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A planet is an object which orbits around a star and whi h does not produce its own energy; instead it is illuminated by reflected sunlight. The Earth is one of nine planets held in orbit around the Sun by the Suns gravity. Because they are relatively close, many of the planets in our solar system appear brighter than the brightest stars, but they do not twinkle like a star. Our turbulent atmosphere causes the pinpoint of light from a star to shimmer, but planets appear as a tiny disk and are not as affected by the atmosphere. Planets are much smaller than the stars. In fact, the Sun composes 99.9% of the mass in the solar system and dictates the motions of the planets.
The planets have been placed in two main categories: solid and gaseous. Solid planets are relatively small and are composed of dense rock, often rich in iron. Gaseous planets are larger and have thick outer layers of hydrogen gas with small rocky cores. In our solar system, the four inner solid planets are known as terrestrial planets because of their resemblance to the Earth. Mercury, Venus, the Earth and Mars are situated closest to the Sun and therefore, receive the most energy. They have solid surfaces with thin atmospheres and have high densities. Much further from the Sun lie the gaseous planets in the solar system. Jupiter, Saturn, Uranus and Neptune are labelled the Jovian planets, and are much larger than the terrestrial planets. Their interiors contain a small rocky core surrounded by liquid hydrogen, and have strong magnetic fields and rapid rotation rates. It would be impossible to stand on a Jovian planet as they have no solid surface; the surface we see are gases covered by a thin upper layer of hazy clouds. The huge mass of these gaseous giants also creates a large gravitational field which has created ring systems around all four planets (Saturns are the most well known) and numerous captured moons. A planets moons are known as satellites, and are pieces of rock pulled in by the planets gravity. The solar system has 98 known satellites including our Moon, and 94 of these are in orbit around the four Jovian planets. Pluto is the furthest planet from the Sun and does not adequately fit into either category of planet as it is a small mass of ice. It is thought that Pluto may be a captured comet, and scientists debate whether or not its status as a planet is justifiable due to its small size and irregular characteristics.
The planets are also labelled according to their position relative to the Earth. The two planets closer to the Sun than the Earth are known as inferior planets, and are visible in the sky only during dusk or dawn (or during the day with a telescope) due to their close proximity to the Sun. Like the Moon they go through phases as they orbit the Sun, although we cannot see a fully lit disk because it is then that they are on the opposite side of the Sun from us. Inferior planets are most easily visible when they are furthest from the Sun in our sky, at maximum or minimum elongation. The six planets further from the Sun than the Earth are known as superior planets, and because they can be on the opposite side of the sky from the Sun, can be in the sky at any time of night. The best observing times for superior planets is during opposition, when they are nearest to the Earth and appear high in the night sky.Load Flash Applet
The closest planet to the Sun is Mercury, a small and barren planet. Mercury is the second smallest planet and appears much like our Moon: grey and pocketed with thousands of impact craters. There is no appreciable atmosphere on Mercury to protect it from small meteorites, and the absence of an atmosphere also affects the surface temperature. The face of Mercury nearest the Sun gets direct and intense sunlight, and as a result temperatures soar to 700K. But without an atmosphere to retain and disperse heat, the temperature drops to 100K in the absence of sunlight, creating the greatest temperature range of any planet. Mercurys proximity to the Sun causes it to race around its orbit in a mere 88 days, the fastest orbital period of any planet. The rotational period of Mercury is nearly 59 days, but the rapid orbital period causes a day on Mercury to last 176 Earth days. From the Earth, Mercury is difficult to observe for a variety of reasons. It is small and its surface reflects less light than any other planet, but it is also an inferior planet and is always very close to the Sun in our sky. Mercury is never in the sky for more than two hours before sunrise or after sunset, and always appears to be somewhat lost in the Suns radiance. A telescope will reveal only a hazy disk with very few surface features. The first detailed images of Mercurys surface came in 1974, when the spacecraft Mariner 10 traveled to the planet. It is the only probe to visit Mercury, and because it could not take images of the dark side we still do not have an image of the entire surface.
