It includes the Sun itself, nine planets orbiting the Sun at various distances (including the Earth), numerous satellites (including the Earth's Moon), and a multitude of asteroids and comets. The Solar System does not include the stars.
The Sun is a star that we see from close up. It is a huge ball of glowing gases with a mass over 330,000 that of the Earth. Indeed all the other objects in the Solar System put together have a mass of less than 1% that of the Sun.
In this essay, I will introduce the reader to the main bodies in the Solar System. Tables will give more detailed information.
| The Sun | |
|---|---|
| Mass | 1.989 × 1030 kg |
| Mass (Earth = 1) | 332,830 |
| Equatorial Diameter | 1,390,000 km |
| Equatorial Diameter (Earth = 1) | 108.97 |
| Mean Density | 1.410 × 103 kg m-3 |
| Mean Density (Earth = 1) | 0.255 |
| Rotational Period | 25 to 36 days |
| Escape Velocity | 618.02 km s-1 |
| Escape Velocity (Earth = 1) | 55.23 |
| Luminosity | 3.827 × 1026 W |
| Apparent Visual Magnitude | -26.8 |
| Mean Surface Temperature | 6,000°C |
| Age | 4.5 × 109 years |
| Chemical Composition |
92.1 % 7.8 % 0.061 % 0.030 % 0.0084 % 0.0076 % 0.0037 % 0.0031 % 0.0024 % 0.0015 % 0.0015 % |
Mass
Mass is the amount of matter that an object contains. On the Earth mass can be measured by weight. The Sun contains thousands of times the mass of all the other objects in the Solar System put together. The Sun's mass is found by measuring its gravitational effect on the planets and other bodies.
Equatorial Diameter
The Sun's diameter is over 100 times that of the Earth. It is the largest object in the Solar System. The visible surface of the Sun is called the photosphere.
Density
Density tells how concentrated matter in a body is. The Sun is a little more dense than water and a quarter the density of the Earth. The figure given is the Sun's average density. This density is similar to those of the
Gas Giant planets. The Sun is gaseous throughout.
Rotational Period
This is how long the Sun takes to rotate once on its axis. The Sun is not a solid object as it is made of gas. Not all parts rotate together. The figures given here are from the pole to the equator. The polar regions rotate more slowly than the equatorial regions. The differential rotation causes the Sun's magnetic field to twist on itself. This produces sunspots.
Escape Velocity
This is the speed that an object must attain in order to escape from the Sun's gravitational field. For the Earth, this speed is about 11 kilometres per second (7 miles per second).
Luminosity
This is the amount of energy that the Sun radiates, in Watts. Every second the Sun loses 6 million tonnes of matter that is converted into energy. The energy is produced by
nuclear reactions in the core of the Sun. Hydrogen atoms combine to form Helium with a small loss of mass that is converted to energy in accordance with Einstein's famous
equation,
Apparent Magnitude
Apparent Magnitude tells how bright an object is as seen from the Earth. It was originally a scale set up for measuring the brightness of the stars. The brightest stars were called First Magnitude, the next brightest were called Second Magnitude, etc.
The brighter a star, the smaller its magnitude. Many stars are brighter than first magnitude. Some stars are so bright they have negative magnitudes. On this scale, Jupiter has a magnitude (at its brightest) of -2.6, Venus -4.4. The faintest stars visible to the naked eye are sixth magnitude. Pluto has a magnitude of +14, far too faint to be visible without a powerful telescope. On this scale the Sun is so bright its magnitude is the high negative value of -27.
Mean Surface Temperature
This is the temperature of the visible solar surface. There are many methods of measuring this temperature.
The first method involves studying the distribution of energy in the Sun's spectrum (Wein's Law)
The second method of measuring the temperature is by the application of Stefan's Law: the temperature is related to the luminosity.
A third method examines the solar spectrum for the condition of various elements. By seeing which elements are ionised and which are absorbing particular wavelengths of energy, the temperature can be found.
