Introduction
Five planets have been known since ancient times. During the 16th and 17th Centuries, the Earth was added to the list of planets when it was realised
that it was also orbiting the Sun. Since then, two more planets have been found making a total of eight. These eight worlds orbit the Sun. Most have
moons or satellites orbiting them.
A number of small Trans-Neptunian icy worlds are now known, including Pluto, which was considered a planet when first found in 1930. The status of
Pluto as a planet was in doubt for several years after the discovery of several similar worlds. Astronomers consider it the most famous example of a
new kind of body called a Kuiper Belt Object. These are essentially asteroid type objects at great distance from the Sun.
There are two types of major planets.
The Terrestrial Planets are small and solid (like the Earth) and are made of rock and metal. The are also called the Inner
Planets because they are fairly close to the Sun. They have no or very few satellites. In order from the Sun, these planets are
Mercury, Venus, Earth and
Mars.
Mercury, a small airless world closest to the Sun.
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Venus, a twin of the Earth in size with a hot, dense atmosphere.
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Earth - the third planet with an atmosphere and oceans.
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Mars, a cool, dry world with a thin atmosphere.
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The Jovian Planets are also known as the Gas Giants. This is because they are large and mainly gaseous. Another
name for them is the Outer Planets, because of their great distances from each other and the Sun. They have large systems of
satellites (like mini solar systems) and ring systems, that of Saturn being particularly spectacular. These planets, in order from the Sun, are
Jupiter, Saturn, Uranus and
Neptune.
Jupiter is the largest planet in the Solar System.
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Saturn with its magnificent ring system.
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Quiet Uranus spins on its side.
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Blue Neptune has a dynamic atmosphere.
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The tables below describe various properties of the planets. Below each table are explanations of the terms used.
Under the column "Number of Moons" there are links to tables with details about the moons. The Kuiper Belt Object,
Pluto, is shown for comparison as it was formerlly classed as a planet.
Orbital Properties
| Planet |
Mean Distance
From
The Sun
(×106 km) |
Mean Distance
From
The Sun
(Earth = 1) |
Period To
Revolve Around
The Sun |
Mean
Orbital
Velocity
(km s-1) |
Orbital
Inclination
(Earth = 0)
| Orbital
Eccentricity |
|---|
| Mercury |
57.91 |
0.387 |
88.0 days |
47.87 |
7° |
0.2056 |
| Venus |
108.21 |
0.723 |
224.7 days |
35.02 |
3.394° |
0.0067 |
| Earth |
149.6 |
1.0 |
365.25 days |
29.78 |
0° |
0.0167 |
| Mars |
227.92 |
1.524 |
687.0 days |
24.13 |
1.850° |
0.0935 |
| Jupiter |
778.57 |
5.204 |
11.75 years |
13.07 |
1.304° |
0.0489 |
| Saturn |
1,433.53 |
9.582 |
29.5 years |
9.69 |
2.485° |
0.0565 |
| Uranus |
2,872.46 |
19.201 |
84 years |
6.81 |
0.772° |
0.0457 |
| Neptune |
4,495.06 |
30.047 |
165 years |
5.43 |
1.769° |
0.0113 |
| Pluto |
5,869.66 |
39.236 |
248 years |
4.72 |
17.16° |
0.2444 |
Mean Distance From The Sun
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.
Period to Revolve Around The Sun
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.
Where
- 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).
As seen from above the north pole of the Earth, all the planets orbit in an anticlockwise direction. This is called Direct Orbital Motion.
Mean Orbital Velocity
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.
Orbital Inclination
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 the Kuiper Belt Object, Pluto), the planets orbit the Sun within a few degrees of the Earth's orbit. The Solar System is essentially flat.
Orbital Eccentricity
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.
Venus has the most circular orbit. The least circular orbits is that of Mercury. The Kuiper Belt object, Pluto, has an orbit 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. Other Kuiper Belt objects have similar orbits.
Physical Properties 1
| Planet |
Diameter
(km) |
Diameter
(Earth = 1) |
Rotational
Period |
Oblateness |
Axial Tilt |
|---|
| Mercury |
4,879 |
0.38 |
58.65 days |
0.0 |
2.0° |
| Venus |
12,104 |
0.95 |
-243.02 days |
0.0 |
177.4° |
| Earth |
12,742 |
1.0 |
23 hrs 56 mins |
0.0034 |
23.45° |
| Mars |
6,780 |
0.53 |
24 hrs 37 mins |
0.005 |
25.19° |
| Jupiter |
139,822 |
10.97 |
9 hrs 55 mins |
0.065 |
3.12° |
| Saturn |
116,464 |
9.14 |
10 hrs 40 mins |
0.108 |
26.73° |
| Uranus |
50,724 |
3.98 |
-17.24 hours |
0.03 |
97.86° |
| Neptune |
49,248 |
3.87 |
16.11 hours |
0.02 |
29.56° |
| Pluto |
2,390 |
0.19 |
-6.38 days |
0.0 |
119.6° |
Diameter
This is the average diameter of each planet.
