In ancient times, sky watchers observed that there were five special points of light in the night sky. All the other points of light always kept their same positions relative to one another. These were the stars. The constellations they formed remained the same year after year, lifetime after lifetime. The five points of light were different. They slowly moved among the stars. The ancient Greeks called these points of light the planetai, a word that means "wandering stars." And the Greeks named them after their gods: Hermes, Aphrodite, Ares, Zeus, and Chronos.
Today we know that these points of light, which we call planets, wander slowly in the sky because they are other "worlds" traveling around our sun. Instead of using the names of the Greek gods, we call these planets by the names of the equivalent Roman gods: Mercury, Venus, Mars, Jupiter, and Saturn.
Our own Earth is also a planet. If you could observe Earth from outer space, you would see that it, too, seems always to be moving among the stars. Planets give off no light of their own. They shine brightly in the night sky because they reflect the sun's light.
What Is a Planet?
Until recently, defining a planet was simple. A planet was any large, rounded object that revolved around a star, such as our sun, in a path called an orbit. However, as astronomers have learned more about our solar system and all the different kinds of objects it contains, this simple definition has proved insufficient.
Today a celestial object must have three characteristics to be considered a planet: It must orbit the sun; it must have enough mass for its own gravity to shape it into a sphere; and it must be massive enough to clear other large objects from its orbit.
Our solar system contains eight planets. These include the Earth, the five known since ancient times, plus Uranus and Neptune. Pluto, discovered in 1930, had been called a planet until 2006. Astronomers then decided it was a dwarf planet, not a true planet. In 2008 Pluto became the first dwarf planet to be called a plutoid. For more information, see the articles Planets, Dwarf and Pluto.
Some of the planets in our solar system have satellites, or moons, that revolve around them. Much of what we now know about the planets and their moons has come from space probes. These probes have orbited, flown by, or even landed on many of these planets and transmitted valuable information back to astronomers on Earth.
In this article, you will travel outward from the sun and visit the planets one at a time. Your first stop is Mercury.
Mercury
Mercury is the closest planet to the sun. Its average distance from the sun is about 35.9 million miles (57.8 million kilometers). The orbits of all the planets are ellipses, or flattened circles. But Mercury's orbit is particularly flattened. As a result, at various points in its orbit Mercury can be as close as 29 million miles (46.7 million kilometers) from the sun or as far away as about 44 million miles (70.8 million kilometers). It takes 88 Earth days for Mercury to complete one trip around the sun. In other words, one Earth year of 365 days is longer than four Mercury years. Mercury rotates on its axis once every 59 Earth days. Its diameter is 3,032 miles (4,880 kilometers).
Mercury appears tiny and almost without features, even when observed with powerful telescopes. Little was known about this planet until Mariner 10 flew past it in 1973 and sent back hundreds of images. These showed thousands of craters, long cliffs called scarps, and a giant impact basin about 800 miles (1,288 kilometers) across.
An extremely thin veil of helium gas has been detected around Mercury. These atoms, which are actually given off by the sun, flow around the planet as it orbits the sun. A very thin atmosphere of oxygen, sodium, and hydrogen gases, which probably escape from Mercury's surface, has also been detected. The magnetic field that surrounds Mercury is much weaker than the Earth's magnetic field. Its gravitational pull is also much weaker than that of the Earth.
Because Mercury is so close to the sun, temperatures can reach about 800°F (427°C) during the day. However, Mercury's thin atmosphere retains little of this heat, so during its long nights the temperature can plunge to −280°F (−171°C). Mercury has no known satellites. For more information, see the article Mercury.
Venus
Your next stop is Venus. This planet travels around the sun in a nearly circular orbit, at an average distance of about 67.2 million miles (108.2 million kilometers). Venus takes 224.7 Earth days to make one trip around the sun. It is nearly the same size as the Earth, and its diameter is about 7,519 miles (12,100 kilometers). Its gravitational pull is almost the same as that of the Earth.
Venus comes closer to the Earth than any other planet. It is only about 26 million miles (41.9 million kilometers) away. Even so, we can see virtually no details in a telescope because Venus is completely covered by thick clouds. These clouds reflect the sun's light. At times Venus is the brightest object in the sky except for the sun and the moon.