The centre of the Sun has a temperature in the millions of degrees.
Age
The Sun's age is the age of the Solar System as a whole.
Chemical Composition
The Sun's chemical composition is found from its
spectrum. Like all stars, the Sun is made up mainly of Hydrogen.
There are two types of planets. The Terrestrial Planets are small and solid like the Earth. The Jovian Planets are also known as the Gas Giants. This is because they are large and mainly gaseous.
The three tables below describe various properties of the nine planets. Under the column "Number of Moons" there are links to tables with more details about the moons.
| Planet |
Mean Distance From The Sun (×106 km) |
Mean Distance From The Sun (Earth = 1) |
Diameter (km) |
Diameter (Earth = 1) |
Period To Revolve Around The Sun |
Rotational Period |
Number Of Moons |
| Mercury | 57.91 | 0.387 | 4,879 | 0.38 | 88.0 days | 58.65 days | 0 |
| Venus | 108.21 | 0.723 | 12,104 | 0.95 | 224.7 days | -243 days | 0 |
| Earth | 149.6 | 1.0 | 12,742 | 1.0 | 365.25 days | 23 hrs 56 mins | 1 |
| Mars | 227.92 | 1.524 | 6,780 | 0.53 | 687.0 days | 24 hrs 37 mins | 2 |
| Jupiter | 778.57 | 5.204 | 139,822 | 10.97 | 11.75 years | 9 hrs 55 mins | 17 |
| Saturn | 1,433.53 | 9.582 | 116,464 | 9.14 | 29.5 years | 10 hrs 40 mins | 22 |
| Uranus | 2,872.46 | 19.201 | 50,724 | 3.98 | 84 years | -17.24 hours | 18 |
| Neptune | 4,495.06 | 30.047 | 49,248 | 3.87 | 165 years | 16.11 hours | 8 |
| Pluto | 5,869.66 | 39.236 | 2,390 | 0.19 | 248 years | -6.38 days | 1 |
Mean Distance From The Sun
Diameter
All the planets rotate. This causes some of them to be flattened at the poles. Planets may have a larger diameter if it is measured through the equator than if it is measured through the poles. The figure given here is an average. One column gives diameters in kilometres, the other relative to the Earth. The planets with diameters equal to or smaller than the Earth are the Terrestrial Planets. The larger planets (Jupiter to Neptune) are much larger than the Earth; these are the Gas Giants.
Period to Revolve Around The Sun
Where
As seen from above the north pole of the Earth, all the planets orbit in an anticlockwise direction. This is called Direct Orbital Motion.
Rotational Period
Number of Moons
Planets travel around the Sun in
elliptical orbits. This means that the distance between a planet and the Sun varies slightly during its orbit. The Mean Distance is an average. This distance is given in two ways. The first is in millions of kilometres. This is a
metric unit. The second compares a planet's distance to the Sun to the Earth's distance. The Inner Planets (Mercury to Mars) are all fairly close together. The Outer Planets (Jupiter and beyond) are more spread out.
This is the average diameter of each planet.
This is a planet's Sidereal Period. How long it takes to complete a single orbit around the Sun relative to the stars. The square of the period of a planet is proportional to the cube of its mean distance from the Sun. This means that the closer a planet is to the Sun, the less time it needs to orbit the Sun. The full relationship is given in the equation below and is called
Kepler's Third Law.
p is the planet's period (in seconds),
a is the mean distance from the planet to the Sun (metres),
G is the Gravitational Constant (6.673 × 10-11 N m2 kg-2),
MSun is the mass of the Sun,
MPlanet is the mass of the planet (masses in kg).
This is how long a planet takes to rotate once on its axis. For most, this is also the length of a planet's day. Most of the planets rotate in an Earth day or less apart from Mercury, Venus and Pluto. As seen from above the north pole of the Earth, most of the planets rotate in an anticlockwise direction. This is called Direct Rotation. A few planets (Venus, Uranus and Pluto) rotate in a clockwise sense. This is called Retrograde Rotation and is shown by the presence of a minus sign.
