Astronomers still debate how the universe originally formed.Everything that exists, exists in the universe. Before you start getting into the details about things that happen in the universe, try to think about how big it is. Start with you. You are only a couple of feet long. Compare yourself to a whale, or a dinosaur. They are enormous. Then think about how big your city is and how much space you take up in your state, your country, or your continent. Then imagine the Earth. You’re not very big now. Hold on it gets worse.

The Earth is pretty small when you compare it to Jupiter. Jupiter is pretty small compared to the Sun. As you go on, there are millions of suns in our galaxy and thousands of galaxies in the universe. No one really knows how many. There are some scientists and mathematicians with theories, but nobody really knows.


Large objects have a greater gravity than smaller objects. Gravity or gravitational forces exist when one object attracts another. We’re not talking about finding someone really cutey-wootie, awwww! We’re talking about the molecules of one object pulling on the molecules of another object. It’s like the Earth pulling on you and keeping you on the ground. That’s gravity at work. Every object in the universe that has mass has a gravitational force. Even you exert gravity. When you compare your mass to the mass of the Earth, your gravitational force isn’t very impressive. Sorry about that.


Obviously gravity is very important on Earth. Other planets also affect our world. Our connection to the Moon’s gravity makes the tides rise and fall. The Earth’s gravity keeps our planet orbiting the Sun, just like the Sun’s gravity pulls on our planet. When the earth spins and gravity pulls on the clouds, weather can be affected. We have to bring up an important idea now. The Earth always has the same pull on every object. If you drop an acorn, or you drop a piano, they will fall at the same speed. It is the Earth’s gravity and pull that make objects speed up when they are falling. The Earth constantly pulls and objects constantly accelerate.


“People always say, “What about feathers? They fall so slowly.” Obviously, there is air all around us. When a feather falls, it falls slowly because the air is in the way. There is a lot of resistance and that makes the feather move slower. The forces at work are the same. If you dropped a feather in a container with no air (a vacuum), it would drop as fast as a baseball.


NASAs Spitzer, Hubble, and Chandra observatories see different wavelengths of light. We’ve already covered the universe. Let’s focus on something smaller, a galaxy. Right now, you’re sitting on a planet that orbits a star in the Milky Way galaxy. As you know, there are loads of galaxies in the universe. Each is different from the other, like fingerprints. We’re going to talk about a few of the common traits of galaxies and how you can look up in the sky and look for other galaxies.


Organized galaxies are made of millions of stars and systems. While we can’t take a picture of our galaxy, astronomers use a variety of telescopes to study nearby galaxies.


As astronomers have studied galaxies in detail, they have determined that there is more than dust, stars, and systems. They currently believe that about 90 percent of the matter in galaxies is called “dark matter.” Dark matter is matter that does not emit any radiation that can be detected using current instruments. We know that dark matter exists because of the gravity exerted on other objects. If you want to work on the cutting edge of astronomy, you might want to look into the study of dark matter and dark energy.


The Hubble telescope viewed the galaxy cluster Abell 2218 in 2003. We spoke about gravity earlier. We explained that gravity holds you to the Earth. Gravity holds the Earth in orbit around the Sun. Even other stars effect our Sun and pull it into the Milky Way galaxy. Now you need to think about the combined gravity of an entire galaxy. The gravity of one galaxy can pull on nearby galaxies and eventually form clusters of galaxies. Most of the gravity that attracts galaxies to each other comes from unseen particles called dark matter.



This is how the Milky Way would look like if seen from above. We live in the Milky Way galaxy. Based on our observations, we have a barred-spiral galaxy. While we can estimate the size of our galaxy, it’s a little harder to determine its age. We know that the Sun is about 5 billion years old. It’s a young star. Based on the observations of other stars in the Galaxy, we think that the Milky Way may be about 12-14 billion years old.

Our galaxy looks like a spiral form because stars and systems are grouped in constantly moving arms that spin around the center. The Solar System is about half way between the edge of the stellar bar and the edge of the galaxy.


We are definitely not alone in our galaxy. There may be as many as 400 billion stars and systems in the Milky Way. Our closest neighbor is Proxima Centauri. It is about 4.2 light years from Earth. Don’t forget that our entire galaxy is 100,000 light years across.

At this time, the Voyager spacecraft are the only manmade objects to leave the Solar System. Launched in 1977, they should reach the edge of the Solar System by 2015. At this pace, they will need to travel another 82 thousand years before reaching the closest star. As you can tell, we are moving pretty slowly right now and travel to another star is not likely in our lifetime.


Millions of galaxies are out there. Ours is impressive because it is ours. The closest galaxy to ours is called the Sagittarius Dwarf. It is a little over 78,000 light years away. If it would take us thousands of years to reach the closest star, you can only imagine how long it would take to reach the closest galaxy.

What about life out there? Most scientists believe that there is some form of life on another planet in the universe. With so many galaxies, systems, and planets, chances are good that we can something alive somewhere. Will we find super-smart aliens? Who knows. Most believe that there will be some form of life, maybe microscopic.


