The Solar System is our local neighborhood in space. This incredible system of celestial objects contains one star, nine planets, more than160 known moons, and a variety of other objects such as asteroids, comets, and dwarf planets. At the center of our solar system is a yellow star known as the Sun. The nine planets and other bodies orbit the Sun and these planets are themselves orbited by a multitude of moons that vary from asteroid-sized chunks of rock to nearly planet-sized worlds with atmospheres of their own. The planets in this system range in size from small, rocky worlds to gigantic balls of gas and ice.
DEFINING A STAR…
A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun! Other stars, mostly found in the Milky Way are visible from Earth during the night (on a ‘starry night’), appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth.
Historically, stars have been important to civilizations throughout the world. They have been important part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable.
By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun. The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar, currently used nearly everywhere in the world, is a Solar calendar based on the angle of the Earth’s rotational axis relative to its local star, the Sun.
OUR OWN SOLAR SYSTEM
Now, let’s take a journey through our own neighborhood in the vast expanse called Space – our own Solar System.
THE SUN – A Ball of Hot Gas
We start our journey through the Solar System at the center. Here we find a gigantic ball of gas so massive that the immense pressure has ignited a fusion reaction. It is an average-sized yellow star known as the Sun. It is one of the most common type of star in the universe. Here in this cosmic furnace, hydrogen atoms under unimaginable pressures are being fused into helium atoms, releasing a tremendous amount of energy. Scientists believe that this reaction has been taking place for almost 5 billion years, and will likely continue for another 5 billion years. At that time, the Sun’s hydrogen supply will have been depleted, and heavier elements will begin to fuse. This will cause the star to swell to the size of a red giant, consuming most if the inner planets in the process. Eventually the Sun will end its life as a cold lightless body known as a black dwarf.
#1. MERCURY – Messenger of the Gods
The next stop on our tour is a small, barren world called Mercury. It is the closest planet to the Sun and the smallest planet in the Solar System; in fact, mercury is about the same size as our Moon. Mercury was named after the ancient Roman messenger of the gods because it moved across the sky much faster than the other planets. In fact, Mercury completes one orbit around the Sun in only 88 days! The planet has a very slow rotational period. It takes 58 days for Mercury to make a complete rotation on its axis. Because of its close proximity to the Sun, it can only be observed in the sky in the east just before sunrise or in the west just after sunset. It never appears more than 27 degrees away from the Sun.
2. VENUS – Goddess of Love and Beauty
Our next stop on the tour brings us to a planet completely covered by wispy white clouds. This is the planet Venus, and it is the second planet from the Sun. Venus is named after the Roman goddess of love and beauty. It is also known as the jewel of the sky, and as the morning star. Venus is so bright in the sky that it has been known since prehistoric times. Venus is sometimes referred to as Earth’s sister planet, as both are nearly the same size and have nearly the same mass. Venus differs from all other bodies in the Solar System because it rotates from east to west. It is, in a sense, upside down. Another odd fact about this planet is that its day is longer than its year! Venus takes 225 Earth days to make one complete revolution around the Sun, while it takes 243 days for Venus to rotate once on its axis.
3. EARTH – A Blue Miracle
As we leave behind the dense clouds of Venus, we next encounter a truly remarkable sight. As we approach the third planet from the Sun, we see a shiny blue sphere of unimaginable beauty, partnered with a singular, large rocky Moon. A delicate veil of thin white clouds encompasses this blue globe. The blue color of this unusual world is caused by liquid water, an rare sight in the Solar System. In fact, this truly unique gift is known to exist only on this one planet. The blue spaces are broken by rocky shapes covered with a color we have never seen before. The beautiful browns of the land masses are painted with various shades of green. These are life forms. This planet is home to living organisms, making it potentially unique in the entire universe. This is the Terran system, comprised of the planet Earth and its lonely Moon!
