What do you know about our solar system's sidekicks? Take the quiz!


There are 88 objects in our solar system that are 200 miles or more in diameter, plus hundreds of smaller objects, including dwarf planets, natural satellites, comets and more. See what else you know about our solar system's sidekicks!

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Question 1 of 20

How old is our solar system?

4.6 billion years old
1.2 billion years old
3.7 million years old

... Scientists have estimated that the solar system is nearly 5 billion years old (4.6 billion, to be more or less precise). The estimate is based on the age of meteors.


Question 2 of 20

More than 90 percent of all matter in the universe is made up of ________ .


... Helium is second to hydrogen as the most common matter in our universe.


Question 3 of 20

What are dwarf planets?

objects that are too small to be considered planets officially
planets with no gravity
planets with debris around them

... Dwarf planets are not necessarily smaller than Earth and its neighbors. They are spherical like planets, but they don't have enough gravitational pull to clear their surroundings of planetary debris (which actual planets have done).


Question 4 of 20

How many moons are there in our solar system?


... Of the 166 moons orbiting the planets in our solar system, 63 of them orbit Jupiter.


Question 5 of 20

How many of the moons scientists have discovered so far could possibly sustain human life?

only one

... Scientists say there's one moon -- so far -- that might be able to sustain human life. It's Europa, one of Jupiter's moons, which has a thin atmosphere composed primarily of oxygen. Scientists believe there's a big chance that water lies beneath Europa's surface as well.


Question 6 of 20

What is Eris?

one of Neptune's moons
the largest comet in orbit
a dwarf planet

... Eris is a dwarf planet that was discovered in January 2005. At first scientists considered calling Eris "the tenth planet."


Question 7 of 20

You can NOT find ________ on the moon.


... The footprints left by astronauts never will disappear because there's no wind or rain on the moon to erase them. Contrary to popular belief, there is gravity on the moon, but it's only one-fifth of Earth's gravity.


Question 8 of 20

Comets sometimes are known as ________ .

dirty snowballs
fire stars
icy shawls

... Comets' composition of ice mixed with small rock particles and dust gave origin to their name as dirty snowballs.


Question 9 of 20

Why was Pluto downgraded to a dwarf planet?

because it was too small
because of the discovery of Eris
because astronomers discovered that it has no atmosphere

... The discovery of Eris sparked a big debate, which led to the creation of the term "dwarf planet." By that definition, Pluto actually is a dwarf planet, so it had to be downgraded to match Eris and others in the group.


Question 10 of 20

What is another name for asteroids?

dwarf planets

... Asteroids that are about 33 feet (10 meters) or more in diameter also are known as planetoids. Smaller asteroids sometimes are known as meteoroids.


Question 11 of 20

A planetary ring, such as the one around Saturn, is made primarily of ________ .

cosmic dust
rock particles

... Scientists are not sure where cosmic dust comes from, but one theory is that the dust that forms rings comes from exploding stars.


Question 12 of 20

What are centaurs?

minor transient planets
asteroids near Neptune
craters found on the moon

... Centaurs are small, transient planets. This means that their orbits are unstable. There are more than 44,000 centaurs in the solar system.


Question 13 of 20

The moons of Mars are among the ________ in the solar system.

least heavily cratered

... Mars has three moons -- Ares, Phobos and Deimos -- and they are among the smallest moons in our solar system. Phobos is about 12 miles (20 kilometers) wide and has a crater that covers nearly half its width. In fact, Phobos and Deimos are lumpy and heavily cratered.


Question 14 of 20

Do comets have atmospheres?


... Comets have temporary atmospheres when they fly close to the sun. At that point, a comet has a thin atmosphere made up mostly of celestial dust and ice. As the comet moves away from the sun again, the atmosphere dissipates.


Question 15 of 20

What's the largest asteroid in the solar system?


... Ceres used to be the largest asteroid, but since it was promoted to the category of dwarf planet, Vesta is now at the top of the asteroid list. Vesta is about the size of Oregon and Texas together.


Question 16 of 20

Except for the moons of Uranus, the moons of every other planet in the solar system are named after mythological figures. What are Uranus' moons named after?

European cities
fictional characters
historical figures

... Uranus' moons are named after fictional characters from the books of Alexander Pope and William Shakespeare. They have names such as Ophelia, Juliet and Titania.


Question 17 of 20

What is the difference between comets and asteroids?

their speeds in space
their sizes
their compositions

... There are several differences between comets and asteroids, but composition is one of the most obvious. Comets are made up mostly of ice and small particles of rock and dust. Asteroids are made up mostly of metal and rock. Both can vary in size, and to make matters more confusing: It's possible that a few asteroids are burnt-out comets.


Question 18 of 20

A comet's tail always points ________ .

toward the sun
away from the sun
neither; comets don't really have tails

... As the sun emits heat, it causes parts of the comet to melt and causes the tail to look like it's pointing away from the sun.


Question 19 of 20

Which of the following is NOT true about our solar system?

All planets, dwarf planets and satellites orbit in the same direction.
Asteroids make up most of the mass of the solar system.
The moon moves away from Earth at a rate of 1.48 inches (3.76 centimeters) per year.

... It's not asteroids, but the sun that makes up most (99.86 percent) of our solar system's mass. All of our planets -- and their manmade satellites and sidekicks -- orbit in the same direction, and the moon is moving away from Earth at a tiny but steady annual rate.


Question 20 of 20

Which of the following names describes an object that burns up as it enters Earth's atmosphere?


... A meteor burns up as it passes through the atmosphere. If it makes it through the atmosphere and crashes against the Earth, it's called a meteorite.


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