Venus is the brightest object in the sky other than the Sun and Moon, and near maximum or minimum elongation will shine in the dusk or dawn sky with a magnitude of about -4. Venus is often called our sister planet, because it is the closest planet to the Earth and its diameter and density are very similar to the Earths. Venus is, however, a planet unlike ours in many respects. Venus has a dense atmosphere of carbon dioxide which covers the entire planet with thick clouds. The clouds create a greenhouse effect, trapping heat from the Sun and raising the temperature to 750K, the hottest planetary surface in the solar system. The surface of Venus is extremely hostile and cannot support life for a variety of reasons. The thick clouds block out sunlight and contain droplets of concentrated sulphuric acid. The atmosphere is so dense and heavy that the pressure at the surface is 90 times that on Earth.
Through a telescope, Venus is featureless but brilliant because the atmosphere reflects more sunlight than any other planet. Surface features on Venus are not visible because the thick clouds obstruct our view, but equipped with radar, spacecraft in orbit around Venus successfully probed the surface between 1983 and 1995 and gave us a detailed topographical map of nearly the entire planet. Most of Venus is a huge and remarkably flat plain, while the remainder of the surface is raised with gently rolling hills and large volcanoes and lava domes. The volcanoes may still be active, contributing to the acidic content in the atmosphere. Several spacecraft from the former Soviet Union have landed on the surface of Venus, revealing a surface composition much like the basalt lava rocks on the Earth and Moon. These probes only operated for a short time, the first for a few seconds, before they succumbed to the harsh conditions on the surface. Radar observations have revealed an extremely long, retrograde rotational period. It takes Venus almost 225 days to complete one orbit of the Sun, but one revolution on its own axis is backwards and takes 243 days. The slow rotation is also believed to be the cause for the absence of a magnetic field surrounding the planet.
The Earth is our home, and is the only planet in the solar system with the exact conditions required to support life. The Earth was formed about 4.6 billion years ago along with the rest of the solar system, but since its beginnings the Earth has been a unique planet. About 71% of the Earths surface is covered by water, and it is the only planet in the solar system with water in liquid form; the amount of energy we receive from the Sun generates a climate ideal for life. We experience such diverse weather patterns on Earth because of our atmosphere and the constant circulation of air due to the Earths rotation. Our atmosphere is unlike any other planets, and has played a role in the Earths ability to sustain life. The air we breathe is rich in nitrogen (77%) and oxygen (21%), unlike the toxic carbon dioxide found on Venus and Mars. Our upper atmosphere blocks harmful radiation from the Sun while still allowing heat to escape, and the weight of the air above us is not heavy enough to crush us as the atmosphere of Venus would. The Earth is dynamic and has been under constant change since its birth; the thick crust of our planet is constantly shifting (plate tectonics), causing Earthquakes and volcanoes to continually reshape the surface. The biological diversity on our planet is incredible, and makes the Earth a wonderful place to live.
For a more detailed explanations of the Earth, including a look at: seasons, northern lights, phases of the moon, eclipses, tides and the moon itself, please see Module 3: The Earth and the Moon.
Mars is the last inner planet but is the closest of the superior planets to the Earth. It shines deep red in our sky and has captured the imagination of the human mind for many years. Mars has several characteristics similar to the Earth, a fact which led to early beliefs that the planet was home to alien life. Through large telescopes, Mars exhibits linear features which were speculated to be irrigation systems build by Martians, but we now know these features are natural landforms. The topography of Mars includes huge canyons, extinct volcanoes, craters and polar ice caps composed of ice and frozen carbon dioxide. The largest volcano on Mars is Olympus Mons, the largest mountain in the solar system with an altitude of 25 kilometres above the surrounding terrain, and a base of about 700 kilometres across. The Earths largest mountain, Mt. Everest, has an altitude of just under nine kilometres above sea level. The surface features of Mars are occasionally blanketed in huge dust storms which can hide vast regions from our view. The interior of Mars is believed to be composed of a small iron core, but the planet does not exhibit a magnetic field. Mars is the only interior planet other than the Earth to have satellites. Its two satellites, Phobos and Deimos, are only several kilometres across and appear to be captured asteroids. Mars appears as a small red disk through telescopes, and features on the surface -- especially the white ice caps -- are best viewed every two years during opposition, when it is at its closest to the Earth.
