The Solar System- Planets and Orbits

 Key Concepts

• Solar system
• Planets
• Inertia
• Asteroid
• Comet
• Lithosphere

Introduction:       

People studied the stars before the invention of telescopes. When they observed the sky in the night, they found out that bright lights in the sky changed their position according to other lights. In those days, astronomers called these objects planets. The name planet comes from the Greek word ‘wanderer’. A planet is a huge body that revolves around a star. A moon is an object that revolves around a planet. Planets and moons are parts of a solar system. A solar system comprises of a star such as the Sun, the planets, the Moons, and other objects revolving around it. 

Planets and Orbits 

We all know that gravity is the force of attraction among all objects. Gravity causes things to fall onto the floor, and this force keeps planets in orbit. The amount of gravitational force between the Sun and the planet depends on its mass. The higher the mass of the object, the greater is the attraction between them. Distance is also one of the factors that affect gravitation. The higher the distance between the planets and the Sun, the lower will be the pull of gravity. 

The planets revolve in orbits due to inertia. Inertia is the tendency of a moving thing to remain in motion. Gravity will pull objects, i.e., planets, towards the Sun, because of the huge mass of the Sun. This does not take place because the planets have inertia, and they are moving. Inertia causes the planets to move in straight lines. The balance between the force of gravity and the inertia of the planets keeps the planets moving in their orbits around the Sun. The movement of planets is in a straight line due to inertia, but gravity pulls the planets and makes their path in a curved shape into orbits around the Sun. 

Solar System: 

• The solar system consists of stars, the Sun and everything attached to it by gravity, and the planets such as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune; dwarf planets like Pluto, Moons and many asteroids, comets, and meteoroids.  
• There are many planetary systems orbiting other stars in the Milky Way. 

 Asteroids 

• Asteroids are present between the orbits of Mars and Jupiter.  
• They are rocky or metallic objects that move around the Sun.  
• The asteroid belt is the region where most asteroids are located.  
• Asteroids move around the Sun the same as planets do.  
• There is a lot of information gathered by scientists about asteroids in recent years. Space investigations have sent back data that gives pictures of these revolving objects. 
• The space probe Galileo flew by two asteroids: Gaspra in 1991 and Ida in 1993.  

What is in the inner solar system? 

• The inner planets are Mercury, Venus, Earth, and Mars. These planets are located close to the Sun. So, they are called inner planets.  
• All the inner planets rotate somewhat slowly, and none of them has rings.  
• The largest of the inner planets is the Earth. 

Mercury 

• The diameter of Mercury is 4,880 kilometers (3,030 miles), and its distance from the Sun is 57.9 million kilometers (36 million miles).  
• The length of the day on Mercury is 59 Earth days, and the length of the year is 88 Earth days. In the solar system, Mercury is the smallest and closest planet to the Sun.  
• The surface temperatures on Mercury in the daytime can go to 800 °F (430 °C), whereas night temperatures can drop to as low as minus 290 °F (-180 °C). This small planet has no Moons. 
• Mercury has a very thin atmosphere of oxygen, sodium, hydrogen, helium, and potassium and cannot stop incoming meteors; hence its surface has craters, just like the Moon. 

Venus 

• The diameter of Venus is 12,100 kilometers (7,500 miles), and its distance from the Sun is 108.2 million kilometers (67.2 million miles).  
• On Venus, the length of the day is 243 Earth days, and the length of the year is 225 Earth days.  
• Venus is the hottest planet in the solar system.  
• Its atmosphere has a thick layer of CO₂ gas that traps the heat and allows the planet’s surface to reach a temperature of 880 degrees F (471 °C).  
• Venus is slightly smaller than the Earth, and it has a core of molten iron. There are no moons revolving Venus. 

Earth 

• The diameter of Earth is 12,750 kilometers (7,922 miles). Its distance from the Sun is 149.6 million kilometers (93 million miles).  
• The length of the day on Earth is 23 hours, 56 minutes, 4 seconds, and the length of the year is 365.24 Earth days.  
• It is the third planet from the Sun and is the only planet in the universe that has life.  
• Earth looks blue due to the abundance of water, and hence it is called a blue planet.  
• Earth’s atmosphere is rich in oxygen 21% and nitrogen 78%. Average temperature on Earth is about 59 °F (15 °C).  
• Earth possesses a strong magnetic field and tectonic activity. Earth has one Moon. 

Mars 

• The diameter of Mars is 6,800 kilometers (4,200 miles).  
• The distance of Mars from the Sun is 227.9 million kilometers (141.6 million miles).  
• On Mars, the length of the day is 24 hours, 37 minutes, 12 seconds, and the length of the year is 687 Earth days.  
• Mars has two moons, and it also has seasons.  
• Temperatures on Mars range from about –193 °F (–125 °C) to 68 °F (20 °C). There is a thin layer of carbon dioxide on Mars. 

What are the outer planets? 