Venus' atmosphere is composed mostly of carbon dioxide. It creates a surface pressure that is about 90 times greater than the pressure at the Earth's surface. The carbon dioxide traps the sun's heat and creates a very strong greenhouse effect. This raises the surface temperature of Venus to more than 800°F (427°C). Venus has no magnetic field and, like Mercury, has no known satellites.
By using radar to penetrate the clouds of Venus, scientists have learned that its surface spins very slowly, completing one rotation in 243 Earth days. However, Venus' clouds can circle the planet in as little time as four Earth days. Another discovery was that Venus rotates from east to west, as Uranus does. All the other planets rotate from west to east.
Radar waves bounced off Venus, from Earth as well as from spacecraft, have allowed scientists to map the surface of this planet in great detail. Such maps show that about 60 percent of its surface is covered by flat, gently rolling plains and a few craters. Two large areas, the size of small continents on Earth, consist of mountainous terrain. Radar maps suggest that, unlike Earth, Venus is made of only one continental plate. There is nothing that resembles the Earth's mid-ocean ridges or similar features. For more information, see the article Venus.
Earth
Beyond the orbit of Venus is Earth. The Earth's orbit is almost as circular as that of Venus. The Earth travels around the sun at an average distance of 93 million miles (150 million kilometers). It completes one orbit around the sun in 365.25 days. It takes 23 hours and 56 minutes for the Earth to rotate on its axis.
The Earth is a ball of rock and metal. Its diameter is 7,923 miles (12,751 kilometers) at the equator. Surrounding the Earth, like a great blanket of gases, is an atmosphere made up mainly of nitrogen and oxygen. As seen from space, however, the Earth's atmosphere appears very thin. If the Earth were the size of an apple, its atmosphere would be thinner than the apple's skin.
Unlike Mercury and Venus, the Earth has a satellite—the moon. It orbits the Earth at an average distance of 240,000 miles (386,400 kilometers). Because the moon rotates on its axis in the same period of time as it revolves around the Earth, it always keeps the same face toward the Earth. The moon is 2,160 miles (3,478 kilometers) in diameter. This is more than one-fourth the diameter of the Earth. For more information, see Earth and Moon.
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The article provides a fascinating overview of planets in our solar system, focusing on their characteristics, orbits, and the elements that define them. Here is a summary of the key points discussed:
Overview of Planets
Definition of a Planet:
A celestial object must meet three criteria to be considered a planet:
It must orbit the sun.
It must have sufficient mass for its gravity to shape it into a sphere.
It must be massive enough to clear other large objects from its orbit.
Our Solar System:
There are eight recognized planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Pluto, once considered the ninth planet, was reclassified as a dwarf planet in 2006 and later as a plutoid in 2008.
Mercury
Closest Planet to the Sun:
Average distance: 35.9 million miles (57.8 million kilometers).
Orbit: Highly elliptical; varies from 29 million to 44 million miles from the sun.
Rotation and Year:
Takes 88 Earth days to complete an orbit.
Rotates on its axis every 59 Earth days.
Physical Characteristics:
Diameter: 3,032 miles (4,880 kilometers).
Features: Many craters, long cliffs (scarps), and a giant impact basin.
Atmosphere: Very thin, contains helium, oxygen, sodium, and hydrogen.
Temperature:
Daytime: Up to 800°F (427°C).
Nighttime: Plummets to -280°F (-171°C).
Satellites: No known moons.
Venus
Second Planet from the Sun:
Average distance: 67.2 million miles (108.2 million kilometers).
Orbit: Nearly circular; takes 224.7 Earth days to complete an orbit.
Physical Characteristics:
Diameter: 7,519 miles (12,100 kilometers).
Gravitational pull similar to Earth.
Atmosphere: Mostly carbon dioxide, creating extreme pressure and a strong greenhouse effect.
Temperature: Surface temperature exceeds 800°F (427°C).
Rotation:
Very slow: 243 Earth days for one rotation.
Rotates east to west, opposite to most planets.
Satellites: No known moons.
Earth
Third Planet from the Sun:
Average distance: 93 million miles (150 million kilometers).
Orbit: Completes one orbit in 365.25 days.
Physical Characteristics:
Diameter: 7,923 miles (12,751 kilometers).
Atmosphere: Composed mainly of nitrogen and oxygen, very thin compared to its size.
