The four inner planets -- Mercury, Venus, Earth and Mars -- share several features in common. Astronomers call them the “terrestrial planets” because they have solid, rocky surfaces roughly similar to desert and mountainous areas on the earth. The inner planets are much smaller than Jupiter, Saturn, Uranus and Neptune, and they all possess iron cores.

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Inner planets are much smaller than the outer planets, and are rocky with an iron core.

Astronomers theorize that the very early solar system formed as a ring of materials surrounding the sun. Heavier elements such as iron and nickel condensed relatively close to the sun, whereas substances such as hydrogen, methane and water condensed in colder regions farther out. The terrestrial planets formed as clumps of rock and heavy elements from the inner ring of materials accumulated due to gravitational attraction; in a similar way, the outer band of gaseous substances produced the outer planets.

Compared to the four gas giant planets that make up the outer solar system, the inner planets all have diminutive sizes. Of the four, Earth is the largest, with a diameter of 6,378 kilometers (3,963 miles) at the equator. Venus is a close second at 6,051 kilometers (3,760 miles). Mars is much smaller with a 3,396-kilometer (2,110-mile) diameter, and Mercury is the smallest terrestrial planet, measuring 2,439 kilometers (1,516 miles) across.

The terrestrial planets all have rocky surfaces that feature mountains, plains, valleys and other formations. The temperatures of the inner planets are low enough that rock exists mostly as a solid at the surface. To different degrees, they also have meteor impact craters, although the dense atmospheres of Venus and Earth protect them from most meteors, and weathering and other factors wipe out all but the most recent craters. Mars has very low atmospheric pressure, and Mercury has almost none, so craters are more common on these planets.

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Astronomers believe all four of the terrestrial planets possess an iron core. During their early formation, the planets were hot blobs of molten metals and other elements; being heavier, most of the iron and nickel ended up on the inside with lighter elements such as silicon and oxygen forming the outside. Geologists have concluded that the earth’s iron core is partly liquid and partly solid by observing the behavior of earthquake waves traveling through the earth. Scientists speculate that the other terrestrial planets may also have partly liquid cores.

Chicago native John Papiewski has a physics degree and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance." Please, no workplace calls/emails!