Our Solar System: One in a Hundred Billion?
Planet Earth is part of a vast space neighborhood
called the solar system. Our solar system is an amazing place. It’s even
more amazing that at least 70 other solar systems have been discovered. And
there could be lots more. Scientists estimate there could be hundreds of
billions of solar systems in our Milky Way galaxy.
Our solar system is one of many groups of planets (planetary systems) that
call the Milky Way Galaxy home.
We may never know the actual number, but all solar systems work the same way.
Planets and other space objects revolve around a star. In our system, the star
is the sun. Eight major planets and their moons,
three “dwarf” planets, and countless asteroids, meteoroids, comets,
and other objects whiz around it. Each object travels in its own egg-shaped orbit.
The shape is called an ellipse.
It Began with a Bang
About 15-20 billion years ago, there was
an enormous explosion called the Big Bang. Whirling clouds of dust and gas
filled the Universe. Over time, gravity made
some clouds start to clump together. Big clumps formed galaxies. Smaller clumps
Then, about 4.5 billion years ago, the clouds that formed our solar system
spun so fast they formed a disk. The center of the disk became so hot it dropped
out. That center became the sun. The dust and gas that didn’t get sucked
into the center stuck together in clumps. These clumps became planets. Our
solar system was born.
The Big Star of Our Show
The eight planets of our solar system revolve around a large star called
The sun is
the center because it’s HUGE. More than one million Earths could fit
inside it. It contains more than 99.8% of all the mass in
the solar system. Its size gives it the gravity it needs to hold the solar
system together. The sun’s gravity pulls the objects down while they
keep trying to move away. It’s a never-ending tug-of-war that keeps the
planets in their orbits instead of flying off into space.
Let’s Take a Spin
It takes Earth 366.6 days to make one revolution around
the sun. That’s about a year. But a year on Earth is not the same as
a year on another planet. The length of a planet’s “year” depends
on its size and its distance from the sun. Smaller planets travel faster. Planets
far from the sun have a longer trip than those close in.
While the planets spin around the sun, each one also spins around its own axis.
The time it takes a planet to make one complete turn, or rotation, is called
a day. A day on Earth is 23 hours and 56 minutes. Days on other planets are
longer or shorter.
Mars Rocks. Jupiter Is a Gas.
Earth is one of four planets in our solar system that have solid, rocky
surfaces. These planets are called terrestrial planets.
©Lunar and Planetary Institute
Mercury, Venus, Earth and Mars are called
the “terrestrial” planets. Terrestrial comes from the Latin word terra,
which means “earth.” All these planets have solid, rocky surfaces.
They’re also called the “inner” planets.
Jupiter, Saturn, Uranus and Neptune are called the “gas giants.” They
do not have solid surfaces. They’re made of dense clouds of gases. These
planets are also called the “outer” planets.
Pluto was once the ninth planet in our solar
system. But in 2006, scientists kicked it out of the lineup. Why? They had
come up with new rules about what makes a “real” planet. Pluto
just didn’t measure up.
The rules are:
- A planet must be shaped like a sphere.
- It must orbit the sun.
- It must have enough gravity to keep it from being pulled into another planet’s
- It must have enough gravity to pull in rocks and other space “junk” orbiting
Pluto is one of three known dwarf planets in our solar system.
Pluto is shaped kind of like a potato. It has a weird orbit that sometimes
crosses into Neptune’s. And it has a lot of debris orbiting around it.
So scientists reclassified Pluto as a “dwarf” planet, along with
Ceres and Eris.
There are more than hundreds of thousands of asteroids in our solar system
and most of them are found within the main asteroid belt.
What else orbits the sun? Asteroids are loose
chunks of rock that scientists think may be left over from the formation of
the solar system. They may be pieces of a planet that broke apart, or just
a small clump that never merged with bigger ones. Most of the asteroids orbit
the sun in a region between Mars and Jupiter known as the Asteroid Belt.
Meteoroids are smaller than asteroids. They may be pieces of planets that
flew off during a space collision. They could be pieces of dust left behind in
the trail of a comet. Or they could be pieces of asteroids that broke apart.
Meteoroids burn up when they enter the Earth’s atmosphere. The
bright trails they leave behind them are called meteors, or “shooting
stars.” Larger pieces that don’t burn up completely
in the atmosphere and make it to the surface of the Earth are called “meteorites.”
Comet McNaught was the brightest comet to shoot across the sky in the Southern
hemisphere in 40 years.
Comets are sometimes called “dirty snowballs.” They’re made
of icy material mixed with rocks and dust. Scientists think that comets,
like asteroids, are left over from when the solar system formed.
Watchers and Wanderers
The earliest astronomers studied
the sun, stars, and planets as the complete universe. They believed Earth was
the center of everything. They noticed that stars did not change positions
except to move as one across the sky. They noticed other star-like objects,
too. These objects seemed to wander among the stars. So they named the
objects “planets,” after the Greek word for “wanderers.”
In the early 1500’s, astronomer Nicholas Copernicus had an idea that
made people very angry. He said that the sun, not Earth, was the center of
the solar system. People didn’t like the idea one bit. How could the
sun be more important than Earth? Copernicus’s theories were unpopular,
but he started modern astronomy.
Later, Galileo Galilei was among the first astronomers to observe the planets
with a telescope.
Twinkle, Twinkle, Little . . . Planet?
Today powerful telescopes let us see things early astronomers could only dream
of. Spacecraft and satellites send back astonishing images from the outer edges
of the solar system. Still, you can study the planets with your own eyes. But
how can you tell if you’re looking at a star or a planet?
Astronomers use powerful telescopes to study the sun, stars, moons, planets,
and other bodies in space.
The sun is our closest star. Its light is close enough to brighten and warm
our world. But light from other stars comes from very, very far away. By the
time it reaches us, it’s just a tiny pinpoint. As we look through the
atmosphere at that point of light, it gets shifted around by all the “stuff” going
on in the atmosphere. That’s what makes it twinkle. Planets
are closer. Their light isn’t so “pointy.” It doesn’t
get shifted around as much. So if you watch a star for a while and it
doesn’t twinkle, it just may be a planet!