Stars often have a number of objects orbiting around them in an adjacent region of space known as a solar system. Our solar system formed about five billion years ago and consists of the Sun, eight planets, at least thee dwarf planets, about 130 satellites, and a large number of comets and asteroids. A planet can be defined according to the following criteria: 1) It is a celestial body that orbits a star, 2) it has cleared the space along its orbital path of objects, 3) self-gravitational force has shaped its surface to be nearly spherical, and 4) it does not have the ability to generate its own light. The definition of a dwarf planet meets all the criteria of a planet, except that it has not cleared the space along its orbital path of objects. Satellites are bodies that orbit around planets and dwarf planets. The Earth’s moon is considered to be a satellite. An asteroid is a small, rocky, planet-like object that orbits the Sun. Several tens of thousands of these objects are found in a dense band between Mars and Jupiter. A comet is a small body of ice and dust that orbits the Sun. The orbit of comets tends to be very elongated and elliptical. When these objects get close to the Sun, the ice vaporizes producing a breathtaking glowing tail.
The orbits of most of our solar system’s planets are almost circular ellipses. Mercury and Pluto are the exceptions and their orbits tend to be more oval shaped. The orbits of the planets are also more or less at the same ecliptic plane. This ecliptic plane is tilted an average of about 7° from the plane of the Sun's equator. Pluto's orbit tilts with an inclination of 17°.
The Inner Solar System
Astronomers usually divide our solar system into two parts: the inner and outer solar system. The inner solar system contains the planets Mercury, Venus, Earth, and Mars (Figure 1). Of this group of planets, Mercury is the closest to the Sun (Figure 2). The distance of Mercury from the Sun varies between 46 and 70 million km or between 28.6 and 43.5 million mi (average about 57.9 million km or 36.0 million mi). Surface temperatures on Mercury vary from -200°C (-328°F) to 430°C (806°F). This is in part due to its closeness to the Sun and the fact that its rotation is relatively slow (about 59 days to complete one cycle). When turned away from the Sun, Mercury’s surface experiences a big drop in temperature because it does not receive sunlight for an extended period of time. No other planet in our solar system has as great a diurnal temperature variation and only Venus is hotter. Mercury has a very thin atmosphere composed mainly of sodium, potassium, and helium. Mercury’s surface is heavily cratered from meteorites. It is also very old and has no tectonic system to renew the surface crust.
Figure 1. The inner solar system consists of the Sun, Mercury, Venus, Earth, and Mars. It is separated from the outer solar system (Jupiter, Saturn, Uranus, Neptune, and Pluto) by a dense zone of asteroids found between the orbits of Mars and Jupiter. (Image Copyright: Michael Pidwirny).
Figure 2. Mariner 10 image of Mercury from 1974. Note how the surface of Mercury shows numerous craters and resembles Earth’s moon. The smooth vertical light toned band represent regions where no image information was recorded, (Image Source: NASA).
Venus is the second planet from the Sun with an average orbital distance of 108.2 million km or 67.2 million mi (Figure 3). The size and mass of this planet are very similar to those of Earth. However, the other characteristics of Venus are quite different from our planet. For example, Venus' rotation is very slow (243 Earth days) and the direction of movement when viewed from the North Pole is in a clockwise direction (Earth’s rotation is counterclockwise when viewed from the North Pole). The atmosphere of Venus is composed of mainly carbon dioxide and is about 90 times more dense then Earth’s air. The carbon dioxide rich atmosphere creates an extreme greenhouse effect that results in surface temperatures of around 467°C (873°F), which is hot enough to melt lead. Venus’ atmosphere also contains a number of cloud layers that are several kilometers thick and composed of sulfuric acid. Radar images from the Magellan satellite indicate that a wide variety of interesting and unique landform features exist on the surface of Venus. One mountain called Maxwell Montes reaches an altitude of 12 km or 7.5 mi. (For comparison, Mt. Everest is 8.8 km or 5.5 mi in elevation.) Planetary geologists speculate that the oldest terrains on Venus are only about 800 million years old. Extensive volcanism at that time is believed to have destroyed the earlier surface and left a number of relatively young volcanic landforms.
Figure 3. Radar image of the surface Venus from the 1990-1994 Magellan mission. Radar was able to penetrate the thick clouds that normally obscure the ground surface of this planet. The Magellan spacecraft imaged more than 98% of Venus at a resolution of about 100 meters (300 ft). (Image Source: NASA).
Our home, Earth, is the third planet from the Sun (Figure 4). What makes Earth so interesting as a planet is that it supports life. As far as we know, no other planet in our solar system has a biosphere. The development of a biosphere can only occur with the right mixture of planetary characteristics. For example, Earth’s atmosphere contains enough oxygen and carbon dioxide to allow for plant photosynthesis, and plant and animal cellular respiration. The particular orbital and rotational characteristics of the Earth help to maintain a range of surface temperatures that encourages the existence of organisms. Lastly, Earth’s unique size and mass cause a gravitation force that is not too strong or too weak for life. A higher gravitational force would crush living cells and would not allow organisms to develop vertical body shapes. A weaker gravitational force would have never caused important gases needed by life to accumulate in the atmosphere.
