Discovery
For millennia, astronomers have followed points of light that seemed to move among the stars. The ancient Greeks named these planets, meaning "wanderers." Mercury, Venus, Mars, Jupiter and Saturn were known in antiquity, and the invention of the telescope added the Asteroid Belt, Uranus, Neptune, Pluto and many of these worlds' moons. The dawn of the space age saw dozens of probes launched to explore our system, an adventure that continues today. The discovery of Eris kicked off a rash of new discoveries of dwarf planets.Formation
Many scientists think our solar system formed from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun. Other particles within the disk collided and stuck together to form asteroid-sized objects named as planetesimals, some of which combined to become the asteroids, comets, moons and planets.The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost planets, leaving behind mostly small, rocky worlds. The solar wind was much weaker in the outer regions, however, resulting in gas giants made up mostly of hydrogen and helium.
The sun
The sun is by far the largest object in oursolar system, containing 99.8 percent of the solar system's mass. It sheds most of the heat and light that makes life possible on Earth and possibly elsewhere. Planets orbit the sun in oval-shaped paths called ellipses, with the sun slightly off-center of each ellipse.
Inner solar system
The four inner four planets — Mercury, Venus, Earth and Mars— are made up mostly of iron and rock. They are known as terrestrial or earthlike planets because of their similar size and composition. Earth has one natural satellite — the moon— and Mars has two moons— Deimos and Phobos.Between Mars and Jupiter lies the Asteroid Belt. Asteroids are minor planets, and scientists estimate there are more than 750,000 of them with diameters larger than three-fifths of a mile (1 kilometer) and millions of smaller asteroids. The dwarf planet Ceres, about 590 miles (950 km) in diameter, resides here. A number of asteroids have orbits that take them closer into the solar system that sometimes lead them to collide with Earth or the other inner planets.
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Earth, our home, is the third planet from the sun. It is the only planet known to have an atmosphere containing free oxygen, oceans of liquid water on its surface, and, of course, life.
Mars is the fourth planet from the sun. Befitting the red planet's
bloody color, the Romans named it after their god of war. The Romans
copied the ancient Greeks, who also named the planet after their god of
war, Ares. Other civilizations also typically gave the planet names
based on its color — for example, the Egyptians named it "Her Desher,"
meaning "the red one," while ancient Chinese astronomers dubbed it "the
fire star." Between Mars and Jupiter lies the Asteroid Belt. Asteroids are minor planets, and scientists estimate there are more than 750,000 of them with diameters larger than three-fifths of a mile (1 kilometer) and millions of smaller asteroids. The dwarf planet Ceres, about 590 miles (950 km) in diameter, resides here. A number of asteroids have orbits that take them closer into the solar system that sometimes lead them to collide with Earth or the other inner planets.
Asteroids are small, airless rocky worlds revolving around the sun that
are too small to be called planets. They are also known as planetoids or
minor planets. In total, the mass of all the asteroids is less than
that of Earth's moon. But despite their size, asteroids can be
dangerous. Many have hit Earth in the past, and more will crash into our
planet in the future. That's one reason scientists study asteroids and
are eager to learn more about their numbers, orbits and physical
characteristics. If an asteroid is headed our way, we want to know that.
Ceres is a dwarf planet, the only one located in the inner reaches of the solar system; the rest lie at the outer edges, in the Kuiper Belt. While it is the smallest of the known dwarf planets, it is the largest object in the asteroid belt.
Outer solar system
The outer planets — Jupiter, Saturn, Uranus and Neptune— are giant worlds with thick outer layers of gas. Nearly all their mass is made up of hydrogen and helium, giving them compositions like that of the sun. Beneath these outer layers, they have no solid surfaces — the pressure from their thick atmospheres liquefy their insides, although they might have rocky cores. Rings of dust, rock, and ice encircle all these giants, with Saturn's being the most famous.
Jupiter is the largest planet in the solar system. Fittingly, it was named after the king of the gods in Roman mythology. In a similar manner, the ancient Greeks named the planet after Zeus, the king of the Greek pantheon.
Saturn is the sixth planet from the sun and the second largest planet in the solar system. Saturn was the Roman name for Cronus, the lord of the Titans in Greek mythology. Saturn is the root of the English word "Saturday."
Uranus is the seventh planet from the sun and the first to be discovered
by scientists. Although Uranus is visible to the naked eye, it was long
mistaken as a star because of the planet’s dimness and slow orbit. The
planet is also notable for its dramatic tilt, which causes its axis to
point nearly directly at the sun.
Comets are often known as dirty snowballs, and consist mainly of ice and rock. When a comet's orbit takes it close to the sun, some of the ice in its central nucleus turns into gas that shoots out of the comet's sunlit side, which the solar wind carries outward to form into a long tail. Short-period comets that complete their orbits in less than 200 years are thought to originate from the disk-shaped Kuiper Belt, while long-period comets that take more than 200 years to return are thought to come from the spherical Oort Cloud.
Trans-Neptunian region
Astronomers had long suspected that a band of icy material known as the Kuiper Belt existed past the orbit of Neptune extending from about 30 to 55 times the distance of Earth to the sun, and from the last decade of the 20th century up to now, they have found more than a thousand of such objects. Scientists estimate the Kuiper Belt is likely home to hundreds of thousands of icy bodies larger than 60 miles (100 km) wide, as well as an estimated trillion or more comets.The Oort Cloud lies well past the Kuiper Belt, and theoretically extends from 5,000 to 100,000 times the distance of Earth to the sun, and is home to up to 2 trillion icy bodies, according to NASA. Past the Oort Cloud is the very edge of the solar system, the heliosphere, a vast, teardrop-shaped region of space containing electrically charged particles given off by the sun. Many astronomers think that the limit of the heliosphere, known as the heliopause, is about 9 billion miles (15 billion kilometers) from the sun.
Pluto, now considered a dwarf planet, dwells in the Kuiper Belt. It is not alone — recent additions include Makemake, Haumea and Eris. Another Kuiper Belt object dubbed Quaoar is probably massive enough to be considered a dwarf planet, but it has not been classified as such yet.Sedna, which is about three-fourths the size of Pluto, is the first dwarf planet discovered in the Oort Cloud.
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Additional Resource:
http://www.kidsastronomy.com/
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