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.
Rotational Period
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 the Kuiper Belt Object Pluto) rotate in a clockwise sense. This is called Retrograde Rotation and is shown by the presence of a minus sign.
The planet Venus has a rotation period of 243 days which is longer than its orbital period around the Sun (225 days).
Oblateness
This is how much the planet is flattened because of its rotation. A value of zero denotes a perfectly spherical planet. Mercury and Venus rotate too slowly to be oblate. The Gas Giants, being gaseous and fast rotators have a high oblateness. Saturn is the most oblate planet.
Axial Tilt
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.
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.
Physical Properties 2
| Planet |
Mass
(Earth = 1) |
Density
(×103 kg m-3) |
Surface
Gravity
(Earth = 1) |
Escape
Velocity
(km s-1) |
Escape
Velocity
(Earth = 1) |
|---|
| Mercury |
0.0553 |
5.43 |
0.378 |
4.3 |
0.384 |
| Venus |
0.815 |
5.25 |
0.907 |
10.36 |
0.926 |
| Earth |
1.0 |
5.52 |
1.000 |
11.19 |
1.0 |
| Mars |
0.107 |
3.95 |
0.377 |
5.03 |
0.450 |
| Jupiter |
317.83 |
1.33 |
2.364 |
59.5 |
5.32 |
| Saturn |
95.159 |
0.69 |
0.916 |
35.5 |
3.172 |
| Uranus |
14.536 |
1.29 |
0.889 |
21.3 |
1.903 |
| Neptune |
17.147 |
1.64 |
1.120 |
23.5 |
2.10 |
| Pluto |
0.002 |
2.03 |
0.059 |
1.1 |
0.0983 |
Mass
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 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.
The very low mass of Pluto is one of the reasons why it is no longer considered to be a major planet. Since the 1980s other Pluto-sized bodies have
been found in the distant part of the Solar System. These are the Kuiper Belt Objects and Pluto is now counted as one of them.
Density
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.
The density of a planet is its mass divided by its volume. The units are kilograms per cubic meter.
Surface Gravity
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.
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
- 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).
Escape 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.
Where
- 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).
Thermal Properties
| Planet |
Solar
Irradiance
(W m-2) |
Solar
Irradiance
(Earth = 1) |
Albedo
(%) |
Surface
Temperature
(° C) |
|---|
| Mercury |
9126.6 |
6.673 |
11 |
467° to -183° |
| Venus |
2613.9 |
1.911 |
65 |
465° |
| Earth |
1367.6 |
1 |
37 |
45° to -60° |
| Mars |
589.2 |
0.431 |
15 |
0° to -100° |
| Jupiter |
50.50 |
0.037 |
52 |
-148° |
| Saturn |
14.90 |
0.011 |
47 |
-178° |
| Uranus |
3.71 |
0.0027 |
51 |
-213° |
| Neptune |
1.51 |
0.0011 |
41 |
-216° |
| Pluto |
0.89 |
0.0007 |
30 |
-223° |
Solar Irradiance
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. The Kuiper Belt Object, Pluto receives less than a thousandth of the energy that the Earth does. The second column shows the figure relative to the Earth.
Albedo
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.
Surface Temperature
The Surface Temperature of a planet depends on several factors. The distance from the Sun determines how much energy is received. The rotation determines how long the surface is heated or remains in the dark: the slower the rotation, the more extreme the temperature changes. The presence of an atmosphere can also remove temperature extremes.
Mercury has extreme surface temperatures because of its slow rotation and lack of atmosphere. Venus has a thick atmosphere which causes a greenhouse effect keeping the temperature high regardless of the time of day or night.
The temperature given for the Gas Giants is of the tops of the visible clouds.
Observational Properties
| Planet |
Synodic
Period
(days) |
Apparent
Diameter
(seconds
of arc) |
Maximum
Apparent
Magnitude |
Colour |
|---|
| Mercury |
115.88 |
4.5 - 13 |
-1.9 |
Silvery |
| Venus |
583.92 |
9.7 - 66 |
-4.4 |
White |
| Earth |
|
|
|
Bluish White |
| Mars |
779.94 |
3.5 - 25.7 |
-2.8 |
Red |
| Jupiter |
398.88 |
29.8 - 59 |
-2.6 |
Pale Yellow |
| Saturn |
378.09 |
14.5 - 20.1 |
-0.5 |
Yellow |
| Uranus |
369.66 |
3.3 - 4.1 |
+5.7 |
Green |
| Neptune |
367.49 |
2.2 - 2.4 |
+8.2 |
Blue |
| Pluto |
366.73 |
0.06 - 0.11 |
+13.7 |
Yellow |
Synodic Period
This period is relative to the Earth.
