John Bergman, age 9, of Tulsa, Okla,, for his question

Which planet has the most gravity?

On the face of the earth, old Andy weighs 100 pounds. If he were on a rocket a million miles out in space, he would weigh nothing at all. On the planet Mars, he would weigh only 36 pounds and on giant Jupiter he would weigh 264 pounds. No. these different surroundings would not cause him to get very fat and very thin. The change in weight would be caused by gravity, that mighty force which hugs us to the face of the earth.

Gravity is a built in force. It is present in every object from a grain of sand to the mighty stars. The strength of an object's gravity depends upon the amount of matter in the object. A pail of stones has more matter than a pail of feathers. We say that it has more mass. A planet with a small mass has weaker gravity than a planet with a lot of mass.

If two planets are the same size and the same weight, the pull of gravity between them is equal. If one is made of lighter materials, then it has weaker gravity than the heavier one. The planets in our Solar System are of different sizes and made of lighter or heavier materials. Since the pull of gravity on the surface of a planet gives an object weight, Andy would have a different weight on each of the nine planets.

Little Mercury is the planet nearest the sun. On the scalesp we would need 20 such planets to equal the weight of the earth. But Mercury is made of lighter materials and takes up one 16th as much space as the earth. This bulk gives it a gravity equal to one quarter of the earths. On Mercury, Andy would weigh about 25 pounds.

Venus, Mars and we suspect Pluto are also smaller and lighter than the earth. On each the gravity is less powerful than earths gravity. On Venus,Andy would weigh 85 pounds and on Mars 36 pounds. We have no accurate facts on Pluto, but we think its gravity is a little less strong than earth's.

Jupiter, Saturn, Uranus and Neptune are all big bulky planets. Neptune is big enough to swallow our earth 60 times, yet it weighs only 17 times more than our world. On ice coated Neptune, Andy would weigh 112 pounds. Uranus is big enough to swallow the earth 64 times, but this big planet weighs only 14i times more than our planet. Uranus is big and bulky. On Uranus, Andy would weigh 96 pounds   four pounds less than he weighs on earth.

Saturn is also a big bulky planet made of light materials. It is big enough to swallow the earth 736 times, yet it weighs only 95 times more than our planet. On Saturn, Andy would weigh 117 pounds.

The remaining planet is giant Jupiter, big enough to swallow 1,318 earth sized planets. Yet on the scales, it weighs only 318 times as much as the earth. This fact means that the pull of gravity at the surface of the biggest planet is more than three times stronger than earth's gravity. On giant Jupiter, Andy would weigh 264 pounds.