Anne Brousseau, age 12, of West Warwick, Rhode Island, for her question:
Which planet is called the earth's twin and why?
The nine planets of our Solar System differ in size and location. No two are exactly alike. But Earth and Venus resemble each other so much that they are called a pair of planetary twins.
Human twins share the same parents and the same birthday. Our Solar System was created by cosmic forces and its birth was billions of years ago. All its nine planets are children of our starry sun. Each is a round, solid body of a certain size, orbiting its own path around the sun at its own speed. Giant Jupiter is bigger than the total bulk of its eight brothers. Chilly Pluto is 100 times farther from the beaming sun than little Mercury. There are five smallish planets and four giants, and two of the smallish planets resemble each other more closely than other pair in the Solar System.
Venus and Earth are the heavenly twins of our solar family. But as with human twins, each is an individual with marked differences. Their similarities are in their sizes and their positions in the Solar System. They occupy the second and the third orbits from the sun and their yearly paths are closer than those of the other planets. At times, they are separated by only 26 million miles. Venus, on the inside traffic lane, gets more light and heat from the sun than does Earth.
The greatest similarities are in their heavenly bodies. The size or volume of a planet can be figured from the length of the diameter line through its equatorial waist. Earth's diameter is merely 366 miles wider than that of Venus and the mass of the earth outweighs Venus by 18 per cent. The density of a planet is estimated by comparing its total weight with an equal amount of water. On this scale, the earth ranks first in the solar system with a density of 5.5. Venus ranks second with 4.9.
A planet's gravitational force is based on a ratio of its mass and volume. Let's borrow a 100 pound, 12 year old girl to check this comparison. Earth's surface gravity gives her a weight of 100 pounds. But transported to the surface of Venus, our girl would weigh only 85 pounds. Compare this with Mercury, where she would weigh only 25 pounds, and giant Jupiter where her weight would be 264 pounds.
The bodies of the twin planets resemble each other, but they have very different planetary motions. Earth's average orbital speed is 66,600 miles per hour. Venus outdistances us by 6,600 miles per hour and completes its yearly orbit in under 225 earth days. Venus has no changing seasons and no 24 hour day. Its rotation remains mysterious and at present the Venusian day is estimated at from 30 to 100 Earth days, or more.
Still greater differences exist on the surfaces of the twin planets. Venus is shrouded in a dense blanket of stifling clouds. Sunbeams fall on these clouds but do not pierce through to its parched, arid surface. Our mild, life giving moisture and oxygen do not exist there. Earth's teeming, population of living things could not sur¬vive on the surface of Venus.