are ejected from the solar system.

The long period comets probably originate from the Oort cloud. This is a conjectured, very extended halo of comets thought to surround the Sun. It probably extends to ~ 100,000 AU and contains trillions of comets.

The long period comets are perturbed into orbits that go into the inner solar system - they then either go back out to the Oort cloud, hit something, or are trapped by Jupiter's gravity into short period orbits.

Comets look a lot like asteroids. The difference is that they have ices covering them, and these vapourise and produce a gas halo around the comet when it comes close to the Sun. Asteroids may be similar to comets that have had the ice layer baked off them, or they may be quite different and much more solid.

Spacecraft will rendezvous with asteroids and comets over the next few years to find out exactly how they are constituted.