The habitable zone is described as the region around a star where a planet can maintain liquid water on its surface. In order to have liquid water, the star has to be at the right distance. If I were in a cold place like Antarctica and I had a cup of hot cocoa and a campfire, I would have to be by the campfire for my hot cocoa to stay a liquid, if I were to go too far away from the campfire my hot cocoa would become ice. It is the same for stars, the planet has to be in the right location relative to the star, so that the liquid on the planet would not freeze or boil. Different stars have different brightnesses, meaning, stars have different locations for the habitable zone . To see why different stars have different habitable zones think of a bonfire and campfire. A bonfire is bright and warm, whereas a campfire is less bright and not as warm. If you were 3 ft away from a campfire you would be comfortable, but if you were 3 ft away from a bonfire you would be very hot, for you to be comfortable you would have to be further away.
The graph below shows on the y-axis the brightness of a star relative to the sun meaning the lowest mass would be a campfire and the highest would be the bonfire. The x axis is the distance from the sun. As shown below Mars is out of the habitable zone because it is too far away from the sun, Venus is too close to the sun, and the Earth is right in the middle meaning it is in the habitable zone.
Astronomers often use this definition as a habitable zone, but a planet’s distance is not the only factor affecting its temperature. An atmosphere also effects a planet’s temperature. A campfire does not only keep you warm but the type of clothing you are wearing also affects your warmth. If you were wearing a parka you would be warmer v.s. wearing a t-shirt. This is because the heat from your body would become trapped inside of the parka. A planet’s atmosphere works the same way, the atmosphere traps the planet’s heat that it releases, this is known as the “greenhouse effect.” If we were to have too much atmosphere here on Earth we would be become very hot and if we did not get enough we would become very cold.
The graph labeled ‘Figure 2’ shows on the y-axis the predicted temperatures a planet should be at if a planet’s temperature just depended on the distance from the Sun. The x-axis shows the actual measured temperature for the planets. The red line on the plot tells where a planet temperature should be at, if a planet is on the red line then the predicted temperature matched the measured temperature. Venus and most of the planets lie to the right of the red line meaning they are hotter than predicted. Venus is much hotter than predicted to be, this is because Venus has a 100 x the greenhouse effect of Earth. For example, if Earth was wearing one parka Venus would be wearing a hundred parkas. The fact that most planets are hotter than predicted proves that the temperature of the planets does not only depend on the distance from the sun (it also depends on the atmosphere).
1. Why Were the parkas and t-shirt used as an example in the text above? How did it relate to the topic being explained?
2. In figure 1 would Venus be considered to be in the habitable zone? Why or Why Not?
3.In your opinion, why do Astronomers often assume the habitable zone depends on a planet’s distance if that is not the only key factor?