Mars has an average surface temperature of about -40 degrees Celsius (230K), and because Mars is tilted like the Earth it also experiences climactic and seasonal changes. There are large amounts of frozen water on the surface in the form of a layer of permafrost as well as polar ice caps which enlarge and shrink with the seasons. The Martian atmosphere is extremely thin and is composed primarily of carbon dioxide, contributing to the belief that life forms needing oxygen to survive cannot currently exist on Mars. There is no flowing water on the surface of Mars, but canyons and dried up river beds with tear drop shaped features suggest that water once flowed in abundance. This evidence raises questions about the possibility of earlier forms of life as well as the difficulty in explaining where the water disappeared to. One possibility is that there may be liquid water under the surface, where the temperature is warm enough for water to be in liquid form.
Because of the possibility of past life on Mars, it has been studied extensively by telescopes and spacecraft which have orbited the planet and landed on its surface. The Viking probes of the mid 1970s showed images of a desolate red surface covered in boulders underneath a pink sky and measured detailed information about the atmosphere. Much of our knowledge of the red planet has come from spacecraft, and future probes to the planet may be able to answer more questions about its history and present. Many scientists still believe that Mars harboured life forms in the past and that some organisms may presently be able to survive under the surface of Mars, where temperatures may be warmer and water is contained in the permafrost. Unfortunately, we still have more questions than answers.
The first of the gaseous giants is the largest planet in our solar system, Jupiter. Jupiter is over 11 times the diameter of the Earth and has a mass 2.5 times that of all the other planets combined. This giant is composed almost entirely of hydrogen (82%) and helium gas (17%). There is no solid surface on Jupiter; instead the surface consists of a dense atmosphere topped with a layer of colourful clouds about 100 kilometres thick. The clouds are made up of bands running parallel to the equator, ranging from white to dark reddish brown, their colour arising from chemical reactions within the clouds. The bands are in constant motion, and create an active surface of red belts and light-coloured zones which rapidly rotate around the planet. Jupiter experiences differential rotation, the bands of cloud rotating at various speeds and in different directions. The rotational rate of the equatorial region is just under ten hours, extremely fast for a planet as large as Jupiter. As a result, Jupiter is noticeably oblate, its equatorial diameter being 6.5% greater than its polar diameter. Two bands traveling in opposite directions create intricate eddies along their border which can become whirlwind storms. Storms are constantly forming and dissipating, but the Great Red Spot has been a huge storm visible for hundreds of years. It is about three times the size of the Earth and is the most recognizable feature on Jupiter, visible even in amateur telescopes.
The interior of Jupiter is most likely composed of a small metallic core surrounded by liquid hydrogen. Currents within the layer of liquid hydrogen cause a strong magnetic field which shields Jupiter from the solar wind and focuses the particles to the poles, causing aurora similar to the aurora on the Earth. The interior of Jupiter is still cooling since its formation and radiates thermal heat. Jupiters core is most likely around 25000K and is slowly cooling, compared to only about 110K at the surface. Had Jupiter been more massive during its formation, it could have become a sister star to the Sun. Four spacecraft have visited Jupiter, two fly-bys by Pioneer in 1974 and two by Voyager in 1979. The probes obtained high resolution images of Jupiter and gave scientists valuable information about the planet and its intricate atmosphere of clouds. One discovery was that Jupiter has a thin and delicate ring structure which is not visible from the Earth. The probes also detected many satellites, and we now know of 28 moons in orbit around Jupiter. Many of the satellites are very small and resemble asteroids, but four of the moons are among the largest in the solar system and were discovered by Galileo Galilei in 1610.
