World Builders™                                                                         Session One  --  Astronomy        
                        Find the Life Zone

                                for Your Planet


Figure out how close to your sun your planet should be to get the same amount of energy as the earth does.

Are you planning to have life on this planet? If so --

Read the page on the Life Zone to find out why you need to put your planet in it.

    Follow these directions to find the best distance between your planet and your star.

The distance will be measured in AUs.

An AU is a distance measure, like a mile or a kilometer. However, space is so immense that we need to use a much longer measure of distance. An AU is the distance from the center of our earth to the center of the sun.This distance is about 93 million miles.

If your sun is brighter and hotter, you need to put your planet farther away to get earth-like intensity of heat and light.

If your star is cool and dim, you need to put your world closer to your star than our earth is to the sun.

Go to the Star Tables now.

Have you chosen your star type? You need a star that has a life span long enough for life to develop.

Suppose that the star that you have chosen is a K4 star.

The part of the star tables that you have chosen might look like this:

K Class Stars: Small, Dim, Red Stars: Could Perhaps Support Life On Inner Planets
Class Temperature in degrees Kelvin  Mass (Mass of our sun = 1)


(Radius of Sun=1)

Terrestrial Equivalent Orbit
in AUs
Lifetime in billions of years
K0 5250 .790 .786 .65  21.100
K1 5080 .766 .788 .61  long
K2 4900 .742 .750 .54  
K3 4730 .718 .762 .51  
K4 4590 .694 .692 .43  very
K5 4350 .670 .684 .39  long
K7  4060 .606  .641 .32  

The yellow row is the one that we have chosen for this example. You can see K4 in the green column.

Look at the row carefully. Notice that the mass and the radius of this star are less than the mass and radius of our sun.

Would you expect this star to be hotter or cooler than our sun?

Check the temperature. The surface temperature of the star is about 4590 degrees Kelvin.

Our sun's surface temperature is 5860 degrees Kelvin.
So the K4 star is smaller and cooler than Sol (our sun)

What will happen if you put your world one AU from the center of this planet? Will it be warm enough to have liquid water?

Well, probably not. Turn your attention to the blue column called Terrestrial Equivalent Orbit
in AUs.
This column tells you where to put your planet so that it will get the same amount of solar radiation as our earth. This is in the life zone.

Look for the number in the blue column that is the Terrestrial Equivalent Orbit in AUs.

See, here it is in the blue column. The number is 0.43 This planet will be closer to its star than earth is to Sol.  This is the number that you will use when computing the year length of your world.

Write down the number of the Terrestrial Equivalent Orbit in AUs for your planet and go on to

Calculate the Year Length When You Know Your Planet's Distance from the Sun

Calculate Your Planet's Distance from the Sun When You Know the Year Length

Kepler's Third Law

Astronomy Information Menu

Design Your Solar System

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© 1998, 2003. Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at