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. |
Venus, a twin of the Earth in size with a hot, dense atmosphere. |
Earth - the third planet with an atmosphere and oceans. |
Mars, a cool, dry world with a thin atmosphere. |
Jupiter is the largest planet in the Solar System. |
Saturn with its magnificent ring system. |
Quiet Uranus spins on its side. |
Blue Neptune has a dynamic atmosphere. |
Planet | From The Sun (×106 km) |
From The Sun (Earth = 1) |
Revolve Around The Sun |
Orbital Velocity (km s-1) |
Inclination (Earth = 0) | 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 |
Where
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.
Planet | (km) |
(Earth = 1) |
Period |
||
---|---|---|---|---|---|
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° |
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.
The planet Venus has a rotation period of 243 days which is longer than its orbital period around the Sun (225 days).
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.
Planet | (Earth = 1) |
(×103 kg m-3) |
Gravity (Earth = 1) |
Velocity (km s-1) |
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 |
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.
The density of a planet is its mass divided by its volume. The units are kilograms per cubic meter.
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
Where
Planet | Irradiance (W m-2) |
Irradiance (Earth = 1) |
(%) |
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° |
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.
Planet | Period (days) |
Diameter (seconds of arc) |
Apparent Magnitude |
|
---|---|---|---|---|
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 |
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.
The planets closest to the Earth (Venus and Mars) vary the most in Apparent Diameter.
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.
Mars is red because its surface is mainly oxides of iron (rust). Venus is white because it is completely covered with uniformly white cloud.
Planet | Composition of Atmosphere | Discovery | 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 |
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.
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.