The Sun is the closest star to the Earth. It is about 93 million miles away. What is a galaxy without thousands of stars? A vacuum. Those thousands of stars grouped together form the galaxies you can see with a telescope. Stars are the objects that heat and light the planets in a system. Everything revolves around stars. Thousands of stars cover our galaxy and when you look at our star, the Sun, you will see it is quite small. Throughout the universe, there are big ones, small ones, and different colored ones. We’re going to discuss the way scientists study stars and how stars affect the planets that orbit them.


So you’re out one night and you look up into the sky. Assuming you aren’t in a city with tons of smog or clouds, you will probably see a sky filled with little dots of light. Those dots (this should not be a surprise) are stars. Some are only a few hundred light years away and some are thousands of light years away. They all have some things in common. You see, stars are huge balls of fire. They aren’t just any fire. That fire is from a constant number of nuclear reactions.


Not all stars have solar systems spinning around them. Some stars are just sitting out in the middle of nowhere. Some stars have a companion star nearby, kind of like a twin. When you have a twin, astronomers call you a binary star.

A star is a huge ball of plasma, usually made of hydrogen (H) and helium (He). That ball of fire also gives off light. All kinds of light. There are visible, infrared, ultraviolet, and X-rays constantly emitted into space. You may find planets that are almost identical to the makeup of stars, like Jupiter, but something has not ignited their nuclear reaction.


All stars require a spark of ignition to become stars. We emphasize the basics at Cosmos4Kids. You’re here to learn about what a star is and a little bit about them. Once again, a star is basically a ball of gas that has nuclear reactions occurring in its atmosphere. As those reactions occur, huge amounts of energy are released into space. The energy is released as electromagnetic radiation (EM). That EM radiation includes visible light, UV (ultraviolet) light, IR radiation (infrared), x-rays and gamma rays. There are also very small particles leaving the star. Those particles are byproducts of the nuclear reactions.


We have a yellow star. That’s the Sun. Some systems have red stars. Others have a blue tint. The color of a star depends on its surface temperature. Bluer stars have a higher surface temperature. Lower temperature stars give off a lot of red light.


Astronomers look at three main characteristics of stars. They study luminosity (brightness), temperature, and radius (size). Each of these three factors can tell you a lot about a star. Take our star for example, the Sun. As far as stars are concerned, it is rather small. The yellow color tells you it has a nice, medium temperature. The small radius also gives an indication that it not much of a powerhouse. Last is the luminosity. On Earth, the Sun may seem bright. Compared to other stars it is only a candle. Astronomers consider our Sun to be in the main sequence of its development.


This bow shock appeared near a young star in the Orion Nebula. Just like living organisms, stars have a life cycle. In the same way that you are born, develop, age and die, stars do the same things. One big difference is that stars don’t need parents. Stars are born from huge clouds of gas and dust. It’s amazing how that gas and dust are probably the most boring things in the universe and they can become everything, asteroids, planets and even stars. So you’ve got that huge cloud of dust and gas. Astronomers call that cloud a nebula. That’s when it all starts to happen.


That nebula starts to condense. Slowly but surely over millions of years, gas particles start to cling to each other, then they attract other particles and molecules. The nebula begins to condense and form a ball. That ball is called a protostar. “Proto” is a prefix that means “early” or “before.” So a protostar is the first step in becoming a full-fledged burning star.


After the star finished the protostar phase, it becomes even denser. The heavy elements move to the center of the star while the light gases stay in the star’s atmosphere. Those gases are usually hydrogen (H) and helium (He). Then something amazing happens, the nuclear fire begins. The star heats up and the gases ignite. This step in the development process is called the main sequence. If you looked, you would see the birth of the star.


In the Solar System section we speak a little about Jupiter. Jupiter is a special planet in that it has a very similar makeup to the Sun. It has a low density and hydrogen and helium are the main components of the atmosphere. It is still missing one thing, nuclear fire. Jupiter could be the star that never was.


The gravity of black holes is so strong that light is pulled in. Black holes are areas in space where there is a huge amount of mass in a very small space. The gravity of this mass is so great that everything in the area is pulled toward the mass. Even light, with its tiny mass, is pulled into the center of the hole. No object can escape the gravitational pull of a black hole.

Have we ever seen a black hole? No. Actually you can’t see a black hole because no light escapes the event. Astronomers use other ways to look for black holes. Since they have large masses and gravities, they affect the surrounding stars and systems. They have found evidence of black holes in the dark centers of galaxies and systems that emit large amounts of x-rays.


As systems develop, there are billions of possible planet types. A system is a star (or two) and a series of objects that revolve around that star. The objects could be gas giants like our Jupiter or small rocky planets like our Mercury. There may also be other non-planetary objects. You will probably find moons around the planets and asteroids/comets zipping around the star in highly elliptical orbits. Just as in biology, a system is made of many parts that all interact with each other.


We have a whole section on our system. Because the name of our star is “Sol”, we live in the Solar System. While there is a debate among astronomers, we currently have one star, eight planets, and several smaller “dwarf planets”. There are also several other large objects and an asteroid belt orbiting the Sun. Our system began to develop several billion years ago with a huge amount of dust and small objects. Over time, the star and planets developed.


As our technology improves, we are able to see further into the universe and study other stars. Seeing a star is pretty easy, they are bright dots in the sky. Seeing a planet is a lot harder. Since planets don’t give off light, we have a tough time seeing them. There are a few ways we detect other planets and possible systems. You might see a big planet because it is reflecting the light of its star.


