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Planets of the Solar System

Overview

As the solar system emerged from its parent nebula, a young star was surrounded by a disk of dust and gas that was starting to clump in various places. As these clumps grew larger, they collapsed under their mutual gravity and to accrete (accumulate) more matter. Clumps closer to the sun were deprived of a lot of gas because the young star's radiation forced most of the lighter materials outward into what would become the outer solar system. While the inner planets became mostly rocky, the outer planets were able to accrete much more gas and to evolve voluminous atmospheres.

Eventually, the sun's wind blew most of the remaining material away, and the new planets could grow no larger. Mercury, Venus, Earth, and Mars became the rocky, terrestrial planets of the inner solar system, while Jupiter, Saturn, Uranus, and Neptune became the gas giants of the outer solar system. Pluto, the "planet" beyond Neptune, is not gaseous like the other planets of the outer solar system, nor is it rocky like those of the inner; it is actually most like a giant comet.

Left-over building blocks were herded into the region between Mars and Jupiter to become the asteroid belt. Other material, farther from the Sun so able to retain more volatile components, formed the comets of the Kuiper Belt and Oort Cloud.

Even though there was no longer much planet-building material left, the solar system looked different than it does today. Rogue asteroids roamed the solar system, as evidenced by the crater patterns on the Earth's Moon. The interplanetary asteroids frequently struck planets and moons until approximately 3.8 billion years ago. After that period, most of the rogue asteroids had either been ejected from the solar system, captured as moons, struck planets or moons, or been shepherded into relatively stable orbits, mostly between Mars and Jupiter in the asteroid belt.

Since then, the solar system has been relatively calm, though each object in it has had a unique evolution that is explored in the pages of this section.

What Is a Planet?

Historically, the solar system was composed of Mercury, Venus, Earth, Mars, Jupiter, Saturn, the Sun, and a smattering of moons. Then comets were realized and asteroids were discovered, and Uranus and Neptune were found. The solar system was still pretty cut-and-dry: There was a star, around which there were eight planets. Between the fourth and fifth was a belt of asteroids that were tiny bodies significantly smaller than any planet. Around most planets were moons.

Pluto's discovery in 1930 and its subsequent classification as a planet was not questioned because another planet was expected to be there. But lately, many more objects have been discovered around, within, and outside of Pluto's orbit. For this reason, and for revised theories of how the solar system, formed and what objects it should contain far from the Sun, most modern astronomers no longer consider a planet, but rather it is the "King Comet" of the Kuiper Belt. The announcement in Summer 2005 that an object had been discovered that is about 25% larger in diameter than Pluto has further pushed this debate into the forefront of semantics.

So what is a planet? Speaking purely by the numbers, one could consider it as a size: Anything larger than Pluto is a planet. But this doesn't work because all four of the Galilean Satellites around Jupiter would then be considered planets, as would Earth's moon. If the cut-off were then instead placed at Mercury's size, Jupiter's moon Ganymede would still be a planet, and it's moon Callisto would be just 30 km shy of planetary status.

Instead, one could think of a planet as a body that only orbits the Sun. This is a problem because then every asteroid and comet would be a planet. So then if one considered eccentricity: Any body with an eccentricity less than Mercury's would be a planet. That would eliminate Pluto, but it would allow many asteroids to be planets. You should be starting to get an idea of how difficult this is and why there has been such debate.

Currently, the International Astronomical Union (IAU) is the only official body that declares the status of an object as a planet, moon, asteroid, etc. If Pluto were discovered today, there is no doubt that it would be considered a Kuiper Belt Object. But it has historically been considered a planet, and thus far, the IAU has not chosen to change its status. The announcement of KBO 2003 UB313 having a size larger than Pluto has cast a new wrinkle into the debate. The IAU has yet to decide whether or not to grant it planetary status. But no matter which way it decides, not everyone will be happy. Until that ruling happens, the definition of "planet" is still open to a wide range of interpretation.

