Part 2: Build Your Own Planet
Lesson 2: Other Habitable Zones
Time: approximately 25 - 35 minutes, or 40 – 50 minutes if students graph their habitable zones
Materials: Text: Lesson 2 – Is Your Planet in a Habitable Zone? (from web site - 1 per group)
Overview
The groups apply the concept of habitable zone to their planets. They determine the habitable zone of the star their planet is orbiting and check where their planet is in relation to their star’s habitable zone. They also check to make sure that their star’s life cycle is as long as the age that they chose on the Planet Preference Survey.
Purpose
There are many factors that influence the average surface temperature of a planet. This lesson examines the impact of MASS and DISTANCE from the nearest star. It also explores the relationship between the MASS of a star and its life cycle.
Standards
A complete list of the standards covered by this lesson is included in the Appendix at the end of the lesson.
Procedure
Students should get in their groups with their folders. Each group will need access to the Planet Temperature Calculator, which is available over the World Wide Web at: www.astro.indiana.edu/~gsimonel/temperature1.html. As a class review the introduction to the lesson:
Are you likely to discover life on the surface of your planet? Could any life survive there? This lesson will explore that question. Recall that a “habitable zone” is an area around a star where a planet or moon would have an average surface temperature between 0ºC and 100ºC. In this lesson we will find out the average surface temperature of your planet and decide whether or not your planet is in a habitable zone.
The groups need their Planet Preference Survey. The choices they made on the Survey will determine the range of values they can enter in the Planet Temperature Calculator.
1) Open the Planet Temperature Calculator:
www.astro.indiana.edu/~gsimonel/temperature1.html
2) Click “continue” in the bottom right corner and then click the “review” button on the next screen. That will take you to the “review” screen. For now, enter 29 for BOND ALBEDO and 1 for GREENHOUSE EFFECT. (These are Earth’s values. You’ll probably be changing them later.)
3) Get out your Planet Preference Survey and look at Question 3. How far away from your star did you place your planet? Enter this number for DISTANCE in the Planet Temperature Calculator.
4) Look at Question 1 of the Planet Preference Survey. What type of star are you orbiting? The type of star you chose will determine the range of numbers that you may enter for MASS in the Planet Temperature Calculator.
If you chose “Low mass” choose a number between 0.1 & 0.4
If you chose “Solar type” choose a number between 0.4 & 1.5
If you chose “High mass” choose a number between 1.5 100
Enter a number in your allowable range then click “calculate.”
Next, the groups check that a star with the MASS that they selected can last as long as the age of the star that they chose on question 2 of their Planet Preference Surveys. This can be a little confusing.
A star can not be older than its life cycle. The Planet Temperature Calculator calculates the life cycle of a star with the MASS that the students input. This number is displayed on the “calculate” screen in the white box to the right of the 3 stacked boxes displaying the average surface temperature.
Question 2 of the Planet Preference Survey restricts students from entering an impossible age, such as a 50 billion year old high-mass star, but it is still possible that a group might enter a MASS for a star that yields a life cycle shorter than its age. For example, on the Planet Preference Survey a group could have chosen a solar type star and an age of 50 billion years. Lesson 2 allows students to enter a MASS of up to 1.5 for a solar type star, but a star with a MASS of 1.5 has a life cycle of only 4.4 billion years. So this group would have to experiment with different values for MASS until they find a value that allows a star to have a life cycle of more than 50 billion years. (A star with a mass of around 0.447 or less would be allowable. This is within the allowable range for a solar type star, but just barely.) All groups should be able to determine an allowable value for MASS, but some groups may need to try a lot of different values for MASS before they find one that allows a life cycle greater than their star’s age.
5) First check the life cycle of the star you are orbiting. The life cycle is how long your star will last before it uses up all its nuclear fuel and blows itself up. This number must be greater than the age of your star (Question 2 of the Planet Preference Survey). Your star can not have a life cycle shorter than its age. If it does then you star has already burnt up all its fuel. No life will survive on your planet and your mission will be unsuccessful. You need to check this every time you change the mass of your nearest star. Try changing the MASS of the star to see how this affects its life cycle. As the MASS of a star increases, what happens to its life cycle? ______
______
6) Now check the average surface temperature of your planet. If the temperature is between 0ºC and 100ºC, congratulations! Your planet is in a habitable zone. If not, you can go back to the “review” screen and try changing the MASS of your nearest star to see if you can get your planet within a habitable zone, but you must keep the MASS of the star within the ranges listed in step #4.
7) Experiment with different numbers for MASS, within your allowable range, until you find one that you like. Remember to check the star’s life cycle to make sure it is greater than the age of the planet.
Don’t get discouraged if you can’t get your planet within a habitable zone yet. Later we will look at other things you can change that might help you.
When you have decided on a suitable number for the MASS of the star you are orbiting, write it here: ______. You will use this number for MASS for the rest of this project.
Many groups will find that their planet is well outside their nearest star’s habitable zone and will want to change the type of star they are orbiting or their DISTANCE from it. Explain to the groups that it is too early to change the choices they made on the planet preference survey because there are other properties that impact a planet’s average surface temperature that they have not looked at yet.
The next step illustrates that as the DISTANCE to the nearest star increases, the average surface temperature of the planet decreases.
