# Wikijunior:Solar System/Solar System

{{  }} !The Hubble Space Telescope. This
telescope is in space. It takes pictures of things that are too far away
to be seen with a regular
telescope.
Do you ever wonder about the things in the sky---the Sun, the Moon, the
stars? People have been watching the sky for a long time, trying to
figure out what is out there. We keep coming up with new ways to learn
more about outer space.

*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*. Far beyond our own Solar System are stars, bodies
like the Sun but in some cases much bigger.

!The eight planets of the Solar System, and the sun. Sizes are to
scale, but distances are
not.{width="180"
height="200"} Thousands of years ago, a man named **Aristarchus** said
that the Solar System moved around the Sun. Some people thought he was
right, but many people believed the opposite: that the Solar System
moved around the Earth, including the Sun (and even the other stars).
This seems sensible because the Earth doesn\'t feel as if it\'s moving,
does it?About 500 years ago, another man named **Copernicus** said the
same thing as Aristarchus: that all the planets moved around the Sun and
the stars were fixed in space.[^1] This time, more people agreed, but
there were still people who thought the opposite. Then, about 100 years
later, a man called **Galileo Galilei** 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. This time, even more people thought
Galileo may be right and that the Earth really did move around the Sun.
Soon, more and more people started using telescopes to study the sky.
However, there were still some people who thought Galileo was wrong, and
he was even arrested and taken to court for lying. All the people who
believed him began to learn how the planets and the other things in the
Solar System moved so they could prove that he was not lying. Thousands
of years after Aristarchus, people finally said \"Okay, the Earth does
move around the Sun\". Galileo couldn\'t be called a liar anymore.[^2]

We can use very large telescopes to see what has happened to other
stars. We can compare pictures of distant stars with pictures of our own
star, the Sun. We live in exciting times because, for the first time, we
have sent people into space, and we also have telescopes in space. These
telescopes in space take thousands of pictures of the planets, our sun,
and the distant stars. On Earth, people use the photos to learn about
all the different things in the Solar System and they try to explain how
the Solar System began. We even have a robot on the red planet Mars that
moves around, and people on Earth tell it where to go and what to
photograph. We also want to know what will happen to the Earth and the
Solar System in the future.

## What is in the Solar System?

!The Solar System, showing the Sun, inner planets, asteroid belt, outer
planets, an outer dwarf planet, and a comet. (Not to
scale!)"){width="250"}
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.[^3]

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,[^4] and neither does Venus. Earth has one. Saturn has more than
80![^5]

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
(smaller than a normal planet) named Ceres. Then come the *outer
planets*: Jupiter, Saturn, Uranus, and Neptune. Farther out there are
two dwarf planets, Pluto and Eris.

The planets have the names of Roman gods that were worshipped by people
thousands of years ago, though no one believes in them now. Did you know
that some days of the week are also the names of ancient gods? Saturday
means \"Saturn Day\". Thursday means \"Thor Day\". Thor was a Viking god
and the son of Odin. Monday and Sunday simply mean \"Moon Day\" and
\"Sun Day\". Some of the months are also named after Roman gods. The
month of \"March\" is named after the Roman god \"Mars\"---he was the
god of War!

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.[^6] (Yes, \"Oort\" was the last name of the person
who first wrote about it.)

!Zodiacal
light.{width="200"
height="200"} 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* or *zodiacal light*.[^7]

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.[^8] Beyond that, there 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 and empty
place![^9]

### What holds it together?

!Sir Isaac Newton, the discoverer of gravity. It is said that he came
up with the concept of gravity when an apple fell on his
head..jpg "Sir Isaac Newton, the discoverer of gravity. It is said that he came up with the concept of gravity when an apple fell on his head."){width="200"
height="220"} Why do all of the planets orbit the Sun? Why do moons
orbit planets? Why doesn\'t the Sun move away 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 orbits the center of our galaxy.
The whole thing stays together because of the force of gravity[^10].

