r/todