I am a middle school teacher. Today I did a lesson in which students determined what their weight would be on the other planets in our solar system. We used a formula I found on the internet in another teacher's lesson Plan; the formula is: Mass (weight on earth) x gravity (different for each planet) = weight on that planet. The gravity chart I used looked like this: Earth: 1 Our Moon 0.17 Venus .90 Mars 0.38 Mercury 0.38 Jupiter 2.36 Saturn 0.92 Uranus 0.89 Neptune 1.13 Pluto 0.07. My students quickly filled in the chart and discovered how heavy they would be on Jupiter, how light they'd be on our Moon, and Pluto, and Mars and Mercury. Since they finished so quickly I asked them to find their Planetary Average Weight. We decided to throw out the Moon (not a planet) and Pluto (not a real planet anymore.) We found that every single student had an average planetary weight that was within one pound of their weight on Earth! I was in awe of this "coincidence." I tried more weights, and the result was always the same: 8 planets, total weight on each, divided by eight ALWAYS equals Earth weight. So here are my questions: What makes this true every time? Is the fact that this is true the reason that Earth is just right for life? Does our home planet have that magic gravity amount that is exactly the solar system average? Do the planets acting together add up to the perfect gravity environment for Earth? Or does this formula have other implications? I am very interested to know your response. Thank you very much.

Answer 1:
This is COINCIDENCE. Put another way... the AVERAGE acceleration due to gravity on the planets you used (i.e. throw out moon and Pluto) turns out BY COINCIDENCE to be equal to 9.8!!! m/s^{2} There is one reason other than divine intervention that could explain this... coincidence!!!!!!!! I never have noticed that...of course if you kept the moon and Pluto the average g would be lower and you would not see this coincidence with planet Earth g=9.8 m/s^{2} I have consulted another source to check you values. My detailed tables give: Mercury 0.38 Venus 0.91 Mars 0.39 Jupiter 2.74 Saturn 1.17 Uranus 0.94 Neptune 1.15A little different but that may be due to rounding ...my source is the latest, and from time to time older sources need updating as spacecraft flybys help get better values of total mass and radius.
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