1. The planets inside the orbit of the earth are called the Inferior Planets: Mercury and Venus.

2. The planets outside the orbit of the earth are called the Superior Planets: Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

3. The planets inside the asteroid belt are termed the Inner Planets (or the Terrestrial Planets): Mercury, Venus, Earth, and Mars.

4. The planets outside the asteroid belt are termed the Outer Planets: Jupiter, Saturn, Uranus, Neptune, and Pluto.

5. The planets sharing the gaseous structure of Jupiter are termed the Gas Giant (or Jovian) Planets: Jupiter, Saturn, Uranus, and Neptune.

The 7 Planets of the Ancients

The term "planet" originally meant "wanderer": it was observed long ago that certain points of light wandered (changed their position) with respect to the background stars in the sky. In ancient times, before the invention of the telescope and before one understood the present structure of the Solar System, there were thought to be 7 such wanderers or planets: Mercury, Venus, Mars, Jupiter, Saturn, the Moon, and the Sun. This list is different in several respects from our modern list of planets:

1. The Earth is missing, because it was not understood that the points of light wandering on the celestial sphere and the Earth on which we stood had anything in common.

2. Uranus, Neptune, and Pluto are missing because they would only be discovered when the telescope made them easily visible.
   • Uranus is barely visible to the naked eye; it was discovered in 1781.

   • Neptune and Pluto are too faint to see at all without a telescope; they were discovered in 1846 and 1930, respectively.

3. The Sun and the Moon were classified as planets because they wandered on the celestial sphere, just like Mars and Jupiter and the other planets.

Stars Look Different from Planets

Planets (and the Sun and Moon) have some observational characteristics that distinguish them from what we would now call the stars:

Distances and periods

Planet	Average distance from Sun (UA)	Period of Revolution	Period of Rotation
Mercury	0.387	88 days	59 days
Venus	0.723	224.7 days	-243 days
Earth	1 (149600 Km)	365.2425 days	23 h 56 m 4 s
Mars	1.524	687 days	24 h 37 m 23 s
Jupiter	5.203	11.86 years	9 h 50 m 30 s
Saturn	9.539	29.46 years	10 h 14 m
Uranus	19.18	84.01 years	-11 h
Neptune	30.06	164.8 years	16 h
Pluto	39.53	247.7 years	6 days 9 h

As discovered by Kepler, the planets orbit on ellipses with the Sun at one focus. In addition, the planets all revolve in the same direction on their orbits (direct orbital motion). Let's now consider the orbits of the planets in more detail.

The Inner Solar System

Here is the inner solar system constructed with the Solar System Live software.

The inner Solar System to scale, Fall, 1996

The sizes and shapes of the orbits are realistic, as is the relative positions of the planets for the date in the Fall, 1996, when the plot was constructed. The sizes of the planetary symbols are not to scale; the planets would be too small to see at this scale as more than dots of light. Notice the eccentricity of the orbits for Mercury and, to a lesser degree, Mars. From this perspective (which corresponds to looking down on the Northern hemisphere of the Earth), the planets all revolve in a counter-clockwise sense, as indicated by the arrow.

Here are the present positions (to scale) of planets in the inner solar system. In this plot, the portion of orbit in blue is above and the portion in green is below the plane of the ecliptic. As noted in conjunction with Kepler's Third Law, motion of the innermost planets is much faster than that of the outermost; this animation illustrates realistic motion of the inner solar system.

The preceding views represent a "top" or North perspective. Here is a side perspective of the inner Solar System showing the tilt of the planetary orbits with respect to the plane of the ecliptic.

Side view of the inner Solar System, Fall, 1996

In this figure the white portion of the orbit is above the ecliptic plane and the yellow portion is below. Notice that the orbits of the inner planets are nearly, but not quite, in the same plane. The orbit of Mercury, in addition to being the most eccentric, has the largest tilt (7 degrees) with respect to the ecliptic plane.

The Entire Solar System

Here is the entire Solar System to scale for the orbits, also in the Fall, 1996:

The Entire Solar System to scale, Fall, 1996

Notice the enormous amount of empty space in the outer Solar System. To show the entire Solar System to scale, the inner Solar System becomes so compressed that the planet orbits almost appear to run together. The very large eccentricity of Pluto's orbit is also obvious.

Here are the present positions (top view, to scale) of all planets in the Solar System. As above, the portion of orbit in blue is above the plane of the ecliptic; portion in green is below the plane of the ecliptic.

The following figure shows the full Solar System to scale from a side view to illustrate the tilt of the orbits.

Side view of entire Solar System, Fall, 1996

Notice that Pluto's orbit is highly tilted (17 degrees) relative to the plane of the ecliptic.

Here is the present position (side view, to scale) of all planets in the Solar System. The portion of the orbit in blue is above the plane of the ecliptic; portion in green is below the plane of the ecliptic. View is from 20 degrees above ecliptic plane to emphasize the tilt of orbits.