All the planets apart from Mercury and Venus have moons (also called satellites). These moons vary from large planet sized objects to small irregular shaped rocks. The Earth has one satellite,
The Moon. Mars has
two, Jupiter has
seventeen, Saturn has
twenty two, Uranus also has
eighteen, Neptune has
eight, Pluto has a
single satellite.
| Planet |
Mass (Earth = 1) |
Orbital Inclination (Earth = 0) |
Orbital Eccentricity |
Axial Tilt |
Density (×103 kg m-3) |
Surface Gravity (Earth = 1) |
Synodic Period (days) |
| Mercury | 0.0553 | 7° | 0.2056 | 0.1° | 5.43 | 0.378 | 115.88 |
| Venus | 0.815 | 3.394° | 0.0067 | 177.4° | 5.25 | 0.907 | 583.92 |
| Earth | 1.0 | 0° | 0.0167 | 23.45° | 5.52 | 1.000 | |
| Mars | 0.107 | 1.850° | 0.0935 | 25.19° | 3.95 | 0.377 | 779.94 |
| Jupiter | 317.83 | 1.304° | 0.0489 | 3.12° | 1.33 | 2.364 | 398.88 |
| Saturn | 95.159 | 2.485° | 0.0565 | 26.73° | 0.69 | 0.916 | 378.09 |
| Uranus | 14.536 | 0.772° | 0.0457 | 97.86° | 1.29 | 0.889 | 369.66 |
| Neptune | 17.147 | 1.769° | 0.0113 | 29.56° | 1.64 | 1.120 | 367.49 |
| Pluto | 0.002 | 17.16° | 0.2444 | 119.6° | 2.03 | 0.059 | 366.73 |
Mass
The mass of planets with satellites can be measured by observing the motions of the satellites and applying
Kepler's Law. For Mercury and Venus, the mass used to be measured by detecting these planet's influence of the Earth, asteroids or comets. Recently, their masses have been measured by probes.
Orbital Inclination
Orbital Eccentricity
Venus has the most circular orbit. The least circular orbits are those of Mercury and Pluto. Pluto's orbit is so elliptical that it sometimes moves closer to the Sun than Neptune. There is no danger of a collision because Pluto's orbit is so highly inclined.
Axial Tilt
Uranus has an axial tilt of 97°. This means that the planet rotates on its side. An axial tilt of more than 90° implies that the planet rotates in a retrograde direction.
Density
The density of a planet is its mass divided by its volume. The units are kilograms per cubic meter.
Surface Gravity
Only Jupiter and Neptune have a stronger surface gravity than the Earth. The Surface Gravity of a planet is proportional to the planet's mass and inversely proportional to the square of the planet's radius.
Where
Synodic Period
If the planet is at its closest to the Earth, the Synodic Period describes how long the planet will take to get back to the same position relative to the Earth. The closer the planet is to the Earth, the longer its Synodic Period. Mars is close to the Earth and at its brightest every 780 days, roughly every 26 months.
Pluto, on the other hand, travels around the Sun so slowly that it takes the Earth a little over a year to orbit the Sun and catch up to it.
Mass is the amount of matter that an object contains. On the Earth mass can be measured by weight. The Gas Giants are far more massive than the Earth. In fact, Jupiter itself has twice the mass of all the other planets put together.
The planets generally revolve around the Sun in almost the same plane. These figures show how many degrees a planet's orbit differs from the Earth's orbit. Apart from Mercury and Pluto, the planets orbit the Sun within a few degrees of the Earth's orbit. The Solar System is essentially flat.
The orbits of all the planets are ellipses. This curve resembles a flattened circle. The eccentricity describes how much the ellipse differs from a circle. An orbit with an eccentricity of 0 is a circle. An orbit with an eccentricity of 1 would be an open curve called a parabola. No planet would stay in orbit with that kind of path. Most planets have orbits with very low eccentricity, close to zero. Their orbits are very nearly circular.
