Mary Drexler, age 12, of Huntsville, Alabama, for her question:
How heavy is each planet?
The constellation Libra means The Scales but naturally this starry group is not up there in the summer skies to weigh planets or other heavenly bodies. That tantalizing job is dumped on the astronomers and they have to figure things out the hard way.
Actually they do not think of weighing a planet as we weigh a pound of cheese. Their notion of weight is mass though mass can be counted by the ton.
We can tackle this question in two ways. One gives the mass of each planet as compared to the mass or weight, of the world. The other gives density, which is how heavy a planet is for its size. The mass problem is more closely related to weight as we know it. A planet's mass is the amount of matter packed into its size or volume.
The earth's mass or weight is estimated to be around 66 sextillion tons. This astronomical number is 6,600 with a tail of 18 zeros. When astronomers compute the mass of the other planets, they take the weight of the world to represent a base unit of one. Four of the other planets are less than this unit, four are much greater. The estimates are not 100 per cent perfect.
The mass of little Mercury is estimated to be 0.05, which means that its weight equals the earth's tonnage divided by~20. The mass of Venus is about 0.82, which is 18 per cent less than the weight of our world. Gars has a mass of only 0.11, which means that the earth weighs almost ten times more.
Next in order of distance from the sun coupes giant Jupiter, more massive than all the other planets put together. In earth units, it is about 318. To translate this into weight as we know it, just multiply 318 by the earth's 6.6 sextillion tons. Next in order of distance and mass comes glorious Saturn. We would need almost 9511 earth type planets to equal its weight.
Though only half the diameter of Saturn, Uranus rates as a giant planet. Its size is 64 times greater than the earth's, yet its mass is merely 14.6. Neptune also is a giant lightweight with a mass of 17.3. It is 60 times larger than the earth, yet it would take only about 17 earths to equal its weight. Little Pluto has been reluctant to divulge its weight, but latest estimates indicate that its mass may be about one tenth that of the earth.
No doubt about it, Jupiter is the heavyweight. But it does not win the density contest, which figures weight in relation to volume. Here the basic unit is an equal volume of water. And the winner is a pretty little planet called Earth which weighs 5.5 times more than an equal volume of water.
For its size, we can claim that our planet is the heaviest in the Solar System. The least dense is whopping Saturn, which weighs only about two thirds as much as an equal amount of water. In a celestial bath tub. Saturn would float rings and all. Jupiter, Uranus and Neptune are slightly heavier than water. Mercury, Venus and liars are dense little planets, though their materials are not quite as heavy as the earth's.