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There are
MANY FIELDS
of science to study our world and the rest of the universe. Astronomy is dedicated to studying the
OBJECTS
and
SPACE
that are outside our planet.
SCIENTISTS
who explore, study, learn and teach others about our
UNIVERSE
and its many objects are called astronomers.
A
binary star
system is a
GROUP OF TWO STARS

that orbit each other or
the same central point.
Astronomers believe at least
HALF THE STARS
in our galaxy are
part of
BINARY SYSTEMS!
An
EXOPLANET

orbiting a binary star system could
have multiple sunrises and sunsets!
When an exoplanet is discovered, one of the first things astronomers want to know is its composition, or the combination of elements that make up the planet.
Composition can help astronomers determine whether an exoplanet could be habitable.
There are more than
10 PROVEN WAYS
to find exoplanets, using telescopes both on Earth and in space! An exoplanet orbiting a star can cause
TINY CHANGES
in how the
STAR APPEARS.
Astronomers use
TELESCOPES
to see these changes and then determine if an exoplanet is present. Some of these changes might affect how
BRIGHT THE STAR
appears, what direction it is moving, and more!
On a clear dark night, you can see
THOUSANDS OF STARS
in the night sky. Each of those stars may have planets orbiting it, called
EXOPLANETS!
Exoplanets do not belong to our
SOLAR SYSTEM.
Some may be similar to our solar system’s planets, but
SOME MAY BE VERY DIFFERENT.
Not every
EXOPLANET

can be bound
to orbiting
a star. If an
exoplanet is ejected
from its star system,
that planet floats
freely in space; these exoplanets are also sometimes called
ROGUE PLANETS!
Scientists estimate there could be hundreds of billions of free-floating planets in the
MILKY WAY
.
Gas giants are huge planets that are made mostly of gases and do not have a surface made of rocky materials.
These planets are much larger than rocky worlds like our Earth. We have four gas giants in our solar system.
Exoplanets orbit their own special star, called a host star. Host stars can come in all different colors, sizes and temperatures.
Sometimes exoplanets even orbit around more than one star in a system! How far away an exoplanet is from its host star helps determine whether organisms could live there.
THERE ARE
MANY, MANY GALAXIES

in our universe. In each galaxy, stars are forming all the time! Planets could be forming around these new stars, too! With so many stars to study and observe, the number of exoplanets is endless!
Hot Jupiters are huge gas-giant exoplanets that are very close to their host star! They may be too close for life to form, and their atmospheres may even be boiling away from the heat! They have very fast orbits because they are so close to their star. One hot Jupiter has an orbit of just four days!

Kepler was the
FIRST
NASA mission
to find
EARTH-SIZE PLANETS
orbiting
nearby
stars. The spacecraft was named after
JOHANNES KEPLER
, a famous mathematician and astronomer who discovered and wrote
MATH EQUATIONS
explaining how the planets in our solar system travel around the sun. The Kepler mission has found thousands of
NEW EXOPLANETS TO STUDY!
Kepler was the
FIRST
NASA mission to find
EARTH-SIZE PLANETS
orbiting nearby stars. The spacecraft was named after
JOHANNES KEPLER
, a famous mathematician and astronomer who discovered and wrote
MATH EQUATIONS
explaining how the planets in our solar system travel around the sun. The Kepler mission has found thousands of
NEW EXOPLANETS TO STUDY!

Here on Earth, we have many
DIFFERENTLY SIZED UNITS
to measure how far away something may be or how big something is.
Astronomers commonly use the
LIGHT-YEAR
, the distance light travels in one year
ONE LIGHT-YEAR
is equal to about
6 TRILLION
miles! One of the closest known exoplanets to Earth is 4.2 light-years away.
Many planets in our solar system have moons. Astronomers believe exoplanets might have moons, too!
They would be called exomoons and are very difficult to find because they are smaller than planets and do not produce their own light. Astronomers are developing new techniques to help make finding them easier.

Some exoplanets
resemble the
GAS PLANETS

in our own solar system, but they’re much smaller! These planets are called
MINI-NEPTUNES
. They are closer in size to Earth than Neptune, but astronomers can tell they’re gas planets because they are
MUCH LESS DENSE
than a rocky world.

