The concept of planet has been with us since the age of the Greeks. Even without telescopes, the ancient civilizations noticed some stars that seemed to move in the sky as opposed to stars that remained fixed in their position year after year. The ancients called them planētēs, “wanderers”. They named these planets after their gods, which seemed to befit their place amongst the stars. The fastest moving one was named Mercury after the god who has winged feet. The shining evening (or morning) star was named after the god of beauty Venus. The red planet was named after Mars, a planet deserving to be named after the god of war. The king of the gods, Jupiter had his own planet as well as well as the god of time Saturn. Aside from these “planets of antiquity”, William Herschel found the blue-green Uranus. Irregularities in the orbit of Uranus led astronomers to find Neptune and finally Clyde Tombaugh became the only American to discover a planet with Pluto.
Last year, astronomers made headlines across the world with their redefinition of the word planet. With better science and technology, we were able to discover hundreds if not thousands of other wanderers revolving around our Sun, some larger than Pluto some only a few kilometers across. These minor bodies could hardly fit in the same league with Zeus or Kronos hence the creation of a new definition which sadly did not include Pluto. To be a planet, a body has to satisfy three conditions. First, it should revolve around the sun. Secondly, it should be large enough such that its gravity pulls it into a round shape. Lastly, it should be large enough to be able to clean out its surroundings by pulling in other material from its orbit leaving the planet as the last celestial body standing. It was the third condition that Pluto failed leaving us with only eight planets in the Solar System, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Mercury is the smallest planet in the solar system and is also closest to the sun at a distance of only 57 million kilometers. The planet goes through three Mercurial days for every two of its 88 Earth-day years. Mercury has no moons and no ring system. It only has 5% of the Earth’s mass and its radius is only 38% as large as Earth’s. Like the Earth, Mercury is a rocky planet with a rocky core that extends up to 75% of the Planet’s radius. Its proximity to the Sun means temperatures as high as 480°C on its sunward face. Its lack of an atmosphere to retain heat means these high temperatures drop to -170°C at night. These temperatures would make the existence of liquid water on its surface impossible though astronomers are speculating on the existence of water ice inside craters at the poles of the planet. The water could be brought by impacting comets and meteors. The lack of an atmosphere has also left its surface filled with craters as revealed by the pictures sent back to Earth by the space probe Mariner X. All in all, the craters, the extreme temperature ranges would make Mercury more similar to the Earth’s moon than Earth itself.
Venus on the other hand is more alike to Earth; the body is most similar to Earth than any other planet on the solar system. Its mass and volume compared to Earth are 81.5% and 85% respectively. However, the similarities end there. A Venus day is 117 earth days long, a Venus year 225 earth days long. Like Earth, Venus has an extremely thick atmosphere that prevents astronomers from seeing the surface of the planet. Scientists have resorted to using radar to penetrate the thick clouds and map the planet’s surface which revealed sand dunes and volcanoes. Venus’ atmosphere is mostly composed of carbon dioxide with traces of sulfuric acid. The high CO2 concentration makes the planet one large greenhouse. The CO2 traps the heat of the sun inside the planet resulting in an average surface temperature of 464°C. The high temperature and highly corrosive atmosphere has resulted in the deaths of all probes that landed on the planet within a few hours from the start of their mission.
Mars is the last of the rocky planets. Even though Mars is much smaller than Earth (10% Earth’s mass, 15% Earth’s volume) it is also the most likely candidate for a second place for life to be found in the Solar System. Percival Lowell’s “discovery” of canal networks on its surface created the notion of “Martians” with their “Martian civilization”. These canals turned out to be large canyons on the Martian surface with the largest – Valles Marineris – stretching a distance equivalent to the distance between New York and Los Angeles. The red hue of the Martian surface is due to the high iron concentration of its surface material. Like Earth, Mars has an active weather system, ice caps on its poles and an active tectonic plate activity. While the idea of Martian invaders may be the stuff of science fiction, the question whether Mars has once harbored life is still one of great scientific interest. A central question is determining whether water, an important precursor to life, has existed in liquid form on Mars’ surface.
Jupiter, the largest of the planets could fit 1,321 earths in its interior. With a mass of 317 earths, Jupiter is a gas giant, a swirling massive ball of gas floating in an orbit around the sun. Even a small telescope would reveal cloud formations on its surface – betraying the complex atmospheric processes at work in the Jovian realm. It has less in common with the Earth and more in common with the Sun. Like the sun, it is composed mainly of hydrogen and helium with traces of ammonia and water. Like the sun, it has a mini solar system orbiting it – Galileo discovered 4 planet sized moons of Jupiter. The most recent count places the number of Jupiter satellites at 63. Jupiter is thought to compose of an atmosphere (where the belts, swirls and the great red spot reside), an inner region where the pressure is so intense that hydrogen takes on a liquid form, and further in is a layer where the pressure is enough to turn hydrogen into its metallic, electrically conducting form.
While all the planets from Jupiter to Neptune have a ring system of some sort, none of them could match the magnificence of Saturn’s ring system. Extending thousands of kilometers from the surface, the rings of Saturn are composed of tiny particles of water ice with some rocky particles covered in ice. The ring is also quite thin with a thickness of less than one kilometer. The planet Saturn on the other hand has a diameter of 120,536 kilometers. While massive at 95 earth masses, Saturn is the least dense planet in the solar system, having a density less than that of water. If a bathtub large enough existed, Saturn could float on the water. Saturn is composed mainly of hydrogen and helium. While its surface is not as active as that of Jupiter, it is still active enough to produce cloud formations and storms. Auroras have also been seen at the poles of Saturn. There are currently 56 known satellites of Saturn. Like Jupiter, Saturn also has a strong magnetic field.
The next two planets are also quite alike. Of the two, Neptune is more massive than Uranus having 17 Earth masses to Uranus’ 14. Uranus though is much larger having 63 times the Earth’s volume compared to Neptune’s 57. Both planets’ atmospheres are composed mainly of hydrogen and helium. Traces of methane gas gives the distinct blue green color to Uranus and the deep blue tinge of Neptune. It is currently unknown why Neptune is of a deeper hue than Uranus. Both planets are also thought to have a homogenous structure, devoid of a kind of layering seen on Jupiter. Uranus’ surface is relatively featureless showing only a monotonic blue green color. An oddity of Uranus is the nearly horizontal tilt of its axis of rotation. Neptune however is a different story. Images from Voyager showed cloud formations and storms on Neptune with a “Great Dark Spot” similar to Jupiter’s Great Red Spot. Both planets have ring systems and have several natural satellites with Uranus’ 27 outnumbering Neptune’s 13.
We could see the variety and similarity of the different members of the solar system. Studying the siblings of the Earth gives us a picture of other possible Earth’s in our solar system. As astronomers keep on finding more and more planets both in and out of the solar system, we wonder how different or how similar to our Earth these new planets are. One thing though is clear; there is still a lot of knowledge to learn about our Universe.
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National Aeronautics and Space Administration. (n.d.). Planets. In NASA's Solar System Exploration. Retrieved June 27, 2007
Williams, D.. (n.d.). Planetary Fact Sheet - Metric. In NASA National Space Science Data Center. Retrieved June 27, 2007