Planets rotate. The line joining the two poles through which the planet rotates is called its axis. If a planet rotated with its axis perpendicular to its orbital plane then its axial tilt would be zero. Such a planet would have no seasons. The Earth's axial tilt is nearly 23.5°. This gives us our seasons and defines the position of the tropics and polar regions.
Density tells how concentrated the matter in a planet is. The Terrestrial Planets are the most dense. They are made mainly of metals and rocks. The Gas Giants are the least dense, being made up of lighter gases. Saturn is less dense than water so it would float!
On the Earth, the acceleration due to gravity is 9.806 65 meters per second per second. This column compares the acceleration of gravity of each planet to that of the Earth.
agravity is the acceleration of gravity (metres per second per second),
G is the Gravitational Constant (6.673 × 10-11 N m2 kg-2),
M is the mass of the planet (kg),
R is the radius of the planet (metres).
This period is relative to the Earth.
| Planet |
Escape Velocity (km s-1) |
Escape Velocity (Earth = 1) |
Solar Irradiance (W m-2) |
Solar Irradiance (Earth = 1) |
Albedo (%) |
Apparent Diameter (seconds of arc) |
Mean Orbital Velocity (km s-1) |
| Mercury | 4.3 | 0.384 | 9126.6 | 6.673 | 11 | 4.5 - 13 | 47.87 |
| Venus | 10.36 | 0.926 | 2613.9 | 1.911 | 65 | 9.7 - 66 | 35.02 |
| Earth | 11.19 | 1.0 | 1367.6 | 1 | 37 | 29.78 | |
| Mars | 5.03 | 0.450 | 589.2 | 0.431 | 15 | 3.5 - 25.7 | 24.13 |
| Jupiter | 59.5 | 5.32 | 50.50 | 0.037 | 52 | 29.8 - 59 | 13.07 |
| Saturn | 35.5 | 3.172 | 14.90 | 0.011 | 47 | 14.5 - 20.1 | 9.69 |
| Uranus | 21.3 | 1.903 | 3.71 | 0.0027 | 51 | 3.3 - 4.1 | 6.81 |
| Neptune | 23.5 | 2.10 | 1.51 | 0.0011 | 41 | 2.2 - 2.4 | 5.43 |
| Pluto | 1.1 | 0.0983 | 0.89 | 0.0007 | 30 | 0.06 - 0.11 | 4.72 |
Escape Velocity
Where
Solar Irradiance
Albedo
Apparent Diameter
The planets closest to the Earth vary the most in apparent diameter.
Mean Orbital Velocity
This is the speed that an object must attain in order to escape from a planet's gravitational field. For the Earth this speed is about 11 kilometres per second (7 miles per second). One column expresses this speed for each planet in kilometres per second; the other relative to the Earth. A planet's escape velocity depends on its mass and radius as shown below.
vesc is the escape velocity (metres per second),
G is the Gravitational Constant (6.673 × 10-11 N m2 kg-2),
M is the mass of the planet (kg),
R is the radius of the planet (metres).
This is the amount of solar energy (in watts) that passes through a square meter of the planet's surface. The closer a planet is to the Sun, the more energy it receives. Mercury receives over six times more energy than the Earth. Pluto receives less than a thousandth of the energy that the Earth does.
This is the percentage of sunlight that is reflected by the planet. Venus reflects 65% of the light that it receives because of its white cloudy atmosphere. Mercury's dark rocks reflect only 11% of the light falling on them.
This is how big the planet appears in the sky as seen from Earth. There is a minimum and maximum value because a planet changes its distance from the Earth. The units are seconds of arc. A degree is divided into 60 minutes. A minute is divided into 60 seconds. The Moon and Sun have an apparent diameter of about half a degree (30 minutes).
This is the planet's average velocity in orbit. A planet will change its velocity as it travels in an elliptical orbit. It moves faster when it is closer to the Sun in accordance with
Kepler's Second Law.
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