We have only confirmed the existence of life on one planet. You can learn more about planets in the Solar System section but let’s give a quick overview that a planet is made up of heavy elements. Stars are mainly made of hydrogen (H) and helium (He). Planets generally have all of the other elements in their crust and core. You will find large amounts of iron (Fe) and minerals. Also, planets do not have the nuclear reactions igniting their atmosphere.


When you think about the characteristics that make a planet a planet, think about the Earth. We have an atmosphere that is not on fire. The Earth is basically a large rock. We have a core of molten iron and plates of rock floating on the surface. Also, most of our mass and diameter are because of the planet, not the atmosphere. Mars and Venus are the same way.

But what about Jupiter, what makes it a planet? It has huge atmosphere compared to the size of the planet. Remember, that there are no nuclear reactions in the atmosphere of the planet. Therefore, Jupiter is a planet. If you go deep enough, you will find rock that exists in the center of that huge, gaseous atmosphere.


As you know, we have completed several missions that study other planets. Even today, engineers across the Earth are developing new spacecraft that will explore our Solar System. Since spacecraft are relatively small, scientists debate the number and types of instruments that will visit each planet. They prioritize mission goals. Some spacecraft focus on studying atmospheres and gases. Other spacecraft are detecting radiation and gravitational fields. If you are interested in space, there are millions of things to discover in our Solar System.


While we are studying the planets in our Solar System, some astronomers are using a variety of telescopes to discover planets in other systems. As of summer 2006, 193 planets have been discovered in other systems. Many of those planets are large gas giants similar to Jupiter. The bigger they are, the easier they are to find.


In a lunar eclipse, the Earth moves between the Sun and the Moon. Eclipses are events that happen when one planet or moon is lined up between a star and another planet or moon. For example, you will see an eclipse when the Earth passes directly between our Sun and the Moon. An eclipse could also happen on the surface of Mars when one of Mars’ moons passes between the surface and the Sun. It’s all about blocking light and creating a shadow.


We’ve been talking about planets passing between the Sun and another object. A lunar eclipse happens when the Earth passes between the Sun and the Moon. Lunar eclipses can only happen when there is a full moon. As the Moon’s orbit takes it behind the Earth, a shadow crosses the face of the full Moon. The shadow does not always cover the entire Moon in a total eclipse. There are also partial and penumbral lunar eclipses. There are two or three lunar eclipses each year. The best pictures come from a total lunar eclipse. If you hear about a lunar eclipse, feel free to look up. They are safe to look at.

In a solar eclipse, the moon moves between the Sun and the Earth.


Solar eclipses are more spectacular than lunar eclipses, but much more dangerous for your eyes. Never look directly at a solar eclipse (or the Sun for that matter). Solar eclipses happen when the Moon passes between the Sun and the Earth. The Moon casts a shadow over a portion of the planet, leaving that area in darkness. If you were watching from space, you could see the shadow pass over the surface. You would also notice that the Moon’s shadow only covers a part of the planet because the Earth is much larger than the Moon. A lunar eclipse can put the entire surface of the Moon in darkness.

There are one or two solar eclipses each year. The shadow moves across different areas of the planet. Depending on the location of the Moon, there are total and annular eclipses. A total solar eclipse happens when the Moon blocks the entire Sun. An annular eclipse only covers some of the Sun. A little ring of the Sun can be seen around the Moon. There are different eclipses because the Moon can be closer or further away from the surface of the Earth.


A variety of planet types orbit the Sun in our solar system. Our system of one star and eight planets was born about 4.6 billion years ago. All of the pieces were created at the same time. But wait! It wasn’t a big “POOF!” and everything was here. It took billions of years for the entire system to develop. All of the gases, dust, and pieces of the system were around at the start. Eventually a star, eight planets, some smaller dwarf planets (like Pluto), and an asteroid belt developed. There wasn’t even a star when the Solar System started out.


The system began as a spinning blob of gases. As the blob spun for millions and millions of years, it began to flatten. It probably looked like that shape for a flying saucer. It was a round, flattened disk with a bulge in the middle. That bulge was the beginning of the Sun. Scientists call that “baby” sun a protosun. The last step for the Sun was the magic that ignited it and caused it to shine. Do you remember that dust and gas swirling around that didn’t become the Sun? The disk flattened even more and the planets began to develop.


Eight planets developed and now orbit the Sun. As you move away from the Sun, you will first find four planets, then a group of small asteroids, and four large Jovian planets. There are also objects called dwarf planets that include bodies such as Pluto and Charon. In the past few years, astronomers have started to discover smaller objects beyond Pluto in the Kuiper Belt. The distance from the Sun to the Earth is considered “1” (scientists call that distance an astronomical unit).The average distance to Pluto from the Sun is 39.5. The Voyager probes launched decades ago are just now reaching the outer edges of our Solar System. That edge, called the heliopause, is far beyond the orbit of Pluto.


