The Jupiter

Jupiter is by far the largest planet within our Solar System: two and a half times larger than all of the other planets put together. It is the fifth planet from the Sun and one of the brightest planets. Jupiter is sometimes called a "gas giant" because most of this planet is made up of liquid and gas.
How big is the planet?
Jupiter is 142,984 km or about 11 Earths in diameter at the equator. That makes it about one tenth as big as the Sun! You could fit about 1,400 Earths into the volume of Jupiter. It is 133,709 km or 10 Earths in diameter from pole to pole. Jupiter's rapid rotation makes it bulge out at the equator.
Jupiter's magnetic field is the largest single planetary thing in the Solar System. It is 26 million kilometers across, making it about 20 times bigger than the Sun. It has a tail that extends past Saturn's orbit. If it could be seen from Earth, it would appear to be five times the size of the full moon.

Surface of Jupiter

What is the surface like on Jupiter ?

The surface we see is not solid. This enormous planet has a relatively small solid and rocky core. Liquids and gases surround this core and blend with the atmosphere.
Jupiter is a cloudy, windy and stormy planet. It is always covered by a layer of clouds, and wind speeds of 600 km/h are not uncommon. The storms are visible as swirls, bands and spots. A particularly violent storm, about three times Earth's diameter, is known as the Great Red Spot. This storm has been in existence for nearly 300 years!
The layer of clouds is divided into several bands. The lighter colored bands are called zones and the darker bands are called belts. The colors are caused by small changes in the temperature and chemistry. Each band rotates in the opposite direction from its neighbors. Along the edges where the bands meet, these winds collide and create swirling patterns.
The stormy atmosphere of Jupiter has flashes of lightning just like on Earth. However these can be up to 100 times more powerful. The lightning is made by water near the tops of the clouds.

Rings Of Jupiter

What are its rings like?

Jupiter's rings are dark and hard to see. They are made of tiny particles that meteors knocked off Jupiter's small inner moons and debris left over from comets and other objects that came close to the surface of Jupiter. In fact, until the Voyager spacecraft arrived near Jupiter and took closeup pictures of the rings of Jupiter, scientists didn't even know that it even had rings at all. Two rings are clearly from material that can be associated with two sets of the inner moons of the planet.

Moons In Jupiter

What are its moons like?

Jupiter has 63 known moons. There are four major moons that were discovered by Galileo in 1610. Those moons are Io, Europa, Ganymede and Callisto. They are called the Galilean moons. There are often eclipses on Jupiter's cloud tops by the Galilean moons.
Amalthea GroupThere are four small moons orbiting inside Io's orbit. That group is called the Amalthea group because Amalthea is the largest one. They are all small and potato shaped. Amalthea is very red. The material of Jupiter's rings came from meteors knocking it off of those moons.

Io

Io is Jupiter's closest major moon. It is 3643.2 km across, slightly larger than Earth's Moon. It has volcanoes and molten sulfur lakes. There are not very many craters if there are any at all because the volcanic activity would cover them up. Io has an iron and maybe iron sulfide core at least 1800 km across. It is surrounded by a silicate shell. There is little water on Io. Maybe it was because when Jupiter was forming, it was hot enough to dry out Io, but not the other major moons. In Roman mythology Io was a beautiful young woman that Jupiter loved.

Europa

Europa is 3,121.6 km across, about ten percent smaller than Earth's Moon. It is made of silicates and has a layer of smooth water ice 10 to 30 km thick. The ice has long cracks in it and very few craters. It looks like the sea ice on Earth. The ice had slid around at the cracks. There is liquid water under the ice up to 100 km below the surface. There are also some large spots on the surface. In Roman mythology Europa was courted by Jupiter in the form of a bull.
Jupiter as seen by the space probe "Cassini". This is the most detailed color portrait of Jupiter ever assembled.