Moon: Earth has one moon, which orbits at an average distance of 240,000 miles (386,400 kilometers) and has a diameter of 2,160 miles (3,478 kilometers).
Conclusion
The article highlights the evolution of our understanding of planets and the complex systems they exist within. Each planet possesses unique characteristics that contribute to the diversity of our solar system. Such knowledge has been largely acquired through space probes and detailed studies, enhancing our understanding of these distant worlds.
Mars
Your next stop is Mars, a planet that shines in the sky with a reddish color and is sometimes called the Red Planet. The orbit of Mars is more oval-shaped than that of the Earth. So the distance between Mars and the sun may be as little as 128.4 million miles (206.7 million kilometers) or as much as 154.8 million miles (249.2 million kilometers). The average distance between Mars and the sun is about 142 million miles (228 million kilometers). It takes Mars 687 Earth days to travel once around the sun. It rotates once on its axis in 24 hours and 37 minutes.
Mars is a small, rocky world that is very dry. Its diameter is 4,200 miles (6,760 kilometers), only a little more than half that of the Earth. Its gravitational pull is weaker than the Earth's.
Mars has two small satellites. Phobos, the larger one, measures about 16 miles (26 kilometers) in its longest dimension. It circles its planet at a distance of 2,462 miles (3,964 kilometers). Phobos completes its orbit in about 71/2 hours, which is less time than it takes Mars to turn once on its axis. Therefore, to a visitor on Mars, Phobos would seem to rise in the west and set in the east. Deimos is smaller than Phobos, measuring about 10 miles (16 kilometers) long. It circles Mars at a distance of 14,700 miles (23,600 kilometers).
Early space probes provided a great deal of information about Mars. They learned that Mars has no magnetic field and that there is very little atmosphere surrounding it. The atmospheric pressure on its surface is 1/600 of the atmospheric pressure on the Earth's surface. Its thin atmosphere is made up almost entirely of carbon dioxide. Temperatures at the planet's equator may occasionally reach 60°F (16°C). But nighttime polar temperatures can plunge to nearly −202°F (−130°C).
The surface of Mars has a variety of features. These include craters, giant extinct volcanoes, and a canyon system almost as long as the width of the United States. In 2004, the robotic rovers Opportunity and Spirit touched down on the surface of Mars. They revealed many details about the planet's geology and topography. They also found signs that liquid water once flowed on the planet, perhaps enough to have supported life. For more information, see the article Mars.
Jupiter
After passing through a belt of asteroids beyond the orbit of Mars, the next stop on your planetary journey is Jupiter. This planet is more than five times farther from the sun than Earth. Jupiter travels around the sun at an average distance of 483 million miles (778 million kilometers). It takes 11.9 Earth years to complete one orbit.
Jupiter is the largest planet. It is one of the four gas giants in our solar system. Its diameter is 89,000 miles (143,000 kilometers). This is more than eleven times greater than that of Earth. Jupiter takes less than ten hours to rotate once on its axis.
What appears to be Jupiter's surface is really the top of a thick, cloudy atmosphere. Nearly 90 percent of it is hydrogen, and most of the remaining 10 percent is helium. There are also very small amounts of methane, ammonia, and some other substances that give Jupiter's clouds their beautiful colors.
The Pioneer and Voyager space probes flew by the planet in the 1970's. They confirmed that at the top of Jupiter's atmosphere it is very cold, about −186°F (−121°C). Deeper down in the clouds, however, temperatures climb to thousands of degrees. The tremendous heat rising through the atmosphere combines with Jupiter's rapid rotation to produce large and violent weather systems. Cloud belts on Jupiter are driven by powerful winds that move at speeds of up to 350 miles (560 kilometers) per hour. The most amazing feature in Jupiter's clouds is an enormous orange-red area known as the Great Red Spot. The Great Red Spot is more than twice the size of our planet.
Deep below the clouds there is no solid surface. Instead, astronomers think there is probably a planet-wide ocean that consists mostly of hydrogen compressed under such great pressure that it forms a liquid with metallic properties. At the center of Jupiter there is probably a core, consisting of iron and silicates, about twenty times the total mass of Earth. The molten metallic materials in the core create a strong magnetic field that extends far out into space. At the top of Jupiter's clouds, this magnetic field is about 14 times stronger than that of the Earth. Jupiter's gravitational pull is about two and one-half times stronger than the Earth's.