The Galilean satellites had never been seen in any detail before the Voyager spacecraft passed by them in 1979, increasing our knowledge of the four moons tremendously. Io, Europa, Ganymede and Callisto are approximately the same size as our Moon (Europa being smaller) and are easily seen from the Earth with binoculars or a telescope as star-like points of light along Jupiters equatorial plane. Their alignment constantly changes on a nightly basis as the moons rapidly circle around Jupiter and dance from one side to the other in a matter of hours. Io is closest to Jupiter and is the most volcanically active body in the solar system with numerous vents ejecting molten sulphur. The thin atmosphere and low gravity of the moon cannot contain the material and it can erupt hundreds of kilometres above the surface. The gravity of Jupiter and the three outer Galilean satellites pull Io in opposite directions and exert tidal forces which heat its interior, generating pressure which is released by the eruption of the internal molten materials. Europa is covered with a smooth tan coloured surface without mountains or craters, suggesting a youthful surface with recent activity. The surface appears to be a layer of frozen water with numerous cracks. Ganymede is larger than Mercury and is the largest satellite in the solar system. Ganymede and Callisto are both dark in colour, icy, and heavily cratered, Callisto being one of the most heavily cratered objects in the solar system.
The second largest planet and the second gaseous planet is Saturn, best known for its beautiful ring system. Saturns rings are composed of millions of highly reflective pieces of ice and rock measuring between a millimetre and several metres across. The rings are less than a hundred metres thick but extend hundreds of thousands of kilometres from Saturns surface. Earth-based telescopes clearly show the rings, but it was not until Voyager 2 passed by that scientists learned the rings were composed of thousands of small and narrow rings, known as ringlets. The ring system extends from the cloud tops of Saturns atmosphere to hundreds of thousands of kilometres beyond the planet, but only two broad portions of the rings are bright enough to be clearly visible from the Earth. The rings are tilted at an angle and from the Earth will appear to vary throughout the planets 30 year orbital period. When tilted at their greatest, the rings are bright and very conspicuous, but every 15 to 17 years the rings turn on edge and are invisible to us. It is believed that the rings were formed when a large rocky object or comet came too close to Saturn, and its gravity broke the object apart into millions of pieces. When an object approaches too close to a large planet, tidal forces exert a higher gravitational pull to the side of the object facing the larger body. This irregularity in the force of gravity causes the object to stretch and as it approaches the planet, it will eventually break apart.
Saturn is composed mostly of hydrogen and helium gas (88% and 11% respectively), and appears pale yellow in colour. Saturn has few atmospheric features because the temperature is too low to induce the chemical reactions that produce the colours on Jupiter, and is also covered in a thin layer of haze which obstructs our view of any detail. Like Jupiter, Saturn has a rapid rate of rotation which contributes to the formation of severe storms in its cloud layer, with wind speeds of up to 1400 kilometres per hour. The atmosphere that forms the surface of Saturn is not as heavy as Jupiters, however, Saturn having the lowest density of all the planets -- so low, in fact, that Saturn could float in water! It is because of its low density that a person would weigh less on the surface of Saturn than on Earth, despite its high mass. Because of these features, Saturn appears even more oblate than Jupiter, being about 11% wider at its equator. Despite its low density, Saturn is still quite massive, and in addition to its ring system it has collected a vast number of satellites. A few of Saturns 30 known moons orbit within the rings. The largest is Titan, which has the thickest atmosphere of any moon in the solar system, covering its surface with a yellowish-orange layer of haze.
The solar system was thought to end at Saturn until Uranus was discovered with a telescope in 1781. Uranus is barely visible to the naked eye under ideal conditions, and although plotted on several earlier star maps it was mistaken as a star. Uranus is about four times the size of the Earth and has a large mass, but also has a low density. It is composed mainly of hydrogen and helium gas with an upper atmosphere containing hazy clouds of frozen water and methane, giving the planet a greenish-blue colour. The observations of Voyager 2 in 1986 did not reveal any surface features or cloud formations, showing Uranus as a plain sphere of gas.