Ganymede

Ganymede is 5262.4 km across, making it 380 km wider than Mercury. It is Jupiter's largest moon and the largest moon in the Solar System. It had plate tectonics like Earth. There are older darker regions and newer areas with grooves where the plates moved. Newer craters have bright rays around them from material thrown up by impacts. Older craters look flat and faded because the icy surface does not hold the shape of the crater as well as rock does over long periods of time. Ganymede may have an iron and sulfur core with a silicate mantle and an icy shell. It may be like Io except with a layer of ice on it. In Roman mythology Ganymede was a beautiful young man who Jupiter kidnapped and made cupbearer to the gods on Mt. Olympus.

Callisto

Callisto is 4820.6 km across, about the same size as Mercury. It has many craters. Like craters on Ganymede, the older craters had faded. The largest crater is Valhalla. It has a bright center 600 km across with rings around it up to 3000 km across. Callisto is made of silicates and ice. There is a 200 km thick icy crust with a liquid water sea under it. In Roman mythology Callisto was turned into a bear by Jupiter's jealous wife Juno. Later Jupiter placed her in the stars as The Great Bear.
Other moonsThe other moons are tiny ones in several groups outside the orbits of the

A Day,Year and Gravitational Pull

How long is a day on this planet?

One Jupiter day is about 10 Earth hours long. You have to say "about" because different parts of Jupiter rotate about its axis at different speeds. This is caused by the fact that Jupiter is mostly gases that are in constant motion and sometimes going in opposite directions. Some efforts have been made to try and measure the rotation speed of the inner rocky core of Jupiter, but that has proved to be quite difficult to accomplish due to the magnetic fields that surround Jupiter and the very active radio energy that is generated by the atmosphere of Jupiter, which interferes with measuring techniques like radar that has been used to measure the surface of Venus and Mars.

How long is a year on this planet?

The Great Red Spot Jupiter EclipsesOne year on Jupiter is 4,335 Earth days or 11.87 Earth years long.
A Jupiter year is about equal to four-tenths (or two-fifths) of a Saturn year. Thus after every two Saturn years, Jupiter has completed five full orbits about the Sun. So after 59 years, Saturn and Jupiter will be back in nearly the same position. When the orbits of two planets are simple ratios of each other like this, it is called a resonance.

How much would Jupiter's gravity pull on me?

If you were floating close to the cloud tops of Jupiter, it would pull you down with a force about two and a half times as strong as the force of Earth's gravity.
Jupiter's rapid rotation causes the equator to bulge out. This would also cancel out about 10 percent of gravity's force on you if you were at the equator. The amount of this counteraction becomes lower the closer you get to the poles.

Jupiter - Who is it named after?

Who is it named after?

Statue of Zeus (Jupiter) in Olympia, Greece
Jupiter is named after the chief of the Roman gods, also called Zeus in ancient Greece. It was so named because of the planet's enormous size, which dominates all the others.

Asteroid Belt

Asteroid Belt Intorduction

The largest asteroid in the solar system is 2001 KX76. In the asteroid belt, the largest asteroid is Ceres, which is 933 kilometers across. The next largest, which is called Vesta, is 530 kilometers across.
Some asteroids are less than a kilometer across. Unofficially the limit has been set at 50 meters, and anything smaller than that is going to be simply called a meteoroid. With advances in telescopes and particularly for objects that travel close to the Earth, some objects smaller than 50 metres have indeed been seen passing nearby the Earth.
There are probably several million asteroids in the solar system. Over 96,000 asteroids have been given numbers. Almost 12,000 of them have names. But even though there are a lot of asteroids, the asteroid belt is mostly empty space. Traveling through the asteroid belt in a space ship would not be very much like what you see in a science fiction film.
The asteroid belt lies between the planets Mars and Jupiter. It contains lumps of rock and metal much smaller than planets. These lumps are called asteroids or minor planets. They are not visible from Earth with the naked eye, but many may be seen through binoculars or small telescopes.

