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An introduction to exoplanets
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.

Described image
Figure _unit7.5.2 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 _unit7.5.2 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.


The Earth falls in the green region, the habitable zone. The average surface temperature is 296 K. To convert this to °C you need to subtract 273, which gives a temperature of 23 °C. Water is a liquid at this temperature. The Earth has a temperature similar to that reported by this interactive application.

Once the greenhouse effect is included, the temperature of the planet does not depend only on its distance from the star. The properties of the planet’s atmosphere are crucial in determining the planet’s temperature. This is one reason why astronomers are very keen to detect and measure the composition of exoplanet atmospheres. You’ll find out how next week.

Explore how much the mass of the Sun would have to change before Earth was no longer in the habitable zone.