Figure 4. Earth is the fifth largest planet with a diameter of 12,756 kilometers (7,928 miles). It is also the only planet in our solar system that has a surface imostly covered by liquid water (some 70% of the surface). (Image Source: NASA).
The last planet in the inner solar system is Mars (Figure 5) with an average distance from Sun of 227.9 million km (141.6 million mi). Mars has especially fascinated scientists because observing it with Earth-based telescope revealed recognizable surface details. For many years, astronomers were convinced that these features could have only been produced by intelligent life. Better observing technologies in the second half of the 20th century suggested that these features were misidentified. Further, several space missions that landed scientific instruments on the surface of Mars in 1976 and in 1997 failed to find conclusive evidence for even microscopic life. However, in 1996, David McKay and his research colleagues announced in the prestigious journal Science the discovery of organic compounds in a meteorite of Martian origin (McKay et al., 1996). These scientists further theorized that these compounds, in combination with a number of other mineralogical features observed in the meteorite, maybe evidence of ancient Martian microorganisms!
Figure 5. Mars the fourth planet from the Sun. The Hubble Space Telescope captured these two dramatically different images of our planetary neighbor Mars. These images show how a global dust storm engulfed Mars with the onset of Martian spring in the Southern Hemisphere. When NASA's Hubble Space Telescope imaged Mars in June, two small storms were seen in the giant Hellas Basin and at the northern polar cap. When Hubble photographed the planet again in early September, the storms had already been raging across the planet for nearly two months obscuring all surface features with a thick layer of dust. This is the largest dust storm seen in several decades of observing Mars. (Image Source: Hubble Space Telescope).
During its orbit, Mars’ distance from the Sun varies by about 42.6 million km (26.5 million mi). This significant change in solar distance causes the Martian average surface temperature to vary by about 30°C (54°F) near the equator annually. In comparison, annual average surface temperatures along Earth’s equator vary at most by 5°C (9°F). Across the Martian globe surface temperatures range from a chilling -133°C (-207°F) at the winter pole under conditions of complete darkness to almost 27°C (81°F) near the equator during summer. Mars has a very thin atmosphere composed primarily of carbon dioxide (95.3%), nitrogen (2.7%), argon (1.6%), oxygen (0.15%), and traces of water (0.03%). The average atmospheric pressure on the surface of Mars is 0.7% of Earth's. Mars has a very interesting landscape. Satellite images have found evidence of water erosion in many places on Mars. Such surface features suggest that flowing water has occurred in the past. Mars is also home to the solar system’s tallest mountain. Volcanic Olympus Mons extends 27 km (16.8 mi) in elevation from its surrounding plain. Mars has two tiny moons that orbit very close to its surface.
The Outer Solar System
The outer solar system contains the dwarf planet Pluto and planets Jupiter, Saturn, Uranus, and Neptune (Figure 6). Jupiter, Saturn, Uranus, and Neptune are quite different from the planets that we have already discussed. All of these planets are quite large and their total mass accounts for more than 99% of the matter found in our solar system (excluding the Sun). Also, these four planets do not have solid surfaces. Instead, their masses are mainly composed of hydrogen and helium gas that becomes increasing dense as you travel from the edge of their atmosphere towards the planet’s interior.
Figure 6. The outer solar system consists of the dwarf planet Pluto and the planets Jupiter, Saturn, Uranus, and Neptune. (Image Copyright: Michael Pidwirny).
Jupiter is the largest planet in our solar system (Figure 7). This gas giant has a volume that is approximately 1000 times greater than the Earth (one-tenth the size of the Sun). Temperatures on the planet’s cloud surface are about -121°C (-186°F). Jupiter radiates more energy into space than it receives from the Sun because the interior of the planet is quite hot. Scientists estimate that the core has a temperature of about 20,000°C (36,000°F). This heat is generated by the slow gravitational compression of the planet. Most researchers believe that Jupiter may have a relatively small solid rocky core that has a mass of 10 to 15 Earths. Above this solid core is a layer of liquid metallic hydrogen (hydrogen with ionized protons and electrons). This unique form of hydrogen can only exist under conditions of extreme pressure and temperature. Such conditions occur only in the deep interiors of Jupiter and Saturn. On top of the liquid metal hydrogen layer is the outermost zone composed of a mixture of hydrogen and helium. At the deepest reaches of this zone, these two elements exist as liquids. Further up in this layer the hydrogen and helium become gaseous. The turbulent atmosphere of Jupiter that we see from space represents the very top of this layer.
Figure 7. Voyager 1 took this photo of the gas giant Jupiter and its moon Io on February 1, 1979, at a distance of about 32 million km (20 million mi). In this image, the atmospheric storm known as the Great Red Spot can be seen just below the center of the planet. (Image Source: NASA).