Asteroid Belt Names After
The first asteroids were named after mythical heroes and gods much like the major planets. The first to be discovered was named Ceres after the Roman goddess of growing plants (particularly grain) and of motherly love. The second asteroid discovered was called Pallas and was named after one of the Greek gods of wisdom. Asteroids are also given a number in the order of their discovery, so Ceres is 1, Pallas is 2, and so forth.
As the number of known asteroids increased, they ran out of mythical names so other names were used instead. Some asteroids were named after countries. For example asteroid number 136 is named Austria. Others were named after plants, for example 978 Petunia. 1620 Geographos was named after the National Geographic Society, in recognition to their efforts at sharing knowledge about the Solar System. Many are named after people, both alive and dead. In a couple of cases, like 2309 Mr. Spock, asteroids were named after the discover's pet cat. This last type of naming is discouraged, but it still happens from time to time. Even fictional characters have been used.
Today, names for asteroids can be suggested by the people who discover them. The names become official after a group of people reviews them to make sure they are not offensive or too much like another name. Because so many asteroids are now being found, most new asteroid discoveries are not even getting a name at all, but rather a numbered code. It is not likely that they will ever be given a formal name, at least in this century.

Asteroid Belt Discovery
Asteroid Belt Facts:
1.The total weight of all the asteroids in the asteroid belt is about 1/35th of that of our moon. 2.Its largest object in the asteroid belt, Ceres, makes about 1/3 of the total weight of all the asteroids. 3.Ceres is the only dwarf planet in the asteroid belt.
The first asteroid to be discovered was Ceres, on 1 January 1801, by Giuseppe Piazzi accidentally. At first he thought this was a comet, then later a planet! When it was realised it was too small Sir William Herschel (the astronomer who discovered Uranus) made up the word "asteroid" to describe it, using the Latin word aster, meaning star, and the -oid, meaning rock or planet. In other words a star-like planet, because he couldn't see any details due to the small size of the object.
By 1807 another 3 asteroids were discovered, but no more were found until 1845 when a persistent asteroid hunter named Karl Ludwig Hencke found a fifth, and sixth asteroid in 1847. Ever since then at least one new asteroid has been found each year.
In 1891, the first pictures of the night sky were taken to find more asteroids. This led to the discovery of many more asteroids. A picture of the same part of the sky is taken on two different nights. When the two pictures are lined up, the stars will be in the same places but an asteroid will have moved.
In our modern times, over 280,000 asteroids have been discovered. Many more are being found all the time. Some of these asteroids pass near the earth and astronomers want to find any that come close to our planet. Large numbers of asteroids are now being discovered by machines.

Asteroid Belt is Made Of?
Three out of four asteroids are made of rock that is rich in carbon. The rest are made of the metals iron and nickel. About half of these are pure iron and nickel; the rest are mixed with compounds of silica, the element that makes up rocks. Each of the larger metal asteroids contains huge amounts of iron: much more than is mined every year on Earth.
Scientists are very interested in what asteroids are made of because it can help them learn how the solar system was formed. Several spacecraft have visited asteroids to learn more about themAre there asteroids outside of the asteroid belt?Most asteroids are found in the asteroid belt, but not all. Some asteroids orbit closer to the Sun, and many asteroids orbit beyond Neptune. Asteroids that closely approach Earth are called Near-Earth Asteroids. Sometimes pieces of asteroids strike the Earth, burning in the atmosphere as a meteor. If they are large enough, they might actually hit the surface and become meteorites.
Asteroid Ida and its moonThere are also some asteroids in the outer solar system that are called Centaurs. But it is hard to tell whether any one Centaur is an asteroid, comet, or Kuiper Belt object. For example, the first Centaur to be discovered was Chiron. But some scientists think it is a comet, not an asteroid. Officially it is called both the asteroid 2060 Chiron and the comet 95P/Chiron!
In most cases when an the orbit of asteroid crosses the path of a planet such as Jupiter, at some point the asteroid will either hit the planet or else be hurled into another orbit. Many of the small moons of some planets may have once been asteroids that were captured by the planet's gravity when they came too close. However there are two points along the orbit of a planet were an asteroid can safely linger. These are found at a point one-sixth (or 60°) of an orbit ahead of the planet, and the same distance behind the planet. These sites are called Lagrange points, and they are found where the force of gravity from the Sun and the planet balance out with the motion of the asteroid's orbit. The asteroids found in these spots are called Trojans, and they move around the Sun at the same velocity as the planet.

