Is Earth the Galaxys Most Unusual and Lucky Planet

Earth is the third planet from the Sun and the fifth-largest in the solar system. Our home planet circles its parent star at an average distance of about 93 million miles (149.6 million km). Earth is one of four so-called terrestrial planets found in the inner portion of the solar system. In addition to Earth itself, the terrestrial (or inner) planets are Mercury, Venus, and Mars. These planets are similar in their physical properties and characteristics to Earth—that is, they are small, relatively high-density bodies that are composed of metals and silicates with shallow (or negligible) atmospheres as compared to the giant, gaseous outer planets (Jupiter, Saturn, Uranus, and Neptune). What makes Earth so unique (or perhaps lucky) is that it is the only planetary body in the solar system currently known to support life. As discussed later in the book, Mars and the Jovian moon Europa are suspected of possibly being life-bearing worlds, but at present, only our home planet is known to host a treasure house of biological activity.

The name Earth comes from the Indo-European language base er, which produced the Germanic noun ertho, and ultimately the German word erde, the Dutch aarde, the Scandinavian jord, and the English earth. Related word forms include the Greek eraze, meaning on the "ground," and the Welsh erw, meaning "a piece of land." In Greek mythology, the goddess

Life on Earth comes in a wide variety of forms, shapes, and sizes. For example, this stingray is just one of the many kinds of marine life that inhabit the waters off the Florida Keys. (NASA)

Life on Earth comes in a wide variety of forms, shapes, and sizes. For example, this stingray is just one of the many kinds of marine life that inhabit the waters off the Florida Keys. (NASA)

of Earth was called Gaia, while in Roman mythology the Earth goddess was Tellus (meaning "fertile soil"). The expression Mother Earth comes from the Latin expression terra mater. Scientists and writers frequently use the word terrestrial to describe creatures and things related to or from planet Earth and the word extraterrestrial to describe creatures and things beyond or away from planet Earth. Astronomers also refer to our planet as Terra or Sol III, which means the third satellite out from the Sun.

From space, Earth is characterized by its blue waters and white clouds, which cover a major portion of it. An ocean of air, consisting of 78 percent nitrogen and 21 percent oxygen, surrounds humans' home planet; the remainder is argon, neon, and other gases. The standard atmospheric pressure at sea level is 14.7 pounds per square inch (101,325 Pa). Surface temperatures range from a maximum of about 140°F (60°C) in desert regions along the equator to a minimum of -130°F (-90°C) in the frigid, polar regions. In between, however, surface temperatures are generally much more benign. The acceleration due to gravity at sea level on Earth is 32.2 feet per second-squared (9.8 m/s2). Some exobiologists suggest that a surface gravity value much larger or smaller than this value would not favor the emergence of complex, possible intelligent life.

Earth's rapid spin and molten nickel-iron core give rise to an extensive magnetic field. This magnetic field, together with the atmosphere, shields people and other living creatures from nearly all of the harmful charged particles and ultraviolet radiation coming from the Sun and other cosmic sources. Furthermore, most meteors burn up in Earth's protective atmosphere before they can strike the surface. Earth's nearest celestial neighbor, the Moon, is its only natural satellite.

While life on Earth is made possible by the Sun, terrestrial life is also regulated by the periodic motions of the Moon. The ocean tides rise and fall because of the gravitational tug-of-war between Earth and the Moon. Throughout history, the Moon has had a significant influence on human culture, art, and literature. For example, the months of the calendar year originated from the regular motions of the Moon around Earth. Even in the space age, the Moon has proved to be a major technical stimulus. The Moon was just far enough away to represent a real technical challenge to reach it; yet this alien world was close enough to allow humans to be successful on the first concentrated effort.

The most recent lunar origin theory suggests a cataclysmic birth of the Moon. Scientists who support this theory speculate that near the end of Earth's accretion from the primordial solar nebula materials (that is, after its core was formed, but while Earth was still in a molten state), a Mars-size celestial object (called an impactor) hit Earth at an oblique angle. This ancient explosive collision sent vaporized-impactor and molten-Earth material into Earth orbit, and the Moon then formed from these materials. Was the formation of Earth's large natural satellite a lucky, very rare random cosmic event, or does this type of cosmic coincidence happen frequently to Earth-like planets around distant stars? The scientific answer to this interesting question may shed additional light on the overall issue of the life, especially intelligent life, in other star systems.

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