Why is the Earth's moon so big?

It could just be me, but it seems like our Moon is WAY bigger than it should be for a planet of our size. If you look at satellite-to-planet mass ratios for the largest moons of Jupiter, Saturn, Uranus, Neptune and then the Earth, the values are on the order of $\mathbf{10^{-5}}$ for Jupiter, $\mathbf{10^{-4}}$ for Saturn, $\mathbf{10^{-5}}$ for Uranus, $\mathbf{10^{-4}}$ for Neptune, but a whopping $\mathbf{10^{-2}}$ for the Earth-Moon system. What gives? Why is the relative size of our Moon two orders of magnitude larger than the average for the solar system?

• That's because technically the Earth-Moon system is a(lmost) a binary planet. The formation mechanism for the Moon is very different from that of the small moons of the large gas planets. The best current hypothesis for the formation is an impact of a Mars sized object into early Earth, while the smaller moons of the gas planets are formed either by condensation of material from the proto-planetary cloud or they are captured by the gas-giants at a later stage. Sep 18, 2014 at 19:10
• Is there something about this in the essay "The tragedy of the Moon" by Isaac Asimov? As far as I can remember, it's only about the consequences of a large moon, not about the causes. Sep 18, 2014 at 20:56
• related (possible dup?): physics.stackexchange.com/questions/68476/… Sep 18, 2014 at 22:18
• @Kyle Definitely related, but I am inquiring about the asymmetry found in our solar system with respect to satellite size, not hypothesizing the effect of a permutation to our current arrangement. Sep 19, 2014 at 0:41
• The mass ratio for the Pluto-Charon system is even $10^{-1}$. Jun 23, 2017 at 13:09

Most moons are captured bits of rock left over from the formation of the planet as it swept its bit of orbit clear.

Our moon is (probably) the result of a very big bit of rock hitting the early Earth and splitting it into two. The smaller bit ended up as the moon.

This also explains the relatively high density of the Earth (it got left with all the heavy core parts) and the low density of the moon (it got all the light surface rocks)

• Was the earth the thing that was split apart, or was the planet that hit the earth the thing that split apart? i.e. is the Moon mostly Earth materials, mostly OtherPlanet materials, or a decent mix of both? Dec 28, 2022 at 2:18

The simplest answer is that the Earth is even MORE massive.

Don't let Earth's lack of stature relative to the Jovian systems fool you. The gas giants are truly massive but are not nor do they have physical land and water at their surface.

This may seem laughable on its face as what could be more massive than a gaseous body in a liquid state?

Nay, veerily...these are not rockets nor the Sun spewing forth tremendous physical energy but are objects "wildly at war with themselves" with truly spectacular countervailing winds and even an "evil eye" beckoning us towards further investigation.

The Earth by spectactural contrast is shockingly calm...travelling at plus 70,000 mph plus rotating on its axis at a leisurely 1000 plus give or take.

Neptune's Moon is the only the one relative to size and mass that I think even comes close...and amazingly Neptune's Moon rotates in the exact opposite direction of Neptune...seeminly belittling the idea of Neptune as a Planet of Importance in so doing.

Earth on the other has this very tempting "delicate air" covering its massive surface...perhaps something that makes the Moon very angry as while she can move entire Oceans it is left to the Sun to "make weather" thus belittling this very placid and nonchalant body slowly being pushed away while still being forced to face no matter her strength, power and wisdom.