Jupiter has 79 known satellites. The largest, Ganymede, is 3,270 miles (5,270 kilometers) in diameter. Callisto is slightly smaller, with a diameter of 3,000 miles (4,800 kilometers). Each of these satellites has an icy surface covered with craters. Another satellite, Europa, which is 1,900 miles (3,100 kilometers) across, is also covered with a layer of ice. Io has a diameter of about 2,200 miles (3,500 kilometers). Its surface is covered with volcanoes and lava flows. The other known satellites range from less than a mile to more than a hundred miles in diameter. Before the Voyager probes, little was known about Jupiter's satellites because they appear only as dots when seen through even the largest telescopes. The later Galileo probe found indications of liquid salt water beneath the ice of Europa, Ganymede, and Callisto. Voyager 1 also discovered a thin, delicate ring of fine particles circling Jupiter. For more information, see the article Jupiter.
Saturn
The next stop on your journey is Saturn, another gas giant. It orbits the sun at an average distance of 885.5 million miles (1.4 billion kilometers). It takes nearly 29.5 Earth years to make one trip around the sun.
Saturn is not quite as large as Jupiter. It has a diameter of 74,500 miles (120,000 kilometers). Like Jupiter, Saturn rotates very quickly; it takes 10 hours and 40 minutes for it to turn once on its axis.
Saturn has a very deep atmosphere, which is made up mainly of hydrogen and helium, with some methane and ammonia. At the top of its atmosphere, the temperature is about −300°F (−185°C). This low temperature causes ammonia in the outer atmosphere to freeze, forming high-altitude haze. This gives Saturn a softer appearance than Jupiter. There are also spots similar to Jupiter's Great Red Spot in Saturn's atmosphere, but they are smaller. The largest is about the size of Earth.
The largest of Saturn's 62 known satellites is Titan. The diameter of Titan is 3,200 miles (5,150 kilometers). This is the only satellite in the solar system that has a thick atmosphere. This atmosphere consists mostly of nitrogen. The pressure at Titan's surface is about one and a half times greater than at the surface of the Earth.
Saturn is one of the most beautiful objects in the sky because of the rings that circle the planet at its equator. The entire ring system is more than 170,000 miles (274,000 kilometers) in diameter. But it is less than 1 mile (1.6 kilometers) thick. The rings may have formed at the same time as the solar system, or they may be the remains of a nearby satellite that was broken up by Saturn's gravitational pull, which is almost the same as that of the Earth. For more information, see the article Saturn.
Uranus
Leaving Saturn and traveling even farther from the sun, you come to the planet Uranus. Uranus is 32,000 miles (51,500 kilometers) in diameter. It orbits the sun at an average distance of 1.8 billion miles (2.9 billion kilometers). It takes about 84 Earth years to complete one orbit. Uranus rotates once on its axis in an east-to-west direction in approximately 17 hours. This is also the rate at which its magnetic field, which is anchored in the planet's interior, and its interior rotate. Uranus' magnetic field is only about two-thirds as strong as that of the Earth.
The blue-green color of Uranus is caused by methane in its thick atmosphere. The temperature at the tops of its clouds is −355°F (−215°C). Scientists think that beneath the atmosphere is a vast ocean of water, methane, ammonia, and other elements. This ocean may surround a core consisting mostly of molten rock and metal. At the tops of Uranus' clouds, the gravitational pull is somewhat weaker than that of the Earth. A system of thin, very dark rings encircles the planet.
Most of the planets in our solar system spin in a more or less straight up-and-down position as they orbit the sun, although some are tilted a little more than others. If the solar system could be placed on a gigantic table, the planets would look like spinning tops as they moved around the sun. Uranus, however, tilts at the extreme angle of 97.8 degrees. You could almost say the planet lies on its side as it orbits the sun. This means that the north and south polar regions of Uranus experience alternate periods of day and night (and summer and winter), each of which is up to 42 Earth years long. If Uranus were much closer to the sun, this could result in very extreme seasons. Because of the planet's distance from the sun and the odd wind patterns in its atmosphere, Uranus has rather even global temperatures. The north and south polar regions are actually a little warmer than the regions around its equator.
