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1. Orbit the Sun.
2. Must be a “round” shape as a
result of its gravity.
3. The object must clear their
orbital path of debris. (This is
where Pluto failed)
Planets are large objects that circle a
star and do not produce light of its
own.
We can see other planets because they
reflect sunlight.
They all rotate on an axis that last one
“day.”
They all revolve around the Sun,
forming their own year.
“Planet” means wanderer.
The inner planets of our solar system are Mercury,
Venus, Earth, and Mars.
All four planets are close to the Sun and are like
Earth in size, density, and in their mostly rocky
make-up.
Because of their similarities they are known as the
Mercury is the Roman messenger of the gods.
Mercury is the closest planet to the Sun and the second
smallest of the nine planets, just a little larger than our
moon.
It has almost no atmosphere.
Temperatures vary from 425* C during the day, to -183* C
during the night. That’s a larger difference than melting
Lead and freezing Carbon Dioxide!
One Mercury year lasts about 3 Earth months. That means
that if you are ten years old on Earth, you would be 40
years old on Mercury!
Venus is the Roman goddess of love and beauty.
Venus is about the same size as Earth and is often referred
to as our sister planet.
It has a dense atmosphere with layers of thick sulfuric acid
clouds and carbon dioxide, keeping the surface
temperature at about 450* Celsius.
It is the hottest planet in our solar system.
The air pressure is 90 times greater than Earth’s air
pressure.
Venus has no known moons.
The atmosphere is made mainly of nitrogen, oxygen, and
carbon dioxide.
Water covers about ¾ of the surface.
The surface temperature rarely goes below -30* C or above
43* C.
It is the only planet in our solar system on which life is known
to exist.
Earth has only one moon, called “Luna.”
It is believed that an object the size of Mars struck the Earth
and created our moon.
It only takes 1.3 seconds for light to reflect off Luna and
reach the Earth!
Mars is named after the Roman god of war.
Mars has two moons that are smaller than our moon.
Mars appears red because of a rust-like dust blowing
around on the surface.
Temperatures range from -124*C to -31*C, or always below
freezing.
The Martian day is only 41 minutes longer than Earth’s,
and the planet has four seasons, just like us!
Mars has the largest volcano in our solar system, called
Olympus Mons.
Mars has two moons, Phobos, meaning “fear” and Deimos,
meaning “terror.”
Olympus Mons is a huge extinct volcano nearly
three times the height of the tallest mountain on
Earth.
The reason that the volcano was able to get so big
is simple. Unlike the Earth, which the surface is
made up of plates that are moving, Mars does not
have plate tectonics.
On Earth, the plates move over a “hotspot” that
form a series of volcanoes and mountains.
Hawaii, for instance, is a island chain formed from
the plate movements over a hotspot. On Mars,
this doesn’t happen, and the volcanic eruptions
The next four planets, Jupiter, Saturn, Uranus, and
Neptune, all have characteristics in common.
They are all much larger than the inner planets;
Jupiter having a diameter over eleven times that
of Earth!
They are made up mostly of substances that
would be gases on Earth. For this reason, they
are referred to as the gas giants.
Jupiter is the Roman ruler of the gods.
Jupiter is the fifth planet from the Sun.
It is the largest planet in our solar system.
It is made up mostly of hydrogen in gas and
liquid form and helium. The gases probably
surround a small, molten Iron core.
Jupiter’s atmosphere has many cloud bands
and the Great Red Spot, a huge circular storm
that’s lasted for centuries!
Jupiter has a set of rings, but they are so thin
and dark, they are almost invisible.
Jupiter has 58 known moons! Io, one of
Jupiter’s moons, is the most volcanically
active object that we know of in our solar
system. Its surface is regularly covered in
This huge storm has
lasted centuries and is as
big three Earths!
Saturn is the Roman god of agriculture.
Saturn is the sixth planet from the Sun and is
the second largest planet in the solar system.
It is best known for its beautiful rings.
Saturn has 31 moons, the largest being Titan.
Saturn is made up of similar elements to
Jupiter.
Strong winds sweep across Saturn constantly,
reaching speeds of 1,100 mph!
Uranus was the Roman god of the skies and husband to
Earth.
Uranus is the seventh planet from the Sun.
Uranus is unique, for it rotates on its side.
It is made mainly of thick, green gases.
It has 11 rings and 15 moons.
The atmosphere is hydrogen and helium.
Temperatures on Uranus are very cold, around - 215*C.
Miranda, one of Uranus’ moons, has been
called the most bizarre object in the solar
system.
Scientists think that the patchwork
appearance of its surface resulted when the
moon was torn apart by collisions and then
Neptune is the Roman god of the seas.
Neptune is the eighth planet in our solar
system.
It is the smallest of the gas giants.
Like Jupiter and Uranus, its rings are too
faint to see.
It has 8 moons, one which being Triton.
Neptune’s face has swirling blue and white
features.
No! Not
that
Pluto!
Pluto is the god of the underworld.
Pluto is the ninth planet in our solar system, or at least it
usually is!
Pluto has an unusual orbit around the Sun. For about 20
years, it is the 8th planet, as it was from 1979-1999. For
about 227 years, it is the 9th planet.
Pluto is also the smallest planet in our solar system. It is
about half the size of our moon.
Not much is known about Pluto, since no space probe has
visited it, although one is on the way.
Pluto has one known moon, Charon.
Because it is so far away from the solar system’s heat
source, the Sun, the temperature on Pluto probably doesn’t
Classifying Pluto is a difficult task for
astronomers.
It is terrestrial, or Earth-like, like the inner
planets, but is obviously the outer-most outer
planet.
It is currently classified as a dwarf planet.
There are
currently five
official dwarf
planets. Pluto,
Ceres, Eris,
Makemake, and
Haumea - for now!
Planet
Distance
from Sun
(in
millions
of km)
Diameter
(in km)
Length of
year
(in Earth
time)
Length of
day
(in Earth
time)
# of
known
moons
Mercury
58
4,880
88 days
59 days
0
Venus
108
12,100
225 days
243 days
0
Earth
150
12,756
365 days
24 hours
1
Mars
228
6,784
687 days
24.6
hours
2
Jupiter
778
143,200
11.9 years
10 hours
58
Saturn
1,429
120,000
29.5 years
10 hours
31
Uranus
2,875
51,400
84 years
18 hours
15
Neptune
4,504
49,500
165 years
18 hours
8