As the planets developed, two types began to emerge. In our system, we have planets that are mainly made of rock and those that are mainly made of gases. The official names are terrestrial (rocklike) and Jovian (those with gases). Of the eight planets in our system, Mercury, Venus, Earth, and Mars are the terrestrial planets. The Jovian planets include Jupiter, Saturn, Uranus, and Neptune. The Jovian planets are all much larger and have a lower density when compared to terrestrial planets. Astronomers have recently decided that there are objects in the Universe that are larger that asteroids and comets, but smaller than real planets. These dwarf planets also orbit the Sun and include Pluto, Charon, and others discovered in the Kuiper Belt. You may also hear the term trans-Neptunian objects used to describe those distant dwarf planets.


The Sun is larger than all of the planets put together. Everything in the Solar System orbits around the Sun. It’s mass is greater than all of the other planets combined. Even though the Sun is huge, it is small when compared to other stars in the galaxy. Even though it is smaller, the Sun provides all of the light for the Solar System. As far as astronomers are concerned, our Sun is named Sol. The entire group of Sun and planets is called the Solar System.


Everything on Earth is affected by the Sun. The Earth’s orientation to the Sun creates the seasons of the year. When your hemisphere of Earth is directed away from the Sun, it is winter. When your hemisphere is pointed closer to the Sun, it is summer. So when you’re in the Northern Hemisphere and it’s summer, kids in Australia might be skiing. As you learn more about the other planets in the Solar System, you will discover that the same idea works for most of them.

The Sun’s energy is spread around the planet, but is focused on or near the equator. That centerline of the planet is where you will find long sunny days, very little seasonal change, and the warmest ocean waters. From the equatorial regions, energy moves north and south as it circulates around the planet. That circulation can happen in the atmosphere or the oceans.


NASAs Messenger mission will soon send back new images of Mercury. If you remember anything about Mercury, remember that it is the closest planet to the Sun and really hot. Temperatures on Mercury get up to 460 degrees Celsius. An average temperature on Earth is about 15 degrees Celsius (although it has a wide range). The Sun beats down on little Mercury all day long.

The amazing thing is that there is a side of Mercury that faces away from the Sun. Temperatures on the dark side of the planet can drop to less than negative 180 degrees Celsius. It’s a whopping 640 degree temperature change from the hottest to the coldest part of the planet. The temperature ranges are a direct result of the very long days on Mercury. It takes 58 Earth days for Mercury to complete one of its days. This slow rotation affects the temperatures on the surface. Very long days allow the temperature to build for long periods of time.

As if the extreme temperatures weren’t enough, Mercury has almost no atmosphere. The loss of atmosphere also allows for extreme temperature changes. Mercury, like the Moon, is covered with craters. Because the planet has no atmosphere, the asteroids never burn off. Imagine if you put our Moon next to the Sun. That comparison helps you understand what Mercury is really like. Tons of space dust and tiny asteroids are always hitting the Earth but our atmosphere helps to burn them up before they hit the planet. Asteroids have hit Mercury for millions of years. Each hit leaves its mark like the ones on our Moon.


When Mariner 10 explored Mercury in the 1970’s we received pictures and discovered that Mercury has a weak magnetic field, but similar to Earth’s because it is a global magnetic field. Scientists think the core of the planet is made of nickel and iron. This iron acts like a huge magnet, changing the way fields interact with the planet. Something else is very interesting. It seems that Mercury lost a huge amount of its mantle/lithosphere millions of years ago. It may have hit another large object while orbiting the Sun. That fact means the layer of rock that covers the core is very thin when compared to other terrestrial planets.


Mercury is very difficult to see from Earth. Why? It is right next to the Sun. Mercury is rarely in the sky in a position where we can see it because it is only visible during the day. Also, Mercury reflects less than 10% of the light that hits it. Scientists use the word albedo to describe how much light a surface will reflect. The low albedo of Mercury tells you that the surface is very dark. We have been able to see some parts of the planet, such as the Caloris Planitia Basin. The CP Basin is a very old crater caused by an asteroid impact millions of years ago. Scientists have also seen long cliffs called scarps. They may have been created when the planet was cooling.


Special cameras have allowed us to see through the thick haze of Venus. The second planet away from the Sun is called Venus. It’s ironic that Venus is named after the Roman goddess of love and beauty. Venus is one of the harshest planets in the Solar System. It’s over 460 degrees Celsius. It has clouds of sulfuric acid in an atmosphere of carbon dioxide. Lava is found across the surface after being spewed from volcanoes. While a harsh place for you, scientists think it’s an amazing planet.


Everything we just told you is about all we know of Venus. The atmosphere is so thick on Venus that we’ve never even seen the surface. The most powerful telescope on Earth or in orbit can’t see through the thick layer of clouds that surround the planet. The Magellan probe went to Venus in the early 1990’s and sent back some interesting photos of the surface, but it had to use wavelengths of light that we can’t see. The probe used ultra-violet and infrared light. This thick atmosphere also creates a greenhouse effect that helps raise the surface temperature of the planet.

Do you remember the clouds of sulfuric acid? Those sulfur compounds in the atmosphere create Venus’ yellow color. The atmospheric reflection also makes it a very bright object in our night sky. It is easier too see with a telescope than the dark planet Mercury.

