Origins: Where Are the Aliens?
Terrestrial and Jovian Planets
With the exception of Pluto, planets in our solar system are classified aseither terrestrial (Earth-like) or Jovian (Jupiter-like) planets. Terrestrialplanets include Mercury, Venus, Earth, and Mars. These planets are relativelysmall in size and in mass. A terrestrial planet has a solid rocky surface, withmetals deep in its interior. In the solar system, these planets are closer tothe sun and are therefore warmer than the planets located farther out in thesolar system. Future space missions are being designed to search remotely forterrestrial planets around other stars.
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The layers of gases surrounding the surface of a planet make up what is knownas an atmosphere. The atmospheres of the terrestrial planets range from thin tothick. Mercury has almost no atmosphere. A thick atmosphere made mostly ofcarbon dioxide covers Venus, trapping heat and raising surface temperatures.Clouds on Venus form from sulfuric acid. Earth”s atmosphere is 77 percentnitrogen, 21 percent oxygen, and 1 percent argon, with variable amounts ofwater vapor, and trace amounts of other gases. White clouds of water vapor hidemuch of Earth”s surface in views of Earth from space. Mars has a very thinatmosphere containing mostly carbon dioxide, with nitrogen, argon, and traceamounts of oxygen and water vapor. The atmosphere also contains thin water andcarbon dioxide clouds, and is frequently affected by dust storms.
Jovian planets include Jupiter, Saturn, Uranus, and Neptune. These planets havelarger sizes and masses. Jovian planets do not have solid surfaces. They aresometimes called gas giants because they are large and made mostly of gases.Small amounts of rocky materials are only found deep in the cores of Jovianplanets. In the solar system, Jovian planets are located farther from the sunthan terrestrial planets, and are therefore cooler. Scientists have found morethan 100 Jovian planets around other stars. The majority of the extrasolarJovian planets that have been discovered so far are closer to their stars thanthe Jovian planets in the solar system are to the sun.
The atmospheres of the Jovian planets in our solar system are made mostly ofhydrogen and helium. Compounds containing hydrogen, such as water, ammonia, andmethane, are also present. Differences in the amounts of these trace gases andvariations in the temperatures of these planets contribute to the differentcolors seen in images taken in visible light. While scientists expect theatmospheres of Jovian planets in other solar systems to be composed mainly ofhydrogen and helium, they have not yet measured the properties of theiratmospheres.
Pluto, the most remote planet in our solar system, might be little more than agiant comet. Pluto resembles the icy, comet-like objects orbiting the Sunoutside of Neptune”s orbit, rather than either the rocky terrestrial planets orthe Jovian planets. Factors that distinguish Pluto from the terrestrial andJovian planets include its composition (ice, rock, and frozen gases), changingatmosphere, small size, comparatively large moon, and its elliptical orbitaround the Sun.