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.
Neptune, on the other hand, travels around the Sun so slowly that it takes the Earth 2.5 days over a year to orbit the Sun and catch up to it.
Apparent Diameter
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).
The planets closest to the Earth (Venus and Mars) vary the most in Apparent Diameter.
Apparent Magnitude
Apparent Magnitude tells how bright a planet (or star) is as seen from the Earth. The magnitude scale was devised by the Ancient Greeks. The brightest stars were called First Magnitude, the next brightest were called Second Magnitude, etc.
In modern times, the scale has been defined mathematically. A star of magnitude 1 is about 2.5 times brighter than a star of magnitude 2 which in turn is 2.54 times brighter than a star of magnitude 3. The brighter a star, the smaller its magnitude. Many stars are brighter than first magnitude. Some stars are so bright they have negative magnitudes. Most of the naked eye planets also have negative magnitudes. The faintest stars visible to the naked eye are sixth magnitude. Uranus is on the limit of naked eye visiblity while Neptune (and Pluto) cannot be seen without optical aid.
The values are for the planets at their brightest.
The brightness of a planet as seen from Earth depends on the closeness of a planet to the Sun (more light to reflect), the albedo (how much light is reflected) and the distance between the planet and the Earth.
Venus is close to the Sun and the Earth and has the highest albedo, being covered by reflective white clouds: it is always the brightest planet and can be 16 times brighter than the brightest star.
Colour
Planets have different colours depending on the type of surface or cloud cover they possess.
Mars is red because its surface is mainly oxides of iron (rust). Venus is white because it is completely covered with uniformly white cloud.
Miscellaneous Properties
| Planet |
Composition of Atmosphere |
Discovery |
Number
Of
Moons |
|---|
| Mercury |
H2 He (trace amounts) |
Mesopotamia |
0 |
| Venus |
CO2 (96%) N2 (3%) H2O (0.1%)
Surface pressure: 90 atm
Clouds: H2SO4 |
Mesopotamia |
0 |
| Earth |
N2 (78%) O2 (21%) Ar (1%)
Surface pressure: 1 atm
Clouds: H2O |
|
1 |
| Mars |
CO2 (95%) N2 (3%) Ar (1.6%)
Surface pressure: 0.02 atm |
Mesopotamia |
2 |
| Jupiter |
H2 (90%) He (10%) CH4 (0.7%) |
Mesopotamia |
63 |
| Saturn |
H2 (97%) He (3%) CH4 (0.05%) |
Mesopotamia |
60 |
| Uranus |
H2 (83%) He (15%) CH4 (2%) |
England (1781) |
27 |
| Neptune |
H2 (74%) He (25%) CH4 (1%) |
Europe (1846) |
13 |
| Pluto |
CH4 N2 (trace amounts) |
USA (1930) |
4 |
Composition of Atmosphere
Most of the planets have atmospheres. The small Terrestrial Planets have relatively thin atmospheres over a rocky surface. For these, the surface pressure is given in atmospheres (atm) where 1 atm is the pressure on the surface of the Earth. Mercury and Pluto have trace atmospheres - essentially none.
The Gas Giants have atmospheres that are very deep and there is no real solid surface. The atmosphere becomes thicker until it liquifies then solidifies. The composition at the top of the clouds is given for these.
Discovery
The five naked eye planets (Mercury, Venus, Mars, Jupiter and Saturn) have been known since ancient times and were probably recognised as different to the stars in the civilisations of Mesopotamia (modern day Iraq).
The Earth was recognised as one of the planets in 1543 when the Polish astronomer, Nicholas Copernicus, published a book that pictured the Earth as another planet orbiting the Sun rather than as the immovable centre of the Universe.
The other three planets were discovered after the invention of the telescope. Uranus was found in 1781 by the German born English astronomer,
William Herschel, during a routine sweep of the sky. Neptune was found by Johann Galle (in what is now Germany) in
1846 after its position was predicted by two mathematicians, John Adams (England) and Urbain Leverrier (France),
who both studied the movement of Uranus and found that was not in accord with theory. Pluto was found by Clyde Tombaugh (USA) after
a systematic search and was initially considered a planet even though its properties were very different to what was expected. It is now recognised
as the brightest member of a swarm of icy worlds beyond Neptune.
Number of Moons
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
sixty three, Saturn has
sixty, Uranus also has
tweny seven, Neptune has
thirteen.
Many of the Kuiper Belt Objects have satellites - Pluto, has
four.
There are over 160 known satellites in the Solar System.
KryssTal Related Pages
Tabulated details about selected minor planets (Astroids, Trojans, Centaurs and Kuiper Belt Objects) with explanations of the terms used.