Famous Asteriods
The dwarf planet Ceres.2001 KX76 is now the biggest asteroid in the solar system, beating out Ceres. Ceres is one of the few asteroids that is shaped like a sphere, and it may have some ice just underneath its dusty surface.
The asteroid Ida has its own moon named Dactyl. A number of asteroid moons have now been found.

Gravity Pull on Mars

Gravity Pull on Mars

If you were on Mars, it would pull you down with a force about two fifths as strong as the force of Earth's gravity. You could lift objects that weigh almost three times as much compared to similar objects here on the Earth. You could jump up almost three times higher, and it would take much longer to fall to the ground from the same height.
Even though it looks as though you would be like a comic-book hero on Mars, you would still have to worry about the mass of an object. A large object that is moving forward would still crush you if you got in its way, just like it would be a problem here on the Earth, and a bullet from a gun would do just as much damage on Mars as it does on the Earth. A vehicle that travels on the surface of Mars would have the same problems trying to stop as it would on the Earth, but an interesting problem would be that because of the reduced gravity a vehicle would not "grip" the ground on Mars as strongly, making it harder to stop if you are traveling at a high speed.

Who was it named after?
In Roman mythology, Mars was the god of war and agriculture. The planet Mars was named this because Mars is red like blood.

Day and Year in Mars

One day on Earth is only 39 minutes and 35 seconds shorter than a day on Mars (1.026 Earth days). A year on Mars is almost two Earth years long (687 Earth days).

Surface of Mars

The surface of Mars is dry and dusty. The highlands of the southern hemisphere have more craters than the lower areas in the northern hemisphere. One of the craters is the huge Hellas Planitia. It is about half the size of the continental United States.
There is an area on Mars called the Tharsis Bulge which has four huge volcanoes. These volcanoes have not erupted for millions of years. The largest volcano is called Olympus Mons. It is 27 km tall, making it the highest mountain in the Solar System, much higher than Mount Everest on Earth. It is 625 km across and takes up an area as big as the US state of Arizona. Mars also has a huge canyon called the Valles Marineris. It is much bigger than the Grand Canyon on Earth. It is 4000 km long, up to 7 km deep and up to 200 km wide. Scientists think that when the Tharsis Bulge was created, the surface of Mars cracked to form the Valles Marineris.
Like the Earth, Mars has ice caps at its poles. They are made from ice and frozen carbon dixoide. During the Martian winter, the southern cap grows as carbon dioxide from the atmosphere freezes.
In some places, there are channels that look like they were made by running water. So, a long time ago Mars may have had lakes and streams made of water.
There is an atmosphere on Mars, but it is very thin. There is also much more carbon dioxide in it than oxygen. (Oxygen is the gas we want when we breathe in, carbon dixoide the gas we get rid of when we breathe out.) So, we would need spacesuits to visit Mars.

Mars

Mars
Mars is nearly 7,000 kilometres (km) wide, just over half the width of the Earth. Its volume is about 15% of the Earth.

Mars is the fourth planet from the Sun. It is called a terrestrial planet because it has a rocky surface like the Earth.

It is thought that Mars has an iron core and a crust made of silicate.

What the Moon is made of?

The Moon is mostly made up of rocks and dust. The outer layer of the Moon is called the crust. The crust is about 70 km thick on the near side and 100 km thick on the far side. It is thinner under the maria and thicker under the highlands. There may be more maria on the near side because the crust is thinner. It was easier for lava to rise up to the surface.
We think the Moon has a small core (center) about 300 km across. The core is composed of solid iron. Because the core is solid, the moon does not have its own magnetic field.

The names Moon and month both come from the ancient Greek name for the Moon, Mene. There have been other names for the Moon, like Selene and Luna. Selene was the Greek goddess of the Moon. Luna was the Roman goddess of the Moon. The Roman people also associated their goddess Diana with the Moon.