The Galileo space probe measured the winds moving in Jupiter’s upper atmosphere at speeds faster than 600 kph (373 mph). Most Earth-based tornadoes have wind speeds less than 175 kph (109 mph). The Great Red Spot is a hurricane like storm that has been raging in Jupiter’s upper atmosphere for more than 400 years. Of the planets in our solar system, Jupiter also has the most rapid rotation. This enormous planet completes one rotation in about 10 hours. Orbiting Jupiter are 63 known satellites, including the four large Galilean moons that are easily visible from a hobby telescope.
Saturn with its rings is considered by many to be one of the most amazing sights in our solar system (Figure 8). From our most powerful telescopes on Earth we can see seven major rings (Jupiter, Uranus, and Neptune also have rings but they are dark in color and difficult to observe). Close-up images from the Voyager 1 and 2 space probes indicated that the rings of Saturn are composed of ice and rock and organized into thousands of ringlets. The average surface temperature of Saturn's is estimated to be around -125°C (-193°F). Like Jupiter, Saturn is a huge planet composed mainly of hydrogen and helium. Saturn's interior is not as hot as Jupiter’s and is estimated to be approximately 12,000°C (21,600°F). As a result of this internal heat energy, Saturn radiates more energy back to space than it receives from the Sun. The structure of Saturn's interior is also similar to Jupiter's consisting of a rocky core, a liquid metallic hydrogen layer, and a gaseous molecular hydrogen outer layer.
Figure 8. Saturn in October 1998. Saturn's equator is tilted relative to its orbit by 27°, very similar to the 23.5° tilt of the Earth. This image shows Saturn during its winter solstice when the North Pole is tilted away from the Sun. (Image Source: Hubble Space Telescope).
Saturn has more than 30 orbiting satellites. Its largest moon, Titan has an atmosphere composed of mainly nitrogen, 6% argon, and a few percent of methane. These atmospheric conditions are very similar to those of ancient Earth when life was first getting started. Consequently, scientists speculate that simple forms of life may exist on Titan. No other satellite in our solar system has an atmosphere.
Uranus is the seventh planet from the Sun with an average orbital distance of 2872 million km or 1785 million mi (Figure 9). Because of this great distance from the solar source of light energy, Uranus is a very cold planet with an average temperature of about -193° C (-315°F). Most of the other planets in our solar system spin on an axis nearly perpendicular to the plane of the ecliptic. However, Uranus' rotational axis is almost parallel to the orbital plane around the Sun. Consequently, each of the planet’s seasons lasts about 20 years (84 years to complete one revolution around the Sun). Uranus is composed primarily of rock, about 15% hydrogen, a little helium, frozen water, methane, and ammonia. Unlike Jupiter and Saturn, the rocky material is not concentrated in the core of the planet but approximately evenly spread out in its mass. The atmosphere of Uranus is estimated to be about 83% hydrogen, 15% helium, and 2% methane. Over twenty moons are known to orbit Uranus.
Figure 9. These two pictures of Uranus, one in true color (left) and the other in false color, were taken by Voyager 2 on January 17, 1986. The false-color image of Uranus reveals a dark polar zone surrounded by a series of progressively lighter concentric bands. One possible explanation for this pattern is that a brownish haze or smog, concentrated over the pole, is arranged into bands by the zonal flow of the upper air winds. (Image Source: NASA).
The eighth planet from the Sun is called Neptune (Figure 10). Neptune has only been visited by one space probe, Voyager 2 on August 25, 1989. Much of our knowledge about this planet comes from this encounter and observations by the Hubble Space telescope. The last of the gaseous planets, Neptune is probably composed of materials similar to Uranus. Its atmosphere is mainly hydrogen and helium with a small percentage of methane. At the surface of the planet, the atmosphere has a temperature ranging from -153° to -193°C (-346° to -391°F). Neptune's atmospheric winds are the fastest in the solar system, reaching speeds as high as 2000 kph (1240 mph). Several disturbances have been observed in Neptune’s active atmosphere. The blue color of the planet is largely caused by the absorption of red light by methane in the atmosphere. Neptune has 13 known satellites.
Figure 10. This image of Neptune was taken by Voyager 2. In the center of the image, we can see Neptune’s Great Dark Spot and its companion bright smudge. (Image Source: NASA).
Not much is known about the dwarf planet Pluto (Figure 11). Because of its great distance from the Sun, it has not been explored by any remote space probes. We do know that temperatures on Pluto’s surface vary from ?-235°C (-391°F) to -210°C (-346°F). Our best views of the planet come from the Hubble Space Telescope. With this instrument, scientists have been able to measure Pluto’s size (and the size of its large moon Charon). Pluto has a highly eccentric orbit that passes inside of Neptune’s orbital path for about 20 of the 248 years it takes to complete one revolution around the Sun.
Figure 11. This Hubble Space Telescope image provided the sharpest view to date of the dwarf planet Pluto and its moon, Charon. This image also allowed astronomers to more accurately measure (to within about 1% error) the diameter of Pluto (2320 km - 1258 mi) and Charon (1270 km - 686 mi). (Image Source: Hubble Space Telescope).
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