The Universe - Part 2

THE UNIVERSE - PART 2

DISTANCE OF A HEAVENLY BODY IN THE SOLAR SYSTEM
 
The distance of a planet can be accurately measured by the radar echo method.
 
The distance s of the planet from the Earth is given by s= Ct/2
 
Where, t is the time taken by the signal in going to the planet and coming back to Earth and C is the velocity of the light.

SIZE OF A PLANET
d = distance × angular diameter

 
SURFACE TEMPERATURES OF THE PLANETS 

• The planets do not emit light of their own. They reflect the Sun’s light that falls on them. 
• The surface temperature T of the planet using Stefan’s law of radiation E = σT4. Where σ is the Stefan’s constant and E is the radiant energy emitted by unit area in unit time. 
• Day temperature of Mercury is 340oC. 
• However Venus is an exception as it has very thick atmosphere of carbon−di−oxide. This acts as a blanket and keeps its surface hot. Thus the temperature of Venus is comparatively large of the order of 480oC.

MASS OF THE PLANETS AND THE SUN

• In the universe one heavenly body revolves around another massive heavenly body. (The Earth revolves around the Sun and the moon revolves around the Earth).
• The centripetal force required by the lighter body to revolve around the heavier body is provided by the gravitational force of attraction between the two.
• The mass of the Sun M can be calculated if the distance of the Sun from the Earth r, the period of revolution of the Earth around the Sun T and the gravitational constant G are known using the relation.

ATMOSPHERE

• The ratio of the amount of solar energy reflected by the planet to that incident on it is known as albedo.
• The albedo of Venus is 0.85. It reflects 85% of the incident light, the highest among the eight planets. It is supposed to be covered with thick layer of atmosphere. The planets Earth, Jupiter, Saturn, Uranus and Neptune have high albedoes, which indicate that they possess atmosphere.
• The planet Mercury and the moon reflect only 6% of the sunlight. It indicates that they have no atmosphere, which is also confirmed by recent space probes.
• There are two factors which determine whether the planets have atmosphere or not        i)Acceleration due to gravity on its surface      ii)The surface temperature of the planet
• The value of g for moon is very small (¼th of the Earth).
• Consequently the escape speed for moon is very small. As the average velocity of the atmospheric air molecules at the surface temperature of the moon is greater than the escape speed, the air molecules escape.
• Mercury has a larger value of g than moon. Yet there is no atmosphere on it. It is because, Mercury is very close to the Sun and hence its temperature is high. So the mean velocity of the gas molecules is very high. Hence the molecules overcome the gravitational attraction and escape.

CONDITIONS FOR LIFE ON ANY PLANET
The following conditions must hold for plant life and animal life to exist on any planet.

•     The planet must have a suitable living temperature range.
•     The planet must have a sufficient and right kind of atmosphere.
•     The planet must have considerable amount of water on its surface.