World Builders™
World Builders™
Session One  --  Astronomy
Session One  --  Astronomy

   How to Design Your Solar System
   How to Design Your Solar System

This third law will allow you to figure out how long your planet's year will be.


1. Choose Your Star:

To decide how far your star should be from earth, read
                                                                                       Distances in Space.
     Return to this page.

Read the page about the Main Sequence of Stars.
     Return to this page.

Go to the page of Star Tables.
 Choose your star from the tables.


 Class Temperature in degrees Kelvin  Mass
(Mass of our sun = 1)


(Radius of Sun =1)

Terrestrial Equivalent Orbit
in AUs
in billions of years
F8 6200 1.190 1.260 1.45  6.880

Copy the information:

Write down the information like this:

Class: F8

Surface Temperature in Degrees Kelvin: 6200

Visual Luminosity (brightness compared to sun): 1.93

Mass (Mass of our sun = 1): 1.190

Radius (Radius of our sun = 1): 1.260

Terrestrial Equivalent Orbit in AUs: 1.45

Lifetime in billions of years: 6.880

Review the notes on what the headings mean.

Underline the important information.

Return to this page.

Now think.

How does this star compare to our sun?
What will this mean to life forms on its planets?
You can write a paragraph about this.

2. Place Your Planet.


Terrestrial Equivalent Orbit in AUs: = 1.45

This means that, for a planet to receive the same intensity of radiation as the earth does, it should be located 1.45 Astronomical Units from the sun that you have chosen. We could make some minor changes here to reduce or increase the radiation that it receives by locating it farther away or closer to the planet.

Check the Statistics about Planets and Moons page to see how much you can vary the distance and still stay in the life zone.

Use the simulation at to check your orbital plans and to get more information about all your planets.

Once you have decided on the number of Astronomical Units, you can find out the year length by going to the page on Kepler's Third Law.

Once you have figured out the orbit and year length for your planet, place the other planets in your system. Assign Astronomical Units to them and use Kepler's Third Law to determine how long their years will be.

Now draw a diagram of your solar system. This part is done!

3. Describe Your Planet.

Now you get to pick the size of your planet. You have a lot of freedom here. However, you want the planet to have enough mass to hold an atmosphere. If your planet is too massive its gravity could make life very difficult for organisms on land.

Check the page on Weight, Mass, and Density.

Check back to the Statistics about Planets and Moons to look at planets and densities to get ideas. You need to decide on your planet's

radius: (use the radius of the earth as 1) (The radius is the distance from the center of the planet or moon to its surface.)

density: the earth's density is 5.52. Your planet might be a little higher or lower, depending on how big the iron core is. The density that you chose will affect your gravity.

gravity: Go to Calculating Your Planet's Gravity when you have decided on either the radius or the Density in grams/cm^3 of your planet.

day length: you have a lot of choice here. Anything over eight hours should work.

axial tilt: check What Causes Seasons?

number of moons, if any: be careful to put your moons well outside Roche's Limit. Keep them well spaced apart from each other so that they will not crash collide!

How often do your moons appear in the sky? Go to Calculating Orbits for Your Moons to find a simple formula for figuring out how quickly your moons orbit your planet.

4. Write Up Your Description

You are nearly finished! Write up a description of your planet and draw a diagram or two of your solar system. If you like, you can put in a picture or two of what it looks like as well. Check out your decisions with the other members of your group, as things like gravity and seasons will affect life forms later. Check with Dr Viau for feedback.

Congratulations, World Builder!

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© 1998, 2003, 2004. 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