4. MARS – The Red Planet
As we leave behind the blue marble of Earth, we next encounter a smaller rocky world that is totally barren. This is the fourth planet from the Sun, and it is a a bright orange-red world pockmarked with deep scars and enormous mountain peaks. Its red color is the result of millions of years of iron oxidation. This planet is rusty. It is capped on both ends by bright sheets of white ice, and is accompanied by two tiny, lonesome, oddly-shaped bodies. This is the planet Mars, and its two moons, Phobos and Deimos.
Mars is the last of the four terrestrial inner planets of the Solar System. It is about half the size of Earth and much less dense. Its red-orange color is caused by iron oxide, commonly known as rust. Both moons orbit very close to the planet. Phobos orbits so close, in fact, that its orbit is slowly decaying. Is it moving closer to the planet each year and will one day eventually crash into the surface.
Scientists believe that liquid water may have once existed on the surface of Mars, and it is possible that life may have once existed there as well. Because many scientists believe that life may have once existed on Mars, it has been explored more than any other body in the Solar System.
ASTEROIDS – Cosmic Leftovers
As we leave the red planet and its moons behind us, we next encounter a strange collection of small planet-like objects. This is the asteroid belt. These asteroids, also known as planetoids, range in size from several hundred miles to several hundred feet. At least 30,000 of these giant rocks are known to be floating in their own individual orbits between Mars and Jupiter. Scientists are not sure why there are asteroids here instead of a planet. Some believe that a large, rocky planet may have existed here in the past. This planet may have then been blasted to pieces by a cataclysmic impact. Others think that the asteroids are composed of the leftover material from which the Solar System was originally formed.
5. JUPITER – The Giant
As we emerge from the scattered fragments of the asteroid belt, we are presented with a truly awesome sight. We have left behind the rocky worlds of the inner planets and have now entered the realm of the gas giants. Here we encounter the fifth planet from the Sun. At over a thousand time the size of Earth, it is also the largest planet in the Solar System. Brilliant colors of red, brown, and orange form bands and swirls in its gaseous atmosphere. A faint ring of delicate, icy material encircles the planet, and many natural satellites of varying sizes can be found in orbit. This is the planet Jupiter and its complex system of moons. Jupiter has been described as “A Solar System in Miniature”. The total number of known moons in the Jovian system is over 63. Four of these moons are nearly planet-sized. Because of Jupiter’s massive size, it has an enormous impact on the objects around it especially on its moon Io. Jupiter’s gravity also had a strong impact on its other moons.
6. SATURN – Jewel of the Solar System
The next stop on our tour brings us to one of the most spectacular sights in the Solar System. The sixth planet from the Sun is a large, bright gas giant surrounded by thousands of delicate, glistening rings. As we get closer to the planet, we can see even more rings. What originally appears a two rings now reveals itself to be a complex system composed of thousands of smaller ringlets. There are rings and there are rings inside of rings. Surrounding all of this stunning beauty is a system of 62 moons, ranging from the tiny to the truly gigantic. Seven of these natural satellites are large enough to warrant further investigation. This is the planet Saturn and its system of rings and moons.
Rings and Moons: Without a doubt, the most striking feature of the Saturnian system is Saturn’s amazing rings. This complex ring system is composed mainly of water ice fragments. These fragments range in size from dust specs to chunks the size of a car. The rings are mostly less than a mile thick, hence they are totally invisible when viewed from the side. Early observations showed two rings, but later missions to the planet revealed that there are actually thousands of smaller rings. Recent observations have shown that the rings are much more complex than previously thought, with unexplained thick and thin areas and elaborate spiral structures. It has also been discovered that some of the rings are kept in place and shaped by the gravitational effects of small moons known as shepherd moons. The Saturnian system contains a large number of moons, nearly as many as the planet Jupiter. So far, 62 moons have been discovered orbiting the planet. Seven of these moons are quite large.