8) Now determine the habitable zone for you nearest star. Enter the number that you just decided on for MASS, enter 29 for BOND ALBEDO and 1 for GREENHOUSE EFFECT. You will keep these three values the same as you change the DISTANCE. Enter a number larger than the number you chose for DISTANCE and click “calculate.” Does the temperature go up or down? ______
Now enter a new number less than your DISTANCE. What happens to the temperature? ______. In your own words, explain how the average surface temperature of a planet changes as the DISTANCE to its nearest star changes. ______
______
The groups are now ready to determine the habitable zone of the star that their planet is orbiting. They have set the MASS of the step in star 7. BOND ALBEDO and GREENHOUSE EFFECT are, for this exercise, set to Earth’s values. The groups need to vary the DISTANCE to determine the inner and outer boundaries of the star’s habitable zone. If a group is having difficulty, remind them to consider what they learned about the relationship between DISTANCE and average surface temperature in step 8 and ask them if they need to increase or decrease the temperature.
Some groups, particularly those that have MASS and DISTANCE values similar to Earth, may finish much sooner than other groups. Groups that finish early might be encouraged to graph the habitable zone of the star their planet is orbiting. The instructions for making a graph are in the previous lesson. Students will need to determine what values to enter for DISTANCE since that will depend on the MASS they have selected for their nearest star.
9) Keep changing the DISTANCE and checking the temperature until you find the DISTANCE at which your average surface temperature is100ºC. (You may have to use decimals.) If you can’t get it exactly 100ºC, just get it as close a possible. This is the inner limit of your star’s habitable zone.
Record this number here: ______AU inner limit.
10) Keep changing the DISTANCE and checking the temperature until you find the DISTANCE at which your average surface temperature is 0ºC., or as close to 0ºC as possible. (You may have to use decimals.) This is the outer limit of your star’s habitable zone.
Record this number here: ______AU outer limit.
11) Finally, see how close your planet is to the habitable zone of your nearest star. Is it in a habitable zone?
_____ yes, yippee!
_____ no, it is _____AU away from the inner/outer limit of the zone. (If your planet is too hot, subtract its DISTANCE from the inner limit of the habitable zone. If it is too cold, subtract the outer limit of the habitable zone from your planet’s DISTANCE.)
You might also want to make a graph of the habitable zone of your nearest star using Microsoft Excel. Choose about 10 to 15 different DISTANCES, a couple of which are outside the boundaries of your star’s habitable zone. See if you can place your planet on the graph. The instructions for making a graph in Microsoft Excel are in Lesson 1.
Appendix
Standards Addressed
Benchmarks (Grades 3 through 5)
1C – The Scientific Enterprise
Science is an adventure that people everywhere can take part in, as they have for many centuries.
2C – Mathematical Inquiry
Numbers and shapes-and operations on them-help to describe and predict things about the world around us.
5D – Interdependence of Life
For any particular environment, some kinds of plants and animals survive well, some survive less well, and some cannot survive at all.
11B – Models
Seeing how a model works after changes are made to it may suggest how the real thing would work if the same were done to it.
11C – Constancy and Change
Things change in steady, repetitive, or irregular ways-or sometimes in more than one way at the same time. Often the best way to tell which kinds of change are happening is to make a table or graph of measurements.
12D – Communication Skills
Use numerical data in describing and comparing objects and events.
Benchmarks (Grades 6 through 8)
2B – Mathematics, Science and Technology
Mathematics is helpful in almost every kind of human endeavor-from laying bricks to prescribing medicine or drawing a face. In particular, mathematics has contributed to progress in science and technology for thousands of years and still continues to do so.
4E – Energy Transformation
Most of what goes on in the universe-from exploding stars and biological growth to the operation of machines and the motion of people-involves some form of energy being transformed into another. Energy in the form of heat is almost always one of the products of an energy transformation.
Heat can be transferred through materials by the collisions of atoms or across space by radiation. If the material is fluid, currents will be set up in it that aid the transfer of heat.
11B – Models
Models are often used to think about processes that happen too slowly, too quickly, or on too small a scale to observe directly, or that are too vast to be changed deliberately, or that are potentially dangerous.
Mathematical models can be displayed on a computer and then modified to see what happens.
11D – Scale
As the complexity of any system increases, gaining an understanding of it depends increasingly on summaries, such as averages and ranges, and on descriptions of typical examples of that system.
Benchmarks (Grades 9 through 12)
1A – The Scientific World View
Scientists assume that the universe is a vast single system in which the basic rules are the same everywhere. The rules may range from very simple to extremely complex, but scientists operate on the belief that the rules can be discovered by careful, systematic study.
1B – Scientific Inquiry
Sometimes, scientists can control conditions in order to obtain evidence. When that is not possible for practical or ethical reasons, they try to observe as wide a range of natural occurrences as possible to be able to discern patterns.
2B – Mathematics, Science and Technology
Mathematical modeling aids in technological design by simulating how a proposed system would theoretically behave.
Mathematics provides a precise language for science and technology-to describe objects and events, to characterize relationships between variables, and to argue logically.
4A – The Universe
Mathematical models and computer simulations are used in studying evidence from many sources in order to form a scientific account of the universe.
9B – Symbolic Relationship
Any mathematical model, graphic or algebraic, is limited in how well it can represent how the world works. The usefulness of a mathematical model for predicting may be limited by uncertainties in measurements, by neglect of some important influences, or by requiring too much computation.
11B – Models
The basic idea of mathematical modeling is to find a mathematical relationship that behaves in the same ways as the objects or processes under investigation. A mathematical model may give insight about how something really works or may fit observations very well without any intuitive meaning.