Day and Year on the Moon

Day
The Moon takes just over 98 Earth days to rotate (spin around) once.
Year
The Moon also takes just over 59.3 days to orbit (move around) the Earth. This is why we always see the same side of the Moon when we look from the Earth. We call this side the near side. The other side we call the far side. In 1959 a probe sent back pictures of the far side. That was the first time anyone saw what it looked like.

Surface of the Moon

Surface of the Moon
The Moon does not have any atmosphere. It also doesn't have any liquid water on its surface. During the day it becomes very hot, but at night it is icy cold. A person visiting the Moon needs an air supply and a special suit.
The Moon has many craters on its surface. The largest one is called the South Pole-Aitken Basin and is roughly 2500 km across.
We think nearly all the craters on moons or planets were made by huge rocks hitting them a long time ago. They are called impacts.
Some of the craters on the Moon look as if they have rays coming out of them. These rays are rocks thrown across the Moon by the impacts that made the craters. Some of the craters around the bottom of the Moon may have ice in them.
There are also darker areas called maria (said "MARR-ee-ah"). These are large pools of lava that cooled a long time ago. Most maria are on the side of the Moon we see from Earth. The lighter areas on the Moon are highlands.

Earth and Moon

Moon Facts
The Moon is our nearest neighbour in space.
When we look at the Moon from Earth, we always see the same side. Until luna3 sent back photos in 1959, no-one knew what the other side looked like. The Moon is nearly twice as big as the planet Pluto.The "Man in the Moon" isn't always seen as a man. People from India see an old woman with a spinning wheel. People from Mexico see a rabbit!
The Moon is not that small compared to the Earth. Sometimes the Earth and Moon together are called a binary or double planet system.

How big is the moon?

Comparison of the size of the Moon and the Earth

Most of the planets in the Solar System are much bigger than their moons, but the Earth and the Moon are much closer in size. The Moon is just under 3,500 kilometers (km) wide and the Earth is about 12,600 (km). That's only about one third of the size of the Earth as you can see in the picture below. Because of this, the Earth and Moon together are sometimes called a binary or double planet system.

Surface of Venus

What is the surface like on this planet?

The surface of Venus is very different from the surface of the Earth. It is very dry and hot enough to melt lead. The pressure on the surface is very strong. It is the same as being 1 km (3,280 feet) below the surface of the sea on Earth.


Channels on the surface of Venus, looking like river channels on EarthChannels that look like rivers have formed on Venus. Scientists think these channels are formed from erupting lava. The lava flows along as it cools down, creating the channels. One feature only Venus seems to have is unusual volcanoes called arachnoids. These are volcanoes that have formed differently from other volcanoes we have found in the Solar System. We don't know exactly how they were formed. Venus also has volcanoes like those on Earth.

Parts of the surface of Venus look something like continents. The largest of these areas is called Ishtar Terra. Deep basins like those under the Earth's oceans have also been discovered. On Venus, though, they have no water. Features like mountain ranges and meteor craters have also been found on Venus. One of the highest mountains on Venus, Maxwell Montes, is about 11 km taller than Mount Everest, the highest mountain on Earth.

Venus

Venus is the second closest planet to the Sun. It is a terrestrial planet. This means that we think it was created in a similar way to our planet Earth and is made of rock.Venus is named after the Roman goddess of love. Sometimes it can be seen shining brightly just before dawn or just after sunset. Some people like the Aztecs and the Greeks, gave Venus two names – one for the morning and one for the evening.
If you were on Venus, it would pull you down almost as strongly as Earth. The atmosphere exerts a pressure at the surface more than 90 times Earth's normal sea-level pressure.

How big is Venus?
Earth and Venus ComparisonVenus is almost the same size as the Earth. This is one reason why Venus is sometimes thought of as Earth's "twin". Venus has a diameter of about 12,100 km.

What is Mercury made of?