• Apart from the asteroid belt is another group of planets that consist of Jupiter, Saturn, Uranus, and Neptune. These planets, known as the outer planets, are very different from the inner planets.  
• The four outer planets are similar in size to one another and are called gas giants. All these outer planets have a small metallic core and a thick layer of atmosphere.  
• The outer planets are larger than the inner planets, and their orbits are very far away from each other.  
• All the outer planets have rings and many moons, and they spin very fast; hence a day on the outer planets is short.  
• Away from the outer planets, there are small icy worlds; one of them that is the largest is called Pluto. It was once known as the ninth planet. For many years there was a discussion among scientists, whether Pluto should be called a planet or not? In 2003, astronomers found out about a similar but slightly bigger world beyond the orbit of Pluto. In 2005, scientists found a moon orbiting this newly found world, and in 2006, the IAU (International Astronomical Union) reclassified Pluto as a dwarf planet. 

Jupiter 

• It has a diameter of 143,000 kilometers (89,000 miles), and its distance from the Sun is 778.4 million kilometers (483.7 million miles).  
• The length of the day on Jupiter is 9 hours, 55 minutes, and the length of the year is about 12 earth years.  
• Jupiter has at least 63 moons, and it is the largest planet. Its atmosphere mostly consists of hydrogen and helium.  
• The great red spot on Jupiter is a “storm” that has lasted more than 300 years.  
• Jupiter’s moon Ganymede is the largest moon present in the solar system. Another of its moon, Europa, may possibly have an ocean of water below its icy crust. 

Saturn

• The diameter of Saturn is 120,500 kilometers (74,900 miles), and its distance from the sun is 1.43 billion kilometers (886.5 million miles).  
• Length of the day on Saturn is 10 hours, 40 minutes, and the length of the year is about 29 Earth years.  
• Saturn has almost 56 moons, and its atmosphere mainly consists of hydrogen and helium.  
• Saturn has a huge storm and a jet stream that blows more than 1,600 km (990 miles)/hour.  
• The special feature of Saturn is that it has a big ring system. Saturn’s largest moon is Titan. It is the only moon in the solar system with a cloudy atmosphere. 

Uranus 

• The diameter of Uranus is 51,000 kilometers (32,000 miles) and its distance from the sun is 2.87 billion kilometers (1.78 billion miles).  
• The length of the day on Uranus is 17 hours, 14 minutes, and the length of the year is about 84 Earth years. Uranus has almost 27 moons and 11 rings, and its atmosphere consists of hydrogen and helium, with a small amount of methane that gives Uranus a blue-green color.  
• Uranus’s moon is Miranda that looks as if broken apart into pieces, and those pieces clumped back together many times as it formed. 

Neptune 

• The diameter of Neptune is 49,500 kilometers (30,800 miles), and its distance from the sun is 4.5 billion kilometers (2.8 billion miles).  
• The length of the day on Uranus is 16 hours, 7 minutes, and the length of the year is about 165 Earth years.  
• Neptune has no less than 13 moons, and its atmosphere mainly consists of hydrogen, helium, and methane. Under Neptune’s cloud, there may be an ocean.  
• The strongest winds are present on Neptune.  
• Triton is one of the moons of Neptune that is larger than Pluto and contains ‘ice volcanoes’ that take material up to 8 km (5miles) high. 

Other objects in the solar system 

The other objects in the solar system are comet, meteor, meteoroid, and meteorite.  

Comet

• It is a ball of rock and ice that revolves around the sun. Comets come from the outside borders of the solar system.  
• When a comet moves towards the sun, the heat of sunlight warms the ice of the comet, which causes the sublimation of ice; that is, ice turns from solid to gas and forms a cloud of gas and dust.  
• The radiation from the sun and sunlight pushes the clouds away, and this results in the formation of a comet tail that points away from the Sun. Some comets come from a part that lies just beyond Pluto’s orbit, known as the Kuiper belt. This belt consists of more than 70,000 objects that are the size of bigger asteroids. 

Meteoroids 

• These are small, rocky, or metallic objects that move around the Sun in both the regions (inner and outer regions) of the solar system.  
• The craters present on the Moon were created by the collision of meteoroids.  

Meteor  

• A meteor is a meteoroid that goes into Earth’s atmosphere.  
• It looks like a bright line in the sky.  
• If a meteor does not break apart and burn up in the atmosphere, then it can fall to Earth’s surface. 

Meteorite 

• When a meteoroid strikes Earth’s surface, it is called a meteorite.  
• Evidence of meteorite impacts can be seen in many places on Earth. 

Summary

• Planets and moons are parts of a solar system. A solar system comprises of a star such as the Sun, and all the planets, moons and other objects revolving around it.
• The amount of gravitational force between the sun and the planet depends on the mass.
• Inertia is the tendency of a moving thing to remain in motion.
• Solar system consists of stars, the Sun and everything attached to it by gravity and the planets such as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune; dwarf planets like Pluto, moons and many asteroids, comets, and meteoroids.
• Meteoroids are small, rocky, or metallic objects that move around the Sun in both the regions (inner and outer regions) of the solar system.