7. URANUS – The Blue Gas World
Continuing our outward journey, we encounter the seventh planet from the Sun. It is a beautiful yet nearly featureless giant, blue green sphere called Uanus. The outer layer of thick, blue gas gives no hint as to what might lie beneath. We have left the realm of the gas giants and entered the domain of the ice giants. This planet is a little bit smaller than its close neighbor Saturn, and is surrounded by a very thin layer of small delicate rings. On closer inspection, we find this mysterious world to be tilted on its side for some strange reason. In orbit around the planet, we find 27 moons of varying sizes and shapes. This is the planet Uranus and its system of rings and moons.
More Rings: The planet Uranus has a very thin and faint system of rings. It was the second planet in the Solar System found to have rings.
8. NEPTUNE – The Blue Gas World
As we leave behind the blue green beauty of Uranus, we encounter another giant blue world of almost the same size. This one is a bit different in appearance, however. Unlike its rather unremarkable distant sister, this giant sphere is marked by wispy white clouds and dark blue spots. Like a giant eye it looms up from the blueness, reminiscent of the great red spot on Jupiter. A few tiny rings and 13 moons encircle this planet. One of these moons,Triton is quite large. This is the planet Neptune and its system of moons.
And Yet More Rings: Like its sister planet Uranus, Neptune has a very thin, dark system of rings.
The planet moves so slowly, and was once thought to be a star.
9. PLUTO – God of the Underworld
As we leave behind us the system of Neptune and its moons, we leave the world of the gas giants and approach the extreme outer limits of our solar system. This region is inhabited by many small, rocky worlds. This region is known as the Kuiper belt, and two of its most famous inhabitants are the planet Pluto and its single moon, Charon. This moon, Charon, is more than half the size of its parent planet Pluto. In fact, many astronomers once referred to Pluto and Charon as “twin planets” because they are so close to each other in size. Because Charon is so large, it does not actually orbit around Pluto. Rather, the two bodies actually orbit around a common center of gravity somewhere between them.
The darkness and isolation of these two worlds has likened them to the underworld. In fact, Pluto was named after the Roman god of the underworld. Charon was named after the mythological figure who ferried the dead across the river Styx to Hades. Pluto was discovered in 1930 by Clyde W. Tombaugh at Lowell Observatory in Arizona. It was much too small to be seen by most telescopes of the time. Astronomers first theorized that a ninth planet might exist when they observed that something seemed to be exerting a gravitational pull on the planet Neptune.
The orbit of Pluto is so highly eccentric that is actually crosses inside the orbit of Neptune. Because of this, Pluto is actually closer to the Sun than Neptune for 20 years of its 248-year orbit. The 17-degree inclination of Pluto’s orbit means that it there is no chance that it will eventually collide with Neptune.
An in-depth sky survey finally turned up the tiny planet. Pluto is so far away from the Sun that it takes 248 Earth years to complete one orbit. The temperature here averages -382° F (-230° C).
The Great Beyond
Not long after we cross the orbit of Pluto, we begin to escape the effects of the Sun. Here we enter interstellar space. From this point on, we will likely not encounter another object until we reach the next nearest star system. The distances between the stars is so incredibly vast, that it would take us 4 years traveling at the speed of light just to reach the closest star to the Sun. The Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 spacecraft are about to be the first man-made objects to leave the Solar System. These spacecraft have cleared the known boundaries of the Solar System and are searching for the heliopause, which marks the end of the Solar wind and the beginning of interstellar space. Both Spacecraft are expected to continue to operate for the next 25 to 30 years, sending back data on magnetic fields and interstellar particles.
The End of the Road
As we pass the outer limits of the Solar System, we are faced with a vast emptiness. Beyond our home system lies the great mysteries of deep space. Here, the distances are so great as to boggle the mind. The nearest star is four light years away. That means that it would take four years to get there if you could travel at the speed of light, which is 186,000 miles per second. We can only look out into that emptiness with our telescopes and be humbled at the wonder of it all. This concludes our Tour of the Solar System.
The Sky – A Tour Of The Solar System (with thanks)