On the opposite end of the spectrum, the many extra-solar planets that have been discovered since 1995 beg for an upper-mass cut-off as well. This has been, relatively unofficially, put at about 13 times the mass of Jupiter. Anything larger is generally considered to be a brown dwarf - a "failed star" that wasn't able to accrete enough mass to start nuclear fusion in its core.

Data for the Planets

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Perihelion (106 km)
46.00
107.5
147.09
206.62
740.52
1352.55
2741.30
4444.45
Mean Orbital Distance (106 km)
57.91
108.2
149.60
227.92
778.57
1433.53
2872.46
4495.06
Aphelion (106 km)
69.82
108.9
152.10
249.23
816.62
1514.50
3003.62
4545.67
Average Orbital Velocity (km/s)
47.87
35
29.78
24.13
13.07
9.69
6.81
5.43
Orbital Inclination (from Earth's Orbit)
7.00°
3.4°
0.0°
1.850°
1.304°
2.485°
0.772°
1.769°
Orbital Eccentricity
0.2056
0.007
0.0167
0.0935
0.0489
0.0565
0.0457
0.0113
Equatorial Radius (km)
2439.7
6051.8
6378.1
3397
71,492
60,268
25,559
24,764
Polar Radius (km)
2439.7
6051.8
6,356.8
3375
66,854
54,364
24,973
24,341
Volume (1010 km3)
6.083
92.843
108.321
16.318
143,128
82,713
6833
6254
Ellipticity (Variation from Sphere)
0.0000
0.000
0.00335
0.00648
0.06487
0.09796
0.02293
0.01708
Axial Tilt (from Earth's geographic North)
0.01°
177.4°
23.45°
25.19°
3.13°
26.73°
97.77°
28.32°
Mass (1024 kg)
0.3302
4.87
5.9736
0.64185
1898.6
568.46
86.832
102.43
Density (water=1)
5.427
5.243
5.515
3.933
1.326
0.687
1.27
1.638
Escape Velocity (km/s)
4.3
10.36
11.19
5.03
59.5
35.5
21.3
23.5
Gravity (m/s2)
3.70
8.802
9.78
3.716
23.1
9
8.7
11
Surface Pressure (bars)
≈ 10-15
92
1.014
0.000636
N/A
N/A
N/A
N/A
Total Mass of Atmosphere (kg)
< 1000
4.8x1020
5.1x1018
2.5x1016
N/A
N/A
N/A
N/A
Sidereal Rotation Period (hours)
1407.6
-5832.5
23.9345
24.6229
9.9250
10.656
-17.24
16.11
Length of Day (hours)
4222.6
2802
24
24.6597
9.9259
10.656
17.24
16.11
Tropical Orbital Period (days)
87.968
224.7
365.256
686.980
4330.595
10,746.94
30,588.740
59,799.9
Bond Albedo 0.119 0.750 0.306 0.250 0.343 0.342 0.300 0.290
Visual Geometric Albedo 0.106 0.65 0.367 0.150 0.52 0.47 0.51 0.41
Visual Magnitude -0.42 -4.40 -3.86 -1.52 -9.40 -8.88 -7.19 -6.87
Solar Irradiance (W/m2) 9126.6 2613.9 1367.6 589.2 50.50 14.90 3.71 1.51
Black-Body Temperature (K) 442.5 231.7 254.3 210.1 110.0 81.1 58.2 46.6
Average Surface Temperature (Celsius) 167° 464° 15° -65° -110° -140° -195° -200°
Number of Moons
Rings? No No No No Yes Yes Yes Yes
Global Magnetic Field Strength (Gs) / Tilt 0.0033 / 169° - / - 0.3076 / 11.4° - / - 4.28 / 9.6° 0.210 / <1° 0.228 / 58.6° 0.142 / 46.9°
Discoverer Unknown Unknown Unknown Unknown Unknown Unknown William Herschel Johann Gottfried Galle
Discovery Date Prehistory Prehistory Prehistory Prehistory Prehistory Prehistory March 13, 1781 September 23, 1846

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