By watching the atmosphere of Venus, astronomers discovered that Venus is upside down when compared to Earth. It has a South Pole on the top of the planet and the North Pole on the bottom. Why? North and South Poles are defined by the direction a planet rotates. Since Mercury rotates in the opposite direction from Earth, its North Pole is actually on the bottom.


The Venera, Pioneer, Mariner and Magellan probes have all flown by Venus. Their instruments determined that the surface is generally smooth. The combination of an active atmosphere and lava flows creates conditions that wipe away evidence of scrapes and scratches. The probes discovered some craters, but there are no mountains. This evidence allowed scientists to conclude that there is very little possibility of plate tectonics.


Earth is the third planet from the Sun The third planet from the Sun is your home. The Earth is the only known planet where life can survive. As far as we know, there is no other planet in the universe like Earth. We have a very narrow temperature range that allows water to remain a liquid. Life has developed over millions of years because of that liquid. What else makes us special? Most of our atmosphere is made of nitrogen (N), a relatively inert gas. If we had clouds of sulfuric acid or methane (like other planets), life may have never developed.


There are also huge landmasses on our planet. The rock plates that float across the surface are called tectonic plates. Those plates float on the liquid region called the mantle. The mantle is an area between the core and the crust that is filled with molten rock. It is kept in a liquid state because of the energy given off by the center (core) of the Earth. Scientists have also discovered that pressure increases as you move towards the center of the planet. The core of the Earth has extreme temperatures and pressures that keep the iron (Fe) and other metals liquid and flowing.

Image of Aurora Australis created over the southern hemisphere


Flowing metal in our planet helps create something called a dynamo effect. Dynamos create large magnetic fields. In the case of the Earth, the magnetic field protects our planet from space. This protective cover is called the magnetosphere. It shields us from the solar winds and solar radiation. You can see where solar winds and the magnetosphere collide when you see the Aurora Borealis (northern hemisphere) and the Aurora Australis (southern hemisphere).


Although many planets in the Solar System have atmospheres, ours protects us from space and encourages life. With an atmosphere made up of 78% nitrogen (N), 21% oxygen (O), and 0.03% carbon dioxide (CO2), life has thrived on this planet. Our atmosphere has many layers divided by different temperatures and pressures. The atmosphere also provides the planet with protection. The ozone (O3) that surrounds Earth filters out ultra-violet light. The density (thickness) of the atmosphere helps to vaporize many solid particles colliding with the planet. As you can tell, the atmosphere serves many purposes.


NASA has several Mars missions planned for the next decade. Mars is the fourth planet from the Sun. It is a very active planet like the Earth. It has evidence of volcanoes, plate tectonics, and liquid water (as opposed to ice) on the planet. It even has polar ice caps like the Earth, with water in the north and solid carbon dioxide in the south. With all of these similarities, Mars is still nothing like Earth. It only has 40% of our gravity because the mass of the planet is so much smaller. Also, its atmosphere is made up of carbon dioxide and has less than 1% of the atmospheric pressure of Earth.


Scientists still hold out hope for life on Mars. Mars had lots of water. Right now, there are ice caps on the surface. At one time, there may have been water in liquid form under the surface of the planet. Recent discoveries by the Mars Exploration Rovers have proven the existence of water by discovering hematite. Where there is water, there could be life. Mars also has weather and a heat source. The atmosphere circulates around the planet and there are volcanoes on the surface.


Many probes have visited Mars through the years because it is so close to Earth. Viking, Pathfinder, Sojourner, and Mariner are only a few. The Viking Lander checked out the surface of Mars in the 1970’s. It sent back some great pictures of the red surface of the planet. The surface is a reddish color because the rocks and crust are chock full of iron compounds. For several years, the two Mars Exploration Rovers have been studying the surface rocks in detail. They have been able to travel to many locations in a small area. With patience, the rovers have made many new discoveries about the geologic history of Mars.

All of these probes have also shown us a great deal of the Martian surface. Mars has the largest volcano ever discovered. It has been named Olympus Mons and is one of the most amazing features of the planet. The Mars Reconnaissance Orbiter will soon photograph the location in higher resolution than before. The probes have also discovered many channels that cross the surface. Those channels may have been created by flowing water and erosion on the surface and the mountains.


Jupiter's Moon Io is tiny when compared to the planet. Jupiter is the largest planet in our Solar System. Its mass is over 300 times the mass of the Earth. If you look at its atmosphere and over 20 moons, you could say that Jupiter is almost like another Sun. There is only one thing missing — Heat. There is a magical time in a star’s life when nuclear reactions start and the star begins to burn. Jupiter never got to that point.

Jupiter is classified as a Jovian planet. The Jovian planets are gas giants that orbit the Sun. Gas giants don’t have solid surfaces of rock like other planets. You will find some rock at the core of the planet, but it is very little when compared to the planet as a whole. As you move deeper into the planet, you find very dense gases that have become liquids. All of the Jovian planets are huge compared to the Earth but tiny when compared to the Sun. Jupiter has a diameter of 85,000 miles while Earth is only 7,600 (the Sun is about 870,000 miles). Jupiter is often one of the brightest objects in the sky because of its size.