#### About mass

Everything is made of matter. The amount of matter is called mass. Two
apples have twice the mass of one apple. 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 an apple because it is so much less than the
pull from the Earth. If you stand on the ground and let go of an apple,
gravity will pull it down towards the center of the Earth. It will hit
the ground. If you could throw the apple 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 apple 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 when very close to that
thing and weaker when 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.[^11]

## 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 about 1500, Nicolaus 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 and only published a full account of his ideas in 1543,
the year of his death.[^12] 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.[^13] Now, almost
everyone on Earth understands that we live in a Solar System. !The
largest objects at the farthest reaches of the Solar System, even
farther than
Neptune!{width="200"
height="200"} People made better telescopes and found more things in the
sky---moons,[^14] new planets,[^15] and asteroids.[^16] More things,
like the dwarf planet Eris, are being found today.[^17]

## How have we explored the Solar System?

!The *Voyager 2*
spacecraft.{width="250"}
!An artist\'s impressions of
*Spirit*.{width="250"}
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. *Observe* is a
more scientific word for *watch*. They observed the Sun and stars to
tell the time of year. In China, they even knew when the moon would
block the Sun[^18]. Most people thought that *celestial bodies* could
cause war or peace on Earth.[^19]

After telescopes were first made in the early 17th century, 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.[^20] 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.[^21] Then they made telescopes better and sent
spacecraft into space, and found that there were no plants or animals on
the Moon[^22] or on Mars.[^23]

Now, we can explore by going to some of the other worlds. Twelve
Astronauts walked on the Moon about 35 years ago. They brought rocks and
dirt back to Earth.[^24] Spacecraft flew by Venus, Mars, and the outer
planets. The pictures they took showed us a lot of what we know about
these worlds.[^25] Robots landed on Mars in 1971, 1976, and 1997. They
took thousands of pictures of the planets. They send photos and movies
back to Earth. They also check rocks to find out what they are made
of.[^26]

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.[^27]

## How was our Solar System formed?

Our Solar System is part of the Milky Way *galaxy*. Galaxies are big
mixes of dust, gas, stars, and other things. Inside our Milky Way galaxy
are clouds of dust and gas where stars are born. Our Solar System was
created in this kind of cloud. A part of the cloud began to get smaller
and less spread out. It formed a big, spinning disk of gas and tiny
pieces of dust. This disk was thickest in the middle. The middle slowly
collapsed until it became the Sun. We are still trying to learn how the
planets were formed. Most scientists think that they were formed from
leftover gas and dust.

200x200px\|framed\|left\|An artist\'s illlustration of how the Solar
System began. This is how it
could have happened. The rest of the disk continued to spin around the
Sun. The tiny pieces of dust hit each other, and some of them stuck
together. Next, the bits of dust slowly collected to form grains, which
in turn joined to form lumps the size of gravel, then pebbles, and then
rocks. The rocks crashed together into mountains. The mountains crashed
together to make bigger things. These big things swept up most of the
rest of the disk to form the planets, moons, and asteroids.[^28]

The Sun got hotter as it collapsed. It began to glow. The temperature at
the center reached a million degrees Celsius. The Sun started to produce
a lot of light and heat. This light and heat swept away most of the
leftover dust and gas between the inner planets. This light and heat are
the sunlight we see and feel every day on Earth. [^29]

## What will happen to the Solar System?

In another five billion years, the Sun will use up most of its hydrogen
fuel. It will enter the final stages of its life. The middle of the Sun
will shrink down and become even hotter. The outer layer of the Sun will
grow much bigger than it is now. It will form a *red giant*.

It will be so big that Mercury and Venus, probably Earth and maybe even
Mars will be inside it. These planets will burn away. Which planets get
destroyed will depend on how much mass the Sun has left.[^30] A strong
solar wind will blow some of the outer layers of gas away from the Sun.
The Sun will have less mass. The Sun's gravity will be less. All of the
planets will move further away from the Sun.[^31]

!The massive, rapidly-aging star *Eta Carinae* throws off a giant cloud
of gas, forming a *planetary
nebula*.{width="250"}

After it has been a red giant for a while, the Sun will start to burn
*helium*. It will shrink down and will not be a red giant any more. It
will use the helium up in about a billion years. Then it will become a
red giant once again. More gas will blow away for a few hundred thousand
years.

A *planetary nebula*[^32] will form. The nebula could last for a few
thousand to a few tens of thousands of years. It will glow in the light
of the Sun.[^33]

At the center, the Sun might shrink into a tiny star called a **white
dwarf**. That kind of star is about the size of Earth. It would take
about 100 of these white dwarfs, stacked end to end, to be as wide as
the Sun is today. The Sun will not have any more fuel to burn. It will
have lots of heat left over and will keep getting cooler and dimmer.
Then its light will go out in a hundred billion years from now.[^34]

Next Topic: The Sun

## Notes

```{=html}
<div style="clear:both;">
```
```{=html}
</div>
```
## References

bs:Wiki junior Sunčev sistem/Sunčev
sistem
bg:Уикиджуниър Слънчева система/Слънчева
система
de:Wikijunior
Sonnensystem/\_Sonnensystem
es:Wikichicos Sistema Solar
fr:Wikijunior:Système
solaire fi:Wikijunior
Aurinkokunta it:Wikijunior Il
sistema solare