Uranus is very unique in that its axis is tilted 98 degrees from vertical, meaning it rotates on its side. The orbital period of Uranus is nearly 84 years, meaning that although the planet rotates on its own axis in 17 hours, it is almost 84 years between sunrises at the poles. Uranus also has nine thin rings around it, which were discovered in 1977 from the Earth. They are not visible from the Earth, but caused a background star to flicker as the rings passed in front of it, allowing astronomers to detect their presence. There are 27 known satellites in orbit around Uranus, the largest being Titania. The large moons have interesting landforms and show evidence of geological formation in the past, as well as lava flows which have partially covered numerous impact craters.
The last of the Jovian planets is Neptune, which was discovered in 1846 after mathematical calculations predicted its presence based on its gravitational effect on Uranus. Neptune is in many ways very similar to Uranus; the two planets have nearly the same size and mass, and both appear greenish-blue because of the methane in their upper atmospheres, although Neptune is a deeper blue. Neither Uranus nor Neptune can be viewed in any detail from the Earth, so our knowledge of these outer gaseous planets is limited. Much of the information we have was also gathered from the spacecraft Voyager 2. The clouds on Neptune had slight patterns of blue bands when Voyager passed the planet in 1989, and like all gaseous planets, Neptune experiences a high wind speed in its upper atmosphere. Voyager photographed a large storm on Neptune much like the Great Red Spot on Jupiter, which has been called the Great Dark Spot and may still be an active storm. Neptune has a clumpy and thin ring system of a few rings, discovered by the Voyager probe and undetectable from Earth. Neptune has 15 known satellites, Triton being one of the largest in the solar system. Triton has a small atmosphere of nitrogen and is geologically active with volcanoes on its surface.
Ancient civilizations knew of six planets that traveled among the stars in the sky. They took detailed observations of their motions and knew a great deal about them. They realized that the planets did not generate their own light, but simply reflected sunlight. The distances and masses of the planets were accurately calculated using Newtonian physics, and many of the physical properties of the planets could be calculated mathematically. It wasnt until the invention of the telescope, however, that details of the planets appearances became known. The telescope allowed astronomers to discover the two planetary types: terrestrial and Jovian. Three new planets were discovered and with the discovery of the Galilean satellites of Jupiter, it was revealed that other planets had moons like the Earth. As technology was enhanced, telescopes became more powerful and we learned more about the solar system. We accurately learned the characteristics of our neighbours in space with the advent of exploratory spacecraft, which allowed us to obtain detailed images of planetary surfaces and studies of soil and atmosphere on Venus and Mars.
Mercury is the closest planet to the Sun, and although it has the greatest temperature range of any planet, it is not the hottest. Venus, the brightest planet in our sky, has a thick atmosphere which traps radiation from the Sun and causes the surface temperature to raise high enough to melt lead. Venus also rotates backwards and has the slowest rotation rate of any planet. The first superior planet is Mars, whose red surface covered in subtle landforms, was long thought to harbour life forms. With the landing of probes on the surface, we realize that this red planet does not support intelligent life, but there is the possibility of bacterial organisms in the layer of permafrost under the surface. Jupiter is the largest planet, a huge sphere of gas that has a surface of active and colourful bands of cloud. There are many storms on Jupiter, including the Great Red Spot, a massive storm that has been raging for years. The first satellites to be discovered in orbit around another planet were the four Galilean satellites around Jupiter, discovered by Galileo in 1610. Also discovered by Galileo and his telescope (although not understood) were the rings of Saturn. The extensive arrangement of the thousands of ringlets around Saturn are an incredible sight in any telescope. Although only about a hundred metres thick, two of the broad rings can be detected easily from the Earth. In addition to its rings, Saturn currently has the most satellites of any planet with 30, the largest being the clouded Titan. Uranus and Neptune both appear greenish blue, and are very similar in size and composition. The furthest planet from the Sun, Pluto, is a small and cold world. It has a very irregular orbit, and could be a dead comet or an escaped moon from one of the Jovian planets. The planets are the largest bodies in orbit around the Sun, but there are millions of smaller objects which are an essential component of the solar system as well.