As you know, the bigger mass you have, the greater effect you have on the surrounding area. Well, Jupiter is huge. There is so much gravity and such a strong magnetic field that scientists discovered hydrogen gas in the outer atmosphere. No big deal. As you go deeper into the atmosphere the pressure and magnetic fields increase. Eventually you get to a layer where you find molecular hydrogen and finally liquid hydrogen right near the core of the planet. It takes an enormous amount of pressure to compress hydrogen into a liquid form.

Just because a planet is big doesn’t mean it can’t have weather and storms like Earth. On Jupiter, the storms are as big as the Earth. When the Voyager probe flew by Jupiter in the 1970s, it took some great pictures of the huge storms that last for hundreds of years. Astronomers can see the swirling and the colored bands because of the different colored compounds in Jupiter’s atmosphere. You’ll find a lot of hydrogen, methane, and ammonia gas.


Saturn isn't the only planet with rings in the solar system. Saturn is the other big planet in our Solar System. You will find its orbit just outside of Jupiter in the sixth position. It is a gaseous planet like Jupiter and those gases give Saturn a very low density. The big astronomy joke is that if you could find a lake big enough and put Saturn in the water, it would float. Its density makes it lighter than water. While knowing Saturn’s density is a fun fact, we know that the thing you will remember is that Saturn is the planet with the big rings. While other planets in our Solar System have rings, Saturn’s can be seen from Earth. The Cassini spacecraft is studying the moons and rings of Saturn for several years.


Saturn is the second largest planet in the Solar System. It’s huge compared to Earth. When you think of a day on Earth, it is 24 hours. A day on Saturn is only about 10.7 hours. It is amazing that a planet that has a diameter almost 10 times that of the Earth spins more than two times as fast. The speeds at the equator are staggering. That speed also gives Saturn a bulge at its equator. It is not like a sphere at all.

Scientists have also discovered that Saturn is like Jupiter in that it has metallic hydrogen swirling through its atmosphere. Its atmosphere is almost 99% hydrogen. There are also helium and ammonia in small amounts. The ammonia gives Saturn its tan color. Again, like other Jovian planets, Saturn does not have a surface like other planets. Its rocky core is small compared to the ice and hydrogen layers.


Only a few probes have made it to Saturn. Pioneer and Voyager I and II all did flybys of the planet and rings. They confirmed a special fact about Saturn and some other Jovian planets. Saturn is self-luminous. Self-luminous means that it gives off more light than it receives. The Sun gives off a certain amount of light and some of it makes it to Saturn. Saturn’s composition actually makes it glow, so its brightness is greater than the amount of light it reflects.

Currently, the Cassini-Huygens probe is studying Saturn. About six months after the probe’s arrival, the Huygens lander was released to study the atmosphere and surface of Saturn’s moon Titan. Titan is the only moon in our Solar System that has a measurable atmosphere. As the lander descended, it sampled the atmosphere and discovered rivers of liquid methane on the surface of the planet.


We don't have many good pictures of the gas giant Uranus. The seventh planet from the Sun is Uranus. William Hershel discovered it in 1781. It is one of the four gas giants in our Solar System, but is much smaller than both Jupiter and Saturn. You will find that Uranus is similar in some ways to Neptune, the eighth planet. Only one Voyager probe has flown by Uranus so very little is known about the planet.

Like other planets we discussed, Uranus is a planet largely composed of gases. It has a light blue color because of the methane in its atmosphere. The atmosphere of hydrogen, helium, and methane is constantly swirling around the planet. As you move from the core to the surface of the atmosphere, you will discover liquid rock, ice, and molecular hydrogen. There is no layer of metallic hydrogen like Jupiter and Saturn.


Scientists also discovered something very special about Uranus’ axis. Instead of orbiting on an axis like the other planets (up and down), Uranus spins almost on its side. When you see pictures of the Earth rotating, you see it spinning a little off-center. Uranus spins on a horizontal axis, not a vertical one.


Astronomers have also determined that Uranus is smooth. There is so much going on in the atmosphere and so little activity in the core that the surface doesn’t change. If an asteroid hits the planet, the swirling storms wipe the craters away. Although the storms swirl around the planet, there are few distinct clouds. The atmosphere moves as a constantly mixing soup.


Neptune is the most distant gas giant from the Sun. Neptune is the eighth planet form the Sun. It wasn’t discovered until 1846. It wasn’t even actually seen with a telescope for several years after that. Astronomers noticed some funny movements in the orbit of Uranus. The changes in the expected orbit were so large than they decided another planet must exist. They made the calculations, looked in the right place, and found Neptune.


Neptune is huge in size compared to Earth. When you compare it to the other gaseous planets like Jupiter and Saturn, it is the smallest. Like the other Jovian giants, Neptune’s atmosphere is made up of hydrogen and helium. It also has large amounts of methane that give it a deep blue tint. It is very similar to Uranus.

See that big swirl in the atmosphere? Just like Jupiter, Neptune has large storms swirling in its atmosphere. When Voyager II passed by, it took pictures of a storm that was big enough to hold the Earth. The storm was moving at more than 500 miles per hour. The speeds inside of the storm could have been more than seven times the speed of the fastest winds on Earth.


Voyager II was the only probe that ever made it to Neptune. It was able to fly by on its way out of the Solar System. It discovered new moons around the planet, bringing the total to eight moons. Its photos also confirmed that Neptune has very thin rings around the planet.


