We compare the properties of the planets to learn how the solar system formed and why the earth is such a special place.

Key points: Terrestrial and giant planets; range of properties with distance from sun and what drives them; retention of gas and atmospheres

Overview of the Solar System

The Planets to Scale; from the sun, the "terrestrial planets" are Mercury, Venus, Earth, and Mars, while the "giant planets" are Jupiter, Saturn, Uranus, and Neptune (Pluto was an oddity at the end*). (from http://www.adamnieman.co.uk/futurelab/)

*On August 24, 2006, the International Astronomical Union - the body that oversees such matters - voted to strip Pluto of its planet status. By official decree, we now have only eight planets.

Terrestrial Planet Properties

• From Mercury to Mars, their distances from the sun range from about 0.4 to 1.5 times the distance of the earth from the sun, defined as an astronomical unit (AU)
• Mercury, Venus, Earth, Mars all have average densities greater than 3 grams/cm³
• All four have rocky surfaces and rocky/metallic interiors
• Atmospheres vary greatly (e.g. none for Mercury to very dense for Venus) but heavier molecules like CO₂ tend to dominate over lighter ones such as H₂
• Only the Earth has an appreciable magnetic field (Venus and Mercury rotate too slowly, Mars apparently no longer has a liquid core)

Here is a rough comparison of their interior structures. They are all variations on a common theme, with solid iron cores inside liquid iron components (except for Mars, where the interior temperature is too low to liquify this zone), surrounded by mantles of silicate rock, overlaid with rocky crusts. (from Nick Strobel's Astronomy Notes.)

Overview of Giant Planets (Jupiter, Saturn, Uranus, Neptune)

Distances from the sun range from about 5 AU (Jupiter) to 30 AU (Neptune)

Average densities are low, similar to water:   Jupiter is 1.3 grams/cm³ and Saturn is 0.7 grams/cm³

Composition similar to Sun -- (especially Jupiter and Saturn)

• 80% hydrogen
• 19% helium
• 1% other elements

Liquid or icy surfaces

Dense atmospheres with violent and long lasting storms

Strong magnetic fields (Jupiter and Saturn)

Here is a comparison of their interiors. They are all built around rocky cores. The interiors of Jupiter and Saturn feature liquid metallic hydrogen (possible because of the immense interior pressures) and then non-metallic liquid hydrogen. The distinction is that the metallic form is electrically conducting.   Uranus and Neptune are largely icy. All these planets have gaseous hydrogen atmospheres at the top. (from Nick Strobel's Astronomy Notes. ). We have put in the earth for comparison. It is not very different in mass than the rocky/metallic cores of the giant planets, indicating that all the planets started with similar reservoirs of raw material. The differing properties have to arise from how the planets formed from these reservoirs.

	The densities and compositions of the planets are correlated with how close they are to the sun, and hence the temperature at which they had to form. (From The Essential Cosmic Perspective, by Bennett et al.)
	Low density ices could only condense in the zone of giant planets, where temperatures were low. (by G. Rieke, some data from Univ. of Michigan Global Change Program, http://www.sprl.umich.edu/GCL/)

The average speed of an atom or molecule goes as the square root of the temperature divided by the mass of the particle. If the speeds are close to the escape velocity, gases will leak away into space. At the high temperatures at the orbit of Mercury (and its relatively small gravity), all gaseous materials escaped. For Venus and Earth, the lower temperature and larger gravity allowed heavier gases to be retained (oxygen, nitrogen) but hydrogen and helium escaped. Mars lost most of its gases because its surface gravity is small. The giant planets were out where temperatures were low enough to retain virtually all the gases.

The frost line for the Solar System lies between Mars and Jupiter. It marks a major divide in planet properties because outside of it, various kinds of frozen light molecules could accumulate on a forming planet as ices (for example, frozen water captures not only oxygen, but some hydrogen). These ices ran up the mass of the planet so it had a chance to retain the light gases, even helium and hydrogen. Thus, the giant planets (around the sun AND the other stars) probably all had to form outside of the frost line.

How do planetary systems form

Test your understanding before going on

Formation of the sun and moon, ceiling painting, St. Isaac's Cathedral, St. Petersburg (photo by G. Rieke)		Pioneering space art by Chesley Bonestell, http://www.bonestell.org/, http://www.dreamstone.com.au
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Click to return to Formation of Planets	hypertext G. H. & M. J. Rieke	Click to go to Exploration of the Solar System