[^1]: <http://www-spof.gsfc.nasa.gov/stargaze/Ssolsys.htm#q21>

[^2]: <http://www-spof.gsfc.nasa.gov/stargaze/Ssolsys.htm#galileo>\
    See also
    \

[^3]: <http://imagine.gsfc.nasa.gov/docs/science/know_l1/sun.html>

[^4]: <http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mercury&Display=Moons>

[^5]: <https://solarsystem.nasa.gov/moons/saturn-moons/overview/?page=0&per_page=40&order=name+asc&search=&placeholder=Enter+moon+name&condition_1=38%3Aparent_id&condition_2=moon%3Abody_type%3Ailike>

[^6]: <http://solarsystem.nasa.gov/index.cfm>

[^7]: <http://www.gsfc.nasa.gov/scienceques2001/20020301.htm>

[^8]: <http://antwrp.gsfc.nasa.gov/apod/ap020624.html>

[^9]: \"Outside Our Solar System\" in
    <http://vathena.arc.nasa.gov/curric/space/spacover.html>

[^10]: \"Gravity is the force responsible for keeping the Earth and
    other planets in our solar system in orbit around the Sun.\" from
    Cosmic Glue,
    <http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970108b.html>

[^11]: Definitions of Mass, Gravity, and Weight from
    <http://ksnn.larc.nasa.gov/webtext.cfm?unit=float>

[^12]: <http://www-spof.gsfc.nasa.gov/stargaze/Ssolsys.htm#q21>

[^13]: <http://www-spof.gsfc.nasa.gov/stargaze/Ssolsys.htm#galileo>

[^14]: Calinger, Ronald S. \"Huygens, Christiaan.\" World Book Online
    Reference Center. 2004. World Book, Inc.
    <http://www.worldbookonline.com/wb/Article?id=ar268300>.;\
    <http://www.nasa.gov/worldbook/huygens_worldbook.html>

[^15]: <http://solarsystem.nasa.gov/planets/profile.cfm?Object=Uranus>

[^16]: <http://solarsystem.nasa.gov/planetselector.cfm?Object=Asteroids>

[^17]: <http://www.space.com/scienceastronomy/050729_new_planet.html>;\
    <http://science.nasa.gov/headlines/y2005/29jul_planetx.xml>;\
    <http://www.jpl.nasa.gov/news/news.cfm?release=2005-126>

[^18]: Eclipse2001 museum <http://museumeclipse.org/about/history.html>

[^19]:
        \
    page 339 \"The Chaldeans \... were also the first to suspect\...
    that the Sun, the moon, the planets and the constellation of stars,
    all affect human life and destiny\.... These beliefs gradually
    spread .. to Egypt, China, Greece, India, and Rome, for example \...
    astrology is still very popular.\"

[^20]: <http://www-spof.gsfc.nasa.gov/stargaze/Ssolsys.htm#galileo>

[^21]: <http://vesuvius.jsc.nasa.gov/er/seh/mars.html>;\
    ;\
    ;\
    From the Earth to the Moon on Project Gutenberg \--
    <http://www.gutenberg.org/etext/83>;

[^22]: <http://www.space.com/reference/mars/history.html>

[^23]: <http://www.hq.nasa.gov/office/pao/History/SP-350/ch-15-4.html>
    (bottom of page)

[^24]: <http://spaceflight.nasa.gov/history/apollo/index.html>

[^25]: <http://www.solarviews.com/eng/sc_hist.htm>

[^26]: <http://marsrovers.jpl.nasa.gov/home/>

[^27]: <http://www.nasa.gov/missions/solarsystem/Why_We_12.html>;
    <http://www.infoplease.com/spot/astronomy1.html>

[^28]: <http://planetquest.jpl.nasa.gov/science/origins.html>

[^29]: <http://rst.gsfc.nasa.gov/Sect19/Sect19_2a.html>

[^30]: Which planets may get destroyed
    <http://www.public.iastate.edu/~lwillson/FuturSun.pdf>

[^31]: Outline of Sun\'s death
    <http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html>

[^32]: A *planetary* nebula was named this because through the earliest
    telescopes, astronomers thought that they looked like planets. The
    name stuck --- but they really have nothing to do with planets.

[^33]: Planetary nebulae <http://www.seds.org/messier/planetar.html>

[^34]: Has information on white dwarf stars
    <http://math.ucr.edu/home/baez/RelWWW/tests.html>