Here is the average separation of the planets from the Sun (in astronomical units) displayed in graphical form,

Orbits, Tilt, Excentricity and Speed

Planet	Tilt on Axis	Tilt on Ellictic	Excentricity	Orbital speed (Km/sec)	Escape speed (Km/sec)
Mercury	28G	7	0.26	47.9	4.2
Venus	3G	3.39	0.007	35.1	10.2
Earth	23G 27'	0	0.017	29.8	11.2
Mars	23G 27'	1.85	0.093	24.1	5
Jupiter	3G 5'	1.3	0.048	13.1	61
Saturn	26G 44'	2.49	0.056	9.6	37
Uranus	97.9G	0.77	0.047	6.8	22
Neptune	28G 48'	1.77	0.009	5.4	25
Pluto	?	17.15	0.248	4.7	10

Here are the eccentricities of the planetary orbits

Last two graphs display clearly the enormous distances in the outer Solar System, and that Pluto and Mercury have by far the most elliptical orbits.

Diameter, Mass, Volume, Density

Planet	Equatorial diameter (Km)	Mass (10^24 Kg)	Volume (Earth=1)	Density (g/m3)
Mercury	4880	.32	.06	5.3
Venus	12104	4.87	.88	4.95
Earth	12756	5.98	1	5.52
Mars	6787	.64	.15	3.95
Jupiter	142800	1900	1316	1.33
Saturn	119300	570	755	.7
Uranus	47100	86.8	67	1.56
Neptune	44600	103	57	2.28
Pluto	6000	0.0129	4	.25

There are many popular misconceptions concerning the size and scale of objects in the Solar System. These mostly have to do with a failure to realize the relative radii of planets and the Sun, and the failure to appreciate how large the outer solar system is relative to the inner solar system. The Sun and the gas giant planets like Jupiter are by far the largest objects in the Solar System. The other planets are small specks on this scale, as the following figure illustrates.

The Sun and planets drawn to scale

Indeed, on this scale the smaller planets like Pluto and Mercury are barely visible.

The masses of the planets are also concentrated in the Gas Giant planets Jupiter, Saturn, Uranus, and Neptune, as the following graph indicates.

However, the large mass of these planets comes from their absolute sizes, not their densities. The inner planets are by far the most dense, as the following graph indicates:

Here is an astrophysical calculator that will display basic astronomical constants and solar system data at the touch of a button, and also allow calculations using these quantities.

Components, Gravity, Temperature

Planet	Main components of atmosphere	Gravity (m/sec2)	Temperature (Kelvin)
Mercury	trace of Hydrogen and Helium	3.6	452
Venus	96% Carbon Dioxide 3% Nitrogen 1.6% Argon	8.5	726
Earth	78% Nitrogen 21% Oxigen 1% Argon	9.81	281-310-260
Mars	95% Carbon Dioxide 3% Nitrogen 1.6% Argon	3.8	310-150
Jupiter	90% Hydrogen 10% Helium 0.07% Methane	26	120
Saturn	97% Hydrogen 3% Helium 0.05% Methane	11.2	88
Uranus	83% Hydrogen 15% Helium 2% Methane	9.4	59
Neptune	74% Hydrogen 25% Helium 1% Methane	15	48
Pluto	Perhaps Methane and Nytrogen	8	37

Satellits

Planet	Satellit	Average distance from Planet (10^3 Km)	Sideral period (days/hrs)	Diameter (Km)	Surface area (10^3 Km2)	Actraction ratio	Propriety	Date of discovery
Mercury
Venus
Earth	Moon	384	27/08	3470	37851	.46	own
Mars	Phobos Deimos	9 24	0/08 1/06	24 13	1.7 0.46	195 32	own own	1877 1877
Jupiter	Amaltea Io Europa Ganimede Callisto Decimo Sesto Settimo Dodicesimo Undicesimo Ottavo Nono	180 420 670 1070 1880 11900 11980 12280 21200 22600 23500 23700	0/12 1/18 3/13 7/04 16/17 264 266 276 625 695 739 758	240 3700 3140 5140 5170 24 160 56 23 31 25 27	81 43674 30995 83123 84532 .81 41 7.6 1.7 1.7 7.6 1.7	18200 3260 1260 490 160 4.4 4.3 4.2 1.3 1.2 1.03 1.03	own own own own own captured captured captured captured captured captured captured	1892 1610 1610 1610 1610 1938 1904 1905 1951 1938 1908 1914
Saturn	Giano Mimas Enceladus Tethys Dione Rhea Titan Hyperion Iapetus Phoebe	158 190 240 295 380 530 1220 1480 3560 12960	0/18 0/23 1/09 1/21 2/18 4/12 15/23 21/07 79/08 550/26	300 500 600 1300 1200 1800 5000 400 1200 140	734 734 3940 3940 8077 68564 326 5184 326	23000 15500 9800 6400 4150 2000 380 260 45 3.5	own own own own own own own own own captured	1967 1789 1789 1684 1684 1672 1655 1848 1671 1898
Uranus	Miranda Ariel Umbriel Titania Oberon	125 190 265 440 590	1/10 2/12 4/03 8/17 13/11	200 900 700 1700 1600	52 734 512 2912 2912	24600 9850 4750 1750 1050	own own own own own	1948 1851 1851 1787 1787
Neptune	Triton Nereid	350 5570	5/21 359/10	5000 300	63868 140	8400 24	own captured	1846 1949
Pluto	Charon							1978

Lithiosphere, Idrosphere, Atmosphere

Planet	Lithiosphere (ray in Km)	Idrosphere (thickness in Km)	Atmosphere (thickness in Km)
Earth	6400	3.3	12.9
Jupiter	29800	27400	12900
Saturn	22500	12900	25700
Uran	11300	9700	4800
Neptun	9700	9700	3200