A key component of sustaining life on a planet is its ability of carrying liquid water. For this, it is important that the radiation it receives from its central star adequate to keep the temperatures of the planet above a value that permits water to be in the liquid state. Because of this characteristic whereby a potentially habitable planet receives heat from a star at the right distance, this planet can be viewed as an engine of life that needs adequate power to start.
Introduce the topic by asking about the Solar System (planets, composition) and where in this system life evolved. Perhaps, initiate a discussion about the conditions required for planets other than Earth (e.g. Venus, Mars) to sustain life. Try to direct the discussion towards the presence of water.
Question 1: Is there life on Mars?
Expected outcomes from discussion: We haven’t found anything that would indicate that there is.
Question 2: Is there liquid water on Mars?
Expected outcomes from discussion: Very little to none. At least not as much that we can expect areas of open water or running rivers.
Question 3: Why are these two things related?
Expected outcomes from discussion: Water is the key to life. Without water, there is no life the way we know it.
Ask the students if they have heard about planets around stars other than the Sun.
Question 4: What does it take to support life on other planets?
Expected outcomes from discussion: There are a few conditions that help in the formation and maintenance of life, but the key is – again – water. Heat is only needed to keep the water liquid, at least in some places. Other conditions that may help are energy sources (light, chemical) or magnetic fields to protect from ionising particle radiation. But since life evolves in oceans, this does not really matter.
Question 5: What conditions are needed to keep water liquid?
Expected outcomes from discussion: Heat and perhaps salt. Salt helps to lower temperatures to keep water liquid.
Question 6: What provides the Earth and other planets with the heat needed to keep water liquid?
Expected outcomes from discussion: Stars, the Sun. The greenhouse effect generated by an atmosphere helps to raise temperatures.
Question 7: What happens to water if it is very cold or very hot?
Expected outcomes from discussion: It freezes or boils and evaporates.
The following experiment is a simple analogy in which the star is represented by a lamp and the planet is represented by a photovoltaic cell combined with a motor.
ACTIVITY 1: ENGINE OF LIFE
Figure 7: Illustration showing the soldered connection between a solar cell and a computer fan. Here, the connection can be interrupted with microplugs. The polarity is indicated next to the soldering points of the photovoltaic cell (M. Nielbock).
1. Solder the motor to the photovoltaic cell. This is usually very simple. The cells provide soldering points with their polarities indicated. Just use cables (often already attached to the motor) and connect them to the corresponding electrical poles (see Figure 7).
2. A coloured cardboard disc attached to the rotation axis improves the visualisation of the motor’s speed as it runs. If a fan is used, the wings may be painted.
3. Plug the lamp into the dimmer and the dimmer into the socket.
Figure 8: Experimental set-up (M. Türk).
Question 8: How will the motor behave if the cell is held at different distances from the lamp?
Expected outcomes from discussion: The rotation speed depends on the distance. Far = slow, near = fast.
1. Switch on the lamp.
2. Hold the photovoltaic cell far away from the lamp. The motor should not move.
3. Moving closer to the lamp, determine the distance at which the motor starts moving.
4. Repeat this procedure for different brightness settings of the lamp by using the dimmer.
1. Write down your observations. Describe the results you get when varying the brightness of the lamp.
Expected result: The distance necessary for the electric motor to begin moving is smaller with a dimmed lamp than with a bright lamp.
2. To compare this model experiment with the configuration of the Solar System, life-sustaining conditions (those under which the motor moves) are possible because the Earth (photovoltaic cell) is close enough to the Sun (lamp). The point at which the motor starts running is the outer edge of the habitable zone. What does this experiment tell us about exoplanets in other planetary systems with different stars that are supposed to harbour life?
Expected result: A planet in a planetary system with a star that is less bright than the Sun needs to be located closer to its host star in order to be in the habitable zone.
For ages 14 and higher:
3. What happens to the motor when the cell is very close to the lamp?
Expected result: It runs very fast. It ‘overheats’.
4. Can we expect planets sustaining life to be at any distance inside the inner edge of the habitable zone?
Expected result: No, because as the distance decreases, the radiation the planet receives is higher. This leads to increased heating of the planet. If the temperature is too high, water cannot remain liquid.
This activity provides a qualitative impression of the nature and basic principle of the habitable zone. But what does a habitable zone actually look like? The next activity gives an example that is close to scientific research. It shows how one can identify the habitable zone and the planets within it.
ACTIVITY 2: THE HABITABLE ZONE OF KEPLER-62
Figure 9: Artistic impression of the Kepler-62f exoplanet (NASA Ames/JPL-Caltech).