Even with advanced telescopes, Pluto is still difficult to see. We didn’t even know that Pluto existed until the 1930’s. Astronomers studied the orbit of other planets in the area and noticed a little wobble. The wobble was big enough for astronomers to start looking for a source. Eventually Clyde Tombaugh found Pluto. In recent years, additional large bodies have been discovered just beyond Pluto’s orbit.

For such a small object, there is a lot of debate about Pluto. Its size is one of the big reasons that astronomers stated that it is no longer considered a true planet. The International Astronomical Union (IAU) met in August 2006 and decided that Pluto would be classified as a dwarf planet. The change comes because of Pluto’s size and eccentric (strange) orbit. It passes in an out of the orbit of Neptune and doesn’t orbit in the same plane as the other planets. It’s a weird little object. Astronomers decided that this weird little object would no longer be a planet. Even though a small object, Pluto has its own satellite (moon) named Charon.


If you say the distance from the Earth to the Sun is 1. Then the average distance from Pluto to the sun is 39.5. That is a really long way. When you are an object that far away from the Sun, you are missing some things. First, there will be very little light. Second, you will get very little energy from the Sun and your planet is going to be mighty cold. Scientists speculate that the average temperature on Pluto is 37 Kelvin. Ice freezes at 273 Kelvin. Brrrr.

Pluto’s orbit is also special. It crosses Neptune’s orbit and is closer to the Sun for several years at a time. This more elliptical orbit has scientists wondering if it is really the largest object in the Kuiper Belt. The Kuiper Belt is the home of thousands of small objects that orbit the Sun beyond Pluto. The recent decisions by the IAU will term many round bodies to be called dwarf planets.


So is it a Jovian or terrestrial? Gas or rocky? Right now, it’s neither. If you had to force it, lean towards terrestrial. The Pluto’s diameter is only about 60% the diameter of the moon. It is so small that it is able to rotate once every six days. Even though small and incredibly cold, astronomers believe it has a very thin atmosphere of nitrogen. In January of 2006, the New Horizons mission launched to study Pluto and objects in the Kuiper Belt. The spacecraft should reach Pluto by 2015 and the Kuiper belt in 2016.


There are many more objects in our solar system than the planets. If you finished the main Solar System section, you now understand that our system has one star and nine accepted planets. Given that basic information, we want to let you know that there is much more in our little system than those ten objects.


Sure, you can see a planet. They are tough to miss. As you continue exploring, you will find moons around almost every planet. While the Earth only has one, other planets have more than twenty moons. You also know that you can see rings made of small pieces of dust and ice that circle Saturn and other gas giants.

As you look for smaller objects in the system, you will find a field of asteroids in orbit around the Sun between Mars and Jupiter. There are also stray asteroids flying through the Solar System. Comets are also found orbiting the Sun. The amazing thing is that there may still be other objects in our Solar System. That’s one of the exciting things about astronomy… There are still millions of things to discover in the universe.


There are also many things we can’t see in the Solar System. There are small particles and energies swirling throughout the system. Many of these unseen energies are created by the Sun. We have a heliosphere that surrounds our entire system and Voyager has just reached the heliopause (the edge). The Sun also gives off small particles called the solar wind, light, and various types of radiation. While Earth needs to be protected from some of these forces, the energy that creates the heliosphere is protecting every object in our system.


Its a cold and empty universe for a single asteroid orbiting the Sun. Asteroids are different from comets. They are like small pieces of planets. Some asteroids that orbit planets are even considered moons. One of the moons of Mars (Deimos) is only four miles across. In our Solar System, you will find asteroids orbiting the Sun in a regular orbit (not like comets, which have stretched elliptical orbits). There is also an Asteroid Belt between the orbits of Mars and Jupiter. The belt holds small pieces of rock that spin around the Sun in a specific orbit. It is almost like a planet that never formed.


Asteroids are made of rocks and metals. Some astronomers consider them to be minor planets (like the moons of Mars and Saturn). Most asteroids are very small but they do have gravity and can affect any objects that come too close. The more iron (Fe) and nickel (Ni) in the makeup of an asteroid, the greater its mass (and gravity). Scientists also use the amount of metal in asteroid classification. Using an infrared sensor, asteroids are classified as light or dark. The lighter ones have more metal than the darker ones.


Beyond our Solar System, asteroids travel through the galaxy from system to system. These asteroids do not necessarily orbit a star but stars influence their direction and speed. Many astronomers believe that asteroids are pieces from the origins of the universe. Some asteroids were large enough and lucky enough to combine with others and form planets. Some asteroids just continued their existence floating through the universe.


Chances are the Earth doesn’t have to worry about a collision with a comet. Asteroids are another matter. Scientists already think that a large asteroid may have hit the Earth when the dinosaurs were alive. That collision may have changed our atmosphere and led to the extinction of the dinosaurs.

We just told you about that asteroid belt just outside Mars’ orbit. There may come a day when one of those asteroids drops out of orbit (maybe from a collision with another asteroid) and heads toward Earth. If it is small enough, it will burn up in the atmosphere. Larger ones will hit the surface of the planet. Hundreds of millions of years ago, collisions with asteroids happened more often. Over time, the number of asteroids in the path of the Earth decreased and collisions became less frequent.


