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Astronomers are a little unsure of the applicability of this index, but NASA’s Planetary Protection Officer is all in favor.
Earth and moon, rejoined again at last!
I mean this does have a practical application, I think. The relative masses of moons to planets does imply the moons ability to sweep junk out of the way before it hits the planet.
This is not based on mass but on volume. For example the moon is only 1% of the earth’s mass yet considerably less dense therefore it makes a thicker layer.
That’s why we live here; the moon protects us!
Mercury in shambles
Finally, Earth is Number One again (well Number Two if you count Pluto, that is)
Pluto-charon shouldn’t count since it is more akin to a binary planetary system than a planet-moon system. Charon and Pluto are so close in size that the system has a barycenter outside of Pluto. This means instead of Charon orbiting a point inside Pluto, Charon and Pluto orbit a point about 800 km above Pluto’s surface.