Explain the criteria for the habitable zone with regard to distance from a star

Question

Explain the criteria for the habitable zone with regard to distance from a star and the importance of sun angle and how it works to cause sunlight across the surface of a planet within the habitable zone. How do time zones, the prime meridian and the International Date Line work as a system to determine the time of day and date for location on the surface and for someone crossing the Pacific Ocean in an airplane? Explain the significance of the Tropic of Cancer, the Tropic of Capricorn, the Arctic Circle and the Antarctic Circle with regard to the annual change of the seasons across the planet.

Explanation / Answer

1. a)

Habitable zone is defined as the specific area around the star which can sustain the liquid form of water and sufficient atmospheric pressure. If we are choosing a planet for the habitable zone then there are seven criteria which we have to consider:

1) Earth Similarity Index:- Earth Similarity Index is calculated on the scale of 0-1, with 1 being exactly like earth. The Earth Similarity Index depends upon the mass, radius and temperature.

2) Habitable Zone Distance:- Habitable zone distance gives the location of the planet in the habitable zone. '-1' habitable zone distance indicates planet is at inner edge and more towards the star, similarly '+1' indicates that the planet is at outer edge of the habitable zone more distant from star. The habitable zone distance depends on the stars luminosity, planets size and temperature of star.

3) Standard Primary Habitability:- The ability of planet to support vegetation is calculated in the standard primary habitability. In this scale '1' indicates the best suitable conditions for the plants and tress growth. It depends on the topographic conditions of the atmosphere.

4) Habitability Zone Composition:- This scale indicates the composition of the planet. Values below '-1' usually indicates that the planet composition contains mostly iron, where as values greater than '+1' indicates that planet composition mainly contains gases.

5) Habitability Zone Atmosphere:- This scale indicates the atmosphere of the habitable planet. Values below '-1' usually indicates that the planet has no atmosphere, where as values greater than '+1' indicates that planet has dense clouds of hydrogen.

6) Planetary and Habitable class:- Planetary class classifies the planet on the basis of cold, warm, hot where as habitable class classifies the planet on the basis of temperature of the planet.

b) The amount of heat energy transfer to the planet through the sun completely depends on the sun angle. If the sun angle is higher then the surface temperature of the planet gets lower and if sun shine with 90 degrees angle then the surface temperature gets higher. The surface temperature of the planet has direct relation with most of the scales for the habitable planet criteria so it does helps to have sunlight variation on the habitable planet.

2.

Prime meridian is the longitude line which is considered as 0 degrees longitude according to international standards. This prime meridian passes through Greenwich,London. International date line follows the 180 degrees longitudinal line which adjusts the date according to the international standards. Someone who crosses international standard date line has to adjust the gain of day or loss of day. If someone is passing in the airplane over the pacific ocean, the he will have to adjust 1 hour for every 15 degrees of longitudinal lines crossed. If person crosses international date line then he have crossed changed the day so he doesn't need to compensate the 1 hour after every 15 degree longitude.

3.

Tropic of cancer is the imaginary circle drawn on earth which is at marking of latitude 23.5 north. At tropic of cancer the sun is exactly overhead at the noon of June 21, that is considered as the beginning of the summer in northern hemisphere.

Similarly the Tropic of Capricorn is the imaginary circle drawn on earth which is at marking of latitude 23.5 South. At tropic of Capricorn the sun is exactly overhead at the noon of December 21, that is considered as the beginning of the winter in western hemisphere.

The Arctic and Antarctic Circles are located at ±66.5 degrees latitude. At December 21 when sun is overhead on tropic of Capricorn, the sun is not visible at arctic circle. The same is true of the Antarctic Circle during Southern Hemisphere winter. At June21 when sun is overhead on tropic of cancer, the sun is not visible at Antarctic circle.

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