Astronomy the Terrestrial Planets Are Defined As Essay

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The Terrestrial planets are defined as rocky planets or telluric planets, and they are Mercury, Venus, Earth and Mars. These planets have a lot of similarities that allow them to be grouped together, especially in contrast to the Jovian or gas planets. The telluric planet is one given type of planet, defined as one that is primarily composed of rocks and heavy metals. Thus, the composition of these planets is similar to each other, which makes for a proper comparison between them (Cessna, 2010).

With respect to composition, telluric planets contain a core, which is made up of molten iron. The core traps energy from the formation of the planet, and this molten core is then surrounded by silicate rock, a layer known as the mantle. There are surface layers of rock as well, and then usually a telluric planet will have an atmosphere. However, the atmosphere of some, like Mercury, is very thin. Mars also has a relatively thin atmosphere, while Venus and Earth have thicker atmospheres.

The size of telluric planets also differentiates them from the Jovian planets, which are much larger. The metal composition of telluric planets is very dense, and Earth especially has a high composition of iron. This emphasis on rock and metal, and the thick density, is characteristic of this type of planet. The Moon has some of these characteristics as well, as does Ceres, a dwarf planet in the asteroid belt between Mars and Jupiter.

Another feature of the telluric planets in our solar system is that they are closer to the Sun than the Jovian planets. The reclassification of Pluto actually made this a neat split between the four Terrestrial planets and the four Jovian ones to the inside and outside of the solar system. The first four planets are all Terrestrial, and the outer four are Jovian. There are dwarf planets beyond Neptune, including Pluto, Eris and Haumea, and these are a different category of celestial object (NASA, 2013)

Lastly, another feature of the Terrestrial planets in comparison to Jovian planets is that they have fewer moons. Earth has one moon, Mars has two moons, while Mercury and Venus do not have any moons. The Jovian planets have dozens of moon between them. NASA has identified 146 confirmed moons, and a further 27 provisional moons, and of these only the Moon, Phobos and Deimos are aligned with Terrestrial planets. This could perhaps indicate that the smaller, high-density telluric planets are not as able to capture moons into their orbit, or perhaps that this part of the solar system does not have as many possible moons to begin with (NASA, 2013).


The Jovian planets are unique from Terrestrial planets in several ways. The Jovian planets are the gas giants Jupiter, Saturn, Uranus and Neptune. While they have rocky cores in common with Terrestrial planets, the Jovian planets have a substantially different composition overall. The temperature at the middle of the planets is very high intensity, so not much has been learned about the cores of these planets. The rocky core of Jovian planets is believed to be molten nickel, rather than iron (Cessna, 2009).

Another distinctive features of the Jovian planets is that they are made up of gas, and are much larger than the Terrestrial planets, hence their designation as gas giants. Jupiter is known for having the greatest mass of all the planets. The rocky/metal core is surrounded by gas clouds that comprise most of the composition of the Jovian planets. The most common gases of these giants are helium and hydrogen, compared with Earth that has mostly oxygen, nitrogen and carbon dioxide in its atmosphere. So the Jovian planets are quite a bit different in their composition even in terms of the gases. This leads on Jupiter, for example, to extreme and intense storms. It is also noted that Neptune has a high percentage of methane and ammonia in its atmosphere, even among its peers.

Their larger size being one distinctive feature, the gas giants are also further in the solar system than are the Terrestrial planets. The four furthest are the giants. There is a fairly substantial gap between Jupiter, the nearest of the giants, and Mars, the furthest of the Terrestrial planets. Thus, these two groups occupy distinctive corners of the solar system, with the Jovian planets taking up a lot of space in the outer part of the solar system. Beyond them is considered to be the outer reaches of the solar system, where only a few dwarf planets orbit.
Another differentiating feature of the Jovian planets is that they have a lot of moons. Almost all of the moons in the solar system are found in the Jovian planets. Jupiter has 50 known moons and Saturn has 53 known moons. Uranus has 27 known moons while Neptune has 13 and all appear to have possible moons as well that still need to be confirmed. Some of these moons are very large. The Neptune moon Triton is the same size as Pluto, and Saturn's moon Titan has a thick atmosphere (NASA, 2013). Gannymede is the largest. Interestingly, the Moon is the fifth-largest moon. However, the common theme is that the Jovian planets tend to have many moons while there are only three moons for the four Terrestrial planets between them.


There are several other members of the solar system. Moons and dwarf planets have already been noted, but there are also asteroids, comets and meteors. Asteroids are pieces of rock that orbit around the sun. They are not planets in the sense that they do not have a molten core and none have an atmosphere. Many of the asteroids in the solar system occupy a space in between Mars and Jupiter known as the asteroid belt. There are others outside of this, but most asteroids are within the belt. Asteroids are basically hunks of rock and metal, so their small size and lack of planetary characteristics makes them distinct from planets. Further, the fact that they orbit the sun and not a planet means that they are not moons either (Choi, 2013).

Meteors are any type of space object or debris that falls into an atmosphere. In particular, when this occurs the object typically burns up as the result of friction in the atmosphere. Meteors are often pieces of asteroids, moons or just other random debris that create light when they fall into the atmosphere (IMO, 2013). Meteors are usually rock or metal but they no longer exist once they have burned up in the atmosphere. Very occasionally, a meteor will strike ground, and this creates a crater that is usually very large in relation to the remaining size of the meteor. Craters are more common on the Moon, where there is no atmosphere, because the meteor does not burn up.

Comets are another type of celestial body, and they are small balls of ice and solid matter. These are usually in orbit around the Sun, but their orbits have a different structure to that of planets. Comets can orbit at great distances from the Sun. As the comet nears the Sun, the heat from the solar radiation causes some of the ice to evaporate, so the comet has a distinctive tail of gases as it travels near to the Sun. Comets tend to orbit a star, but their composition is quite different from other celestial bodies because of the ice. There was concern not long ago that a comet might have melted as it passed by the Sun (Amos, 2013).

While asteroids and meteors are believed to have their origins in the formation of the solar system -- possibly created when other bodies are struck by meteors, launching debris into space -- comets are believed to have a different origin. They are thought to originate outside of the solar system. However, there is actually quite little known about the origins of comets. Many of the longer comets are thought to originate in the Oort Cloud, which lies between the solar system and the next star, while other comets are from within the solar system past Neptune.


The Drake Equation is an estimation of how many other civilizations might exist. The inputs to the Drake Equation are the rate of formation of stars suitable for the development of intelligent life, the fraction of those stars with planetary systems, the number of planets per system that can support life, the fraction of planets where life appears, the fraction of civilizations that develop technology and the length of time such civilizations release detectable signals into space. Put together, this equation will highlight the likelihood of our finding other intelligent life.

The Equation stimulates curiosity if nothing else, but it has also guided the search for answers to the specific inputs. As each individual input needs to have assumptions for it, scientists are pursuing ways to refine such assumptions, giving us a better sense of what the odds are for finding intelligent life......

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