Kepler-62 is a star that is a little cooler and smaller than the Sun. It is part of the constellation Lyra. In 2014, it was discovered by the Kepler Space Telescope and has five planets orbiting it. The details about Kepler-62 are summarised in Table 1.
Table 1: Properties of Kepler-62.
Figure 10: Comparison between the Solar System and the Kepler-62 planetary system (NASA Ames/JPL-Caltech).
Some of the five exoplanets of Kepler-62 are suspected to be Earth-like. The main properties of the five planets are given in Table 2. All planetary orbits are nearly circular and listed in astronomical units (AU). This is the mean distance between the Sun and Earth.
Table 2: Properties of the five exoplanets of the Kepler-62 system.
Name | Orbital radius (AU) | Mass (Earth masses)
Kepler-62b | 0.0553 | ca. 2.1
Kepler-62c | 0.093 | ca. 0.1
Kepler-62d | 0.120 | ca. 5.5
Kepler-62e | 0.427 | ca. 4.5
Kepler-62f | 0.712 | ca. 2.8
Tasks (Drawing a scaled model)
Determine or discuss a suitable scale that allows you to put the entire system on a sheet of paper.
Fill the table (Table 3) with the scaled orbital radii. Round up the values to full millimetres.
The model will show the planetary system from a bird’s eye view. Use the compass to draw the scaled circular orbits around the assumed position of the host star Kepler-62.
In the next step, you will add the habitable zone. First, you can apply the simple equation
which only depends on the luminosity of the star. Note that this equation tells you where an Earth-like planet would be located around a Sun-like star of lower luminosity. Use Table 1 to calculate the distance of the habitable zone from Kepler-62. Note that the distance of the habitable zone scales with the square root of the stellar luminosity.
Q: Using this simple relation, where with regard to the Solar System would the habitable zone be if a star had four times the luminosity of the Sun?
A: It would be at twice the distance as compared to the habitable zone of the Solar System.
Calculating the proper boundaries of habitable zones can be tricky and needs sophisticated models. There is an online tool at
that performs the calculations when the luminosity and surface temperature of the star are provided (This can be done by the teacher instead of providing computer access for all students.). All you have to do is enter the surface temperature Teff and the luminosity of the star in the upper two fields, as shown in Figure 11. Note that after entering a value, you have to click in the field again.
Figure 11: Screen shot of the online tool to calculate the dimensions of a habitable zone. The example above shows the values of the Sun. Important: After inserting the values, you have to click in the field to submit the entry (http://depts.washington.edu/naivpl/sites/default/files/hz.shtml).
Use the values of Kepler-62 from Table 1. Use the result labelled as ‘Conservative habitable zone limits (1 Earth mass)’ and ‘HZ distance from the star (AU)’. Add the distances for the inner and outer edges of the habitable zone to the table below and calculate the scaled radii.
Table 3: Orbital parameters of the Kepler-62 planetary system (The scaled values are optimised for a sheet of A4 paper with a width of 18 cm. They are not provided with the worksheets).
Name | Orbital radius (AU) | Scaled radius (cm)
Kepler-62b | 0.0553 | 0.60
Kepler-62c | 0.093 | 1.01
Kepler-62d | 0.120 | 1.30
Kepler-62e | 0.427 | 4.64
Kepler-62f | 0.712 | 7.74
Habitable Zone (inner) | 0.456 | 4.96
Habitable Zone (outer) | 0.828 | 9.00
Q: How well does the simple distance of the habitable zone agree with the model calculation?
A: The simple value is close to the inner edge of the more accurately calculated habitable zone.
Q: Can you think of a reason why the simple solution is so close to the extreme value of the modelled solution? In which way do the two approaches differ? (Hint: If the students have difficulties with the answer, let them use the online calculator with the solar surface temperature instead of the stellar luminosity.)
A: The simple approach does not take into account the surface temperature.
Q (perhaps better suited for higher term classes): How does the missing parameter (surface temperature) influence the radiation characteristics of Kepler-62?
A: Lower temperatures mean redder spectra. The spectrum of Kepler-62 contains a relatively higher amount of infrared radiation than that of the Sun. This is more effective in directly heating planetary atmospheres.
Now add the inner and outer edges of the habitable zone to the scaled model of the planetary system.
You can colour the area between the inner and outer edges of the habitable zone. Green would be appropriate.
Figure 12: Model of the Kepler-62 system (solution not presented to the students).
Q: Which of the planets are inside the habitable zone?