What is Mercury made of?
The center of Mercury is made of iron in partly-molten (liquid) form. We know that it is iron at the center because the planet generates a magnetic field. It contains more iron for its size than any other planet in the Solar System. The rest of Mercury, its thick crust, is made of a special type of rock called silicate rocks. There are craters near the poles that are constantly in shadow. Some of these craters contain ice. There is a huge crater on Mercury called Caloris Basin. It was formed when a comet hit the planet and lava or molten rock filled the impact crater. The round wall of this crater is over 2km tall.

A Year in Mercury

How long is a year on Mercury?
Mercury has the shortest year in the Solar System. It is about 88 Earth days long.

It used to be believed that the same side of Mercury always faced the Sun. In order for this to be true, Mercury would have to take the same amount of time to rotate (spin around) as it does to circle the sun. Through careful observation we now know that Mercury's rotation is somewhat faster than its orbit. Because of the way the orbit and rotation work together, on Mercury, a day is actually almost twice as long as a year.

Merucury's Surface

Mercury has craters like those on the Earth's moon. The largest crater on Mercury is the Caloris Basin. It is about 1300 km wide. It was created by a huge asteroid hitting Mercury. The asteroid was probably 100 km wide, but it hit Mercury's surface so hard that it made a much bigger hole.

The surface also has big cliffs called scarps. They were made long ago when Mercury cooled down. It shrank, causing the surface to get wrinkled in some places. This wrinkling created the scarps.

There may also be ice on the top and bottom of Mercury. Like the Earth, these areas (called poles) don't get much warmth from the Sun. Any ice there won't melt.

It is very hot during the day (over 400°C) because Mercury is so close to the Sun. At night it is very cold because Mercury loses almost all its heat since there is almost no atmosphere to keep the warmth there. The temperature can fall to almost -275 degrees

How long is a day on this planet?
Mercury rotates (spins around) much more slowly than the Earth. It take Mercury 58 days to spin once. Because Mercury orbits the sun very quickly, a day on Mercury lasts longer than 58 days. If you were standing on Mercury, at the equator and timed how long it took the sun to go from directly overhead to sunset to sunrise and then rise directly overhead again, it would take 176 Earth days. These long days and nights allow for the temperatures to rise as high, and fall as low, as they do.

Mercury

The closest planet to the Sun is the Mercury. It is a terrestrial planet. That means a planet made from rock like Earth.Only one spacecraft, Mariner 10, has visited Mercury. Much of what we know about this planet came from this mission.

How big is Mercury?

Comparison of the size of Mercury to the Earth Mercury is 4879 km across. Mercury's diameter is just less than half the diameter of the Earth. It is the smallest planet in the Solar System. Only dwarf planets like Pluto are smaller. Mercury's small size and its closeness to the Sun sometimes make it difficult to see without a telescope or binoculars. The best time to spot Mercury in the sky is just after the sun goes down at night or shortly before it rises in the morning.

The Sun

The Sun is a star—the closest one to Earth. It is a large ball of very hot gas. The air we breathe and the Helium in a balloon are both gases. It is over 5,500 °C at the surface, and much hotter at the center, about 15 million °C. The Sun is made of mostly hydrogen (70%) and helium (28%). It turns much hydrogen into helium every second, thus creating heat and light.

The Sun makes light and heat that warms the surface of the Earth and allows plants to grow. We can get food from plants, and we can burn wood and other parts of plants to cook, warm our houses, and make cars go. Without the Sun there would be no life on Earth.

How have we explored the Solar System?

Before the telescope, people explored the sky with their eyes. They saw how the planets seemed to wander through the sky. They learned to predict where the Sun, the moon, and planets would be in the sky. They built some observatories -- places for watching the sky. They watched the Sun and stars to tell the time of year. In China, they even knew when the moon would block the Sun. Most people thought that celestial bodies could cause war or peace on Earth.

After telescopes were first made, people kept making them better. Astronomers saw that planets are not like stars. They are worlds, like the Earth. They could see that some planets have moons.They began to think about what these worlds were like. At first, some thought that the other planets and moons had people or animals living on them. They thought about how it would be to live on these other worlds. Then they made telescopes better and saw that there are no plants or animals on the Moon or on Mars.


