Science, Maths & Technology

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An introduction to exoplanets

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# 5.3  Why isn’t the Earth frozen?

Clearly the interactive application in Activity 5 is not telling the whole story. It makes a simple calculation assuming all the sunlight heats the surface of the Earth, which then cools according to the simplest equation describing how warm objects cool. In fact, all the sunlight does not reach the surface of the Earth: the Earth’s atmosphere absorbs and scatters some of the sunlight. But the atmosphere also absorbs heat energy from the surface that would otherwise escape directly back into space, as shown in Figure 6. This keeps the surface warmer than it would otherwise be. This is the ‘greenhouse effect’, the importance of which has been widely discussed in recent years. It is now clear that the Earth is gradually warming because human industry is changing the mix of gases in the Earth’s atmosphere.

Figure 6  The greenhouse effect. Heat energy in the form of ‘infrared radiation’ is given off by the Earth’s surface and absorbed by gases in the atmosphere.

## Activity 6  With a greenhouse effect, is your planet habitable?

Timing: Allow about 10 minutes

This interactive application includes the effect of an atmosphere which causes a simple greenhouse effect. It appears similar to the one you encountered in Activity 5, but the equations used to calculate the planet’s average surface temperature are different. Consequently, the results it displays are different. As before, the habitable zone, where the presence of liquid water is possible, is indicated in green.

Active content not displayed. This content requires JavaScript to be enabled.
Interactive feature not available in single page view (see it in standard view).

Change the star’s mass to explore again how the location of the habitable zone can change for different stars.

Set the slider values to correspond to the Sun and Earth.

Answer the following question:

In which zone is the Earth, and what is the value of its surface temperature? Convert this from K into °C and comment on your findings.