Different types of scientists have
DIFFERENT PLACES
where they conduct their research and collect their data. Astronomers use telescopes to collect data and study celestial objects. These telescopes are housed in observatories.
Ground-based observatories
use visible light and radio to study space, and are located in various places on the
SURFACE OF THE EARTH
. Many observatories have special times when guests can visit! Check out a local observatory near you!
The
CLOSEST
exoplanet to Earth orbits the star Proxima Centauri. It’s called Proxima Centauri b, and it is only a little over
FOUR LIGHT-YEARS
away! It would take many years to travel to this exoplanet because we cannot move as fast as light, but
IMAGINE
what new information we could discover!
With astronomers discovering
NEW WAYS
to find exoplanets and new observatories being built, the quest for
NEW WORLDS
is just beginning! Many questions are being asked, scientists are studying data and new missions are being designed to help find
ANSWERS
.
Rocky worlds are
PLANETS
that have a solid surface and are made of rocky materials. They are similar to Mercury, Venus, Earth and Mars. Rocky worlds are much
SMALLER THAN GAS PLANETS.
If we are going to find life like we have here on our Earth, a rocky world would be a
GOOD PLACE TO LOOK!
A super-Earth
is a planet that
is
MUCH, MUCH,
LARGER

than Earth
but not as large
as a gas giant.
Don’t let the name
super-Earth fool you!
A super-Earth might not have features and composition similar to Earth’s, but gets its name because it is closer in size to Earth than a gas giant.
When an exoplanet orbits in front of its
HOST STAR
, it
BLOCKS
some of the
LIGHT
. Astronomers call this a transit. Each transit will block a different amount of light because exoplanets can be
DIFFERENT SIZES
and
DIFFERENT DISTANCES
from their host star. This is one way for astronomers to find new
EXOPLANETS








.
Everything we know that exists is located in the universe. The universe is the term astronomers use to describe all of space. Astronomers believe it is growing and growing!
How big will the universe get?
Our galaxy, the Milky Way, is one part of the universe. How many exoplanets are in our galaxy?
One of the ways astronomers
DISCOVER
exoplanets is called
RADIAL VELOCITY
.
STARS
aren't completely still in space when an exoplanet is orbiting them. The planet
TUGS
on the star ever so
SLIGHTLY
, causing it to move in a small circle. These movements affect a star’s
LIGHT SPECTRUM
. When moving
TOWARD US
, the
COLORS
will appear
SHIFTED
toward the color
BLUE
. When moving
AWAY
from us, the color spectrum is
SHIFTED
toward
RED
. These shifts can be measured and show a planet is in
ORBIT
.
A water world, or an ocean planet, is a planet that astronomers think could be entirely
COVERED BY WATER
. With all of that water, it may be hard to have
LAND-BASED LIFE FORMS.
However, if the planet could have life, imagine all of the new types of
WATER LIFE
that could be discovered!
Astronomers call the light we see with our eyes
"VISIBLE LIGHT,"
but visible light only makes up a small portion of all the light in the universe! X-rays are a
SPECIAL TYPE OF LIGHT
astronomers use to observe exoplanets transiting their host star. X-rays provide different information than visible light.
The time it takes for a planet to
ORBIT
around its star is called a
YEAR
, or an
ORBITAL PERIOD
. Different planets have different year lengths. The length of the planet’s year depends on how close it is to its
HOST STAR
. The closer the planet is to its host star, the shorter its year will be.
Every star has
three
SPECIAL ZONES

surrounding it. These zones are
DIVIDED BY TEMPERATURE.
Just like
in Goldilocks, there is a zone that’s too
HOT
for life, a zone that’s too
COLD
, and one that is
JUST RIGHT!
The middle zone is the “just right” zone, called the habitable zone. Planets there are the most likely to support life!

1. How are planets in our solar system different than exoplanets? How might they be similar?

2. Kepler was a famous mathematician and astronomer. Can you name any others?

3. Why would a rocky world be a good place to look for life?

4. What makes a super-Earth similar to Earth? What makes a super-Earth different from Earth?

5. What kind of life forms do you think would live on a water world?

6. What planets have longer years than Earth in our solar system?

7. What questions do you have about exoplanets?
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