Twelve Astronauts walked on the Moon about 35 years ago. They brought rocks and dirt back to Earth.Spacecraft flew by Venus, Mars, and the outer planets. The pictures they took showed us a lot of what we know about these worlds.Robots landed on Mars in 1971, 1976, and 1997. They took thousands of pictures of the planets. Two robots, "Spirit" and "Opportunity", are working on Mars right now. They send photos and movies back to Earth. They also check rocks to find out what the rocks are made of.

So far, we have not found any life except on Earth. Maybe tiny one-celled life once lived on Mars. Maybe there is life under the ice on Jupiter's moon Europa. New spacecraft are being planned to look for life on these worlds.

Discovery of the Solar System

Who discovered the Solar System?

Anyone who looks up at the sky enough can see seven bright objects. These are the Sun, our Moon, Mercury, Venus, Mars, Jupiter, and Saturn. People have known about them for a very long time. Ancient people thought they were related to gods. In Babylon, they named the days of the week after them. Almost everyone was sure that all these things were orbiting the Earth. They did not know we lived in a Solar System.
In 1543, Nicholaus Copernicus figured out that the planets orbit the Sun. Only the Moon orbits the Earth. But he was afraid to say so for most of his life. Then Galileo Galilei pointed a telescope at the sky. He found moons orbiting Jupiter. He was certain Copernicus was right, and he got in trouble for saying so. It took seventy years to convince scientists that the planets orbit the Sun. Now, almost everyone on Earth understands that we live in a Solar System.
People made better telescopes and found more things in the sky – moons, new planets, and asteroids. More things are being found today. Recently, astronomers found some things a lot like Pluto. One of them, called Eris, is bigger than Pluto. Maybe it should be called a planet. Maybe Pluto should not be called a planet any more.

About Mass

About mass
Everything is made out of matter. The amount of matter is called mass. Two bananas have twice the mass of one banana. The more mass a thing has, the more gravity pulls it and the more its gravity pulls other objects. We don't notice the pull from a banana because it is so much less than the pull from the Earth. If you stand on the ground and let go of a banana, gravity will pull it down towards the center of the Earth. It will hit the ground. If you could throw the banana hard enough at the right angle, it would go into orbit around the Earth. That is how rockets put astronauts into orbit. If you threw the banana really, REALLY hard in the right direction, it would fly away from Earth and never come back; but our arms are not that strong.
The force of gravity from anything is strongest very close to that thing, and weaker further from it. Scientists use weight to mean how hard gravity pulls us. Astronauts weigh less on the moon because it has less mass. It does not pull as hard. We actually weigh a tiny bit less on top of a tall mountain than we do in a lower place. This is because we are farther from most of the Earth.

Our Solar System

Our Solar System

Planets are big balls of rock or gas that move around stars. We live on one we call the Earth, which moves around a star we call the Sun. There are at least seven other planets moving around the Sun and a lot of other smaller things as well. All these things together are called a system. The Latin word for the Sun is Sol, so we call this system the Solar System.

A long time ago, people didn't realize that all these things in the Solar System move around the Sun. They thought everything moved around the Earth, including the Sun. This seems sensible, because the Earth doesn't feel as if it's moving, does it?

About 500 years ago, however, a man named Copernicus suggested that all the planets moved around the Sun.[1] Then, about 100 years later, a man called Galileo began looking at the sky with a new invention: the telescope. He showed that it was very likely that all the planets moved around the Sun. Soon, more and more people started using telescopes to study the sky. They began to learn how the planets and the other things in the Solar System moved.
Now, we send rockets into space to learn more. Astronauts travel around the Earth. Some of them have landed on the the Moon. Robots can fly to other planets to take pictures. We can see things that people like Copernicus and Galileo could only dream about.