The surfaces of comets become more active as they approach the Sun. Comets are small chunks of ice and dirt that fly through systems in long, wild orbits. They are created when a planetary system develops. Nearly all comets orbit a star while asteroids may move through the galaxy as remnants from the creation of the universe. Comets are closely related to the formation of systems. They are left over pieces of frozen gases and water that were never absorbed by the forming planets.


You may know that planets move in elliptical orbits around their sun. An ellipse is a shape that is like a narrow circle. Comets move in very narrow and stretched out ellipses. They sometimes cross the orbits of several planets on their trip around their sun. Comets also have tails. Those tails are streams of gas and dust that follow the comets for thousands of miles. Comets aren’t really on fire. The light you see is reflected light from the closest star. The light is reflected off the gases, ice, dirt, and dust that travel with the comet.


Every comet is special and unique. They each have distinct orbits around a star and a unique chemical makeup. Even though they are all different, scientists categorize them by how long it takes them to complete an orbit around the Sun. There are long-period comets and short-period comets. Long-period comets usually take over 200 years to orbit the Sun. Short-period comets make it around in less than 200. Halley’s Comet is famous and orbits the Sun once every 76 years. Comet Kohoutek has a period of 75,000 years.


In the same way that we send probes to look at other planets, we also spacecraft to study comets. In 2004, the Stardust mission encountered Comet Wild2 and collected samples. Astronomers hope to learn more about the origins of the Solar System and the universe from those particles. The most interesting compounds will be those that contain carbon because they may hold the keys to the origins of life. Comets could have hit the Earth millions of years ago and helped seed organic (carbon-based) life. The Deep Impact mission sent a projectile to collide with a comet. The spacecraft then analyzed the scattered dust and ice.


Our asteroid belt can be found between the orbits of Mars and Jupiter. The Solar System has eight planets, smaller dwarf planets (like Pluto), and a star. Between the fourth (Mars) and fifth (Jupiter) planets is a special place. Astronomers call this region the asteroid belt because there are millions of tiny asteroids in this region. These objects circle the Sun just like a planet might. Sometimes they fall out of orbit and head towards the Sun, crossing the orbital path of the Earth.

As we said, the asteroid belt is made of millions of pieces of rock and dust. There could be over 40,000 larger ones. The largest discovered asteroid is about 900 miles across. As time passes, the size of the asteroids will decrease. As they orbit the Sun, they constantly hit each other and break into smaller pieces. Those collisions are sometimes the cause of asteroids being knocked out of orbit.


Astronomers think that the asteroids are leftover pieces of the Solar System. As other large objects in the system combined to form planets, there were some smaller pieces left over. Their location between Mars and Jupiter could also influence the pieces. They may have never formed a small planet because of the gravitational pulls of Jupiter (very strong) and Mars (very weak). The pulling of the two planets keeps the asteroids separated and circling the Sun in a doughnut shaped area.


Astronomers think there may be other very small asteroid belts in our system. Many small pieces of rock are in orbit between Earth and Mars. There are also pieces in orbit between Mercury and the Sun. The smaller fields are not as large as the one outside of Mars, but they do seem to hold a larger number of asteroids than the very empty areas of the Solar System. There are even hints that other systems have similar fields of asteroids.


There may be many small dwarf planets beyond pluto in the Kuiper belt. Is there anything beyond Pluto? Yes. Thousands of small objects orbit the Sun beyond Pluto. Most are found in an area called the Kuiper Belt. You will find asteroids, comets, dwarf planets, and other small solar system bodies. The objects you find out there are also called Trans-Neptunians (beyond Neptune). Astronomers have only begun to examine the area since 1992. It takes special probes and telescopes to study the area.

The Kuiper Belt is like the asteroid belt in some ways. A very large area looks like a flattened doughnut. The shape is also called a toroid. Astronomers believe the area holds pieces that have remained the same since the beginning of the Solar System. You start to encounter objects in the Kuiper Belt at about 50 times the distance from the Earth to the Sun. The edge of the Solar System is about 100 times the distance from the Earth to the Sun.


As we said, there are many asteroids found in the Kuiper Belt. Many of them have diameters more than 50 miles across. Astronomers also believe that many short-period comets begin their lives in the belt. An example of a short-period comet is Halley’s Comet with a period of 76 years. Short-period comets make a complete elliptical orbit in less than 200 years. Comets that take longer to complete a trip around the Sun are called long-period and are usually found in the Oort Cloud.


As time passes, astronomers will learn more about the objects in the Kuiper Belt. They currently have records of over 70,000 objects (mainly asteroids). Because they are so far away, they are very difficult to study. Imagine trying to look at a black rock (in black space) that is only 50 miles across from about 46.5 billion miles away. That’s what scientists have to deal with.

In January 2006, the New Horizons mission was launched to study Pluto and objects in the Kuiper Belt. The spacecraft should reach Pluto in 2015 and then continue through the belt in 2016. There are many discoveries to be made in the farthest reaches of our Solar System.

-Now, enjoy these fantastic activities and videos about the universe and the solar system! (Build your own solar system) (An interactive solar system) (Click a planet to choose your destination) Journey to the Edge of the Universe

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