Uranus has 27 known satellites. The largest is Titania. It has a diameter of about 1,000 miles (1,600 kilometers). The smaller satellites range in size from about 12 to 96 miles (20 to 155 kilometers) in diameter. For more information, see Uranus.
Here's a summary of Mars, Jupiter, Saturn, and Uranus based on the provided text:
Mars
Nickname: The Red Planet due to its reddish appearance.
Orbit:
Average distance from the sun: 142 million miles (228 million kilometers).
Distance ranges from 128.4 million miles (206.7 million kilometers) to 154.8 million miles (249.2 million kilometers).
Takes 687 Earth days to complete one orbit around the sun.
Rotation: Completes a rotation on its axis in approximately 24 hours and 37 minutes.
Physical Characteristics:
Diameter: 4,200 miles (6,760 kilometers), about half that of Earth.
Weak gravitational pull compared to Earth.
Satellites:
Phobos: Diameter about 16 miles (26 kilometers), orbits Mars at a distance of 2,462 miles (3,964 kilometers) in about 7.5 hours.
Deimos: Smaller, about 10 miles (16 kilometers), orbits at a distance of 14,700 miles (23,600 kilometers).
Atmosphere and Temperature:
Very thin atmosphere (1/600 of Earth's pressure), mostly carbon dioxide.
Temperatures may reach 60°F (16°C) at the equator and drop to −202°F (−130°C) at night in polar regions.
Surface Features: Includes craters, giant extinct volcanoes, and a vast canyon system. Evidence suggests that liquid water once flowed on the planet.
Jupiter
Orbit:
Average distance from the sun: 483 million miles (778 million kilometers).
Takes 11.9 Earth years for one orbit.
Physical Characteristics:
Diameter: 89,000 miles (143,000 kilometers), the largest planet in our solar system.
Rapid rotation: about 10 hours.
Atmosphere: Dominantly hydrogen and helium, with traces of methane and ammonia. The temperature at the top of the atmosphere is about −186°F (−121°C).
Weather Systems: Features large storm systems and cloud belts driven by winds up to 350 miles (560 kilometers) per hour. The Great Red Spot is a massive storm larger than Earth.
Core and Magnetic Field: Likely consists of a planet-wide ocean of metallic hydrogen and a core of iron and silicates. Strong magnetic field, about 14 times stronger than Earth's.
Satellites: 79 known, including Ganymede (largest), Callisto, Europa, and Io, with features ranging from icy surfaces to volcanic activity.
Saturn
Orbit:
Average distance from the sun: 885.5 million miles (1.4 billion kilometers).
Takes nearly 29.5 Earth years to complete one orbit.
Physical Characteristics:
Diameter: 74,500 miles (120,000 kilometers).
Quick rotation: about 10 hours and 40 minutes.
Atmosphere: Rich in hydrogen and helium with ammonia and methane. The temperature at the top is around −300°F (−185°C).
Appearance: Known for its stunning rings, which are about 170,000 miles (274,000 kilometers) in diameter but less than 1 mile (1.6 kilometers) thick.
Satellites: 62 known, with Titan being the largest (3,200 miles/5,150 kilometers in diameter) and the only moon with a significant atmosphere.
Uranus
Orbit:
Average distance from the sun: 1.8 billion miles (2.9 billion kilometers).
Takes about 84 Earth years to complete one orbit.
Physical Characteristics:
Diameter: 32,000 miles (51,500 kilometers).
Rotates once on its axis in about 17 hours.
Atmosphere: Methane gives it a blue-green color. Temperature at the tops of its clouds: −355°F (−215°C).
Unique Tilt: Axial tilt of 97.8 degrees causes extreme seasonal variations, with polar regions experiencing prolonged periods of day and night for 42 Earth years.
Satellites: Has 27 known, with Titania being the largest (1,000 miles/1,600 kilometers in diameter).
Summary
This journey through the solar system reveals the unique characteristics and captivating features of each planet. Mars, known for its red color and potential past water, leads to the massive gas giants Jupiter and Saturn, each with their spectacular atmospheres and numerous satellites. Finally, Uranus stands out with its extreme axial tilt and unique blue-green hue due to methane. The diversity of these planets illustrates the complexity of our solar system.
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