We can use very large telescopes to see what has happened to other stars. We compare pictures of distant stars with pictures of the Sun. We can use thousands of pictures of the planets to learn more about Earth. We use what we learn about all the things in the Solar System to figure out how it was formed.
A long time ago, people didn't realize that all these things in the Solar System move around the Sun. They thought everything moved around the Earth, including the Sun. This seems sensible, because the Earth doesn't feel as if it's moving, does it?
About 500 years ago, however, a man named Copernicus suggested that all the planets moved around the Sun. Then, about 100 years later, a man called Galileo began looking at the sky with a new invention: the telescope. He showed that it was very likely that all the planets moved around the Sun. Soon, more and more people started using telescopes to study the sky. They began to learn how the planets and the other things in the Solar System moved.
Now, we send rockets into space to learn more. Astronauts travel around the Earth. Some of them have landed on the the Moon. Robots can fly to other planets to take pictures. We can see things that people like Copernicus and Galileo could only dream about.

Our Solar System
The Sun
Mercury
Venus
Earth
Moon
Mars
Asteroid belt
Jupiter
Saturn
Uranus
Neptune
Pluto
Comets
Kuiper Belt
Oort Cloud
Glossary
We can use very large telescopes to see what has happened to other stars. We compare pictures of distant stars with pictures of the Sun. We can use thousands of pictures of the planets to learn more about Earth. We use what we learn about all the things in the Solar System to figure out how it was formed. We can also guess what might happen to it in the future.

What is the Solar System?
At the center of the Solar System is the Sun. It is a star, like the billions of other stars in the sky. The other stars are very, very far away, so they look tiny. The Sun is important to us because it gives us heat and energy that allows life. None of the life on Earth could exist without the Sun.
The rest of the things in the Solar System orbit (travel around) the Sun. The planets are the largest of these. Each planet is a little like the Earth. But the planets are also very different from each other.
Many of the planets have moons. A moon orbits a planet. Mercury has no moons. Earth has one. Jupiter has 63!
The planets closest to the Sun are called the inner planets. These are Mercury, Venus, Earth, and Mars. Then comes a big ring of asteroids, chunks of rock much smaller than planets. This ring is called the asteroid belt. Within the asteroid belt, there is a dwarf planet named Ceres. Then come the outer planets: Jupiter, Saturn, Uranus, and Neptune. Farther out there are two dwarf planets, Pluto and Eris.
Beyond the orbit of Neptune is another big ring of things like the asteroids, called the Kuiper belt. Kuiper (said "KYE-per") was the last name of the person who first wrote about it. Most of the things in the Kuiper belt are hard to see through telescopes.
After the Kuiper belt comes the Oort cloud. Scientists think this is where comets come from. It is very far away, many times farther away than Pluto is from the Sun (over a thousand times). It is near the edge of the Solar System.(Yes, "Oort" was the last name of the person who first wrote about it.)In between all the other things is dust. The pieces of dust are very far apart, but they shine in the light of the Sun. Before dawn, in September or October, they glow in the East. We call this the zodiacal glow.
When pieces of space dust hit the Earth's atmosphere, they burn brightly. We call them shooting stars or meteors.
The Sun creates solar wind—a kind of gas that blows away from the Sun into space. This gas travels out past the planets into outer space. The edge, where the solar wind meets the wind from other stars, is called the heliopause. That is about 100 times as far from us as the Earth is from the Sun. Beyond that is a lot of empty space. The nearest star to our Sun is thousands of times farther away than the size of the entire solar system. The Universe is a really huge place!

What holds it together?

Why do all of the planets orbit the Sun? Why do moons orbit planets? Why doesn't the Sun move off and leave the planets behind? The answer to all of these questions has to do with gravity. Gravity is a force that is a property of mass. It pulls things together.
We don't notice the pull from the Sun because it also pulls on the Earth by the same amount. But the Sun's gravity is strong enough to keep the Earth from shooting away. Even though the Earth is going fast, it keeps turning to go around the Sun. It is like they were tied together with an invisible string. In the same way, moons orbit many of the planets. They are kept there by gravity. The Sun itself does not sit still in space. The entire Solar System is orbiting the center of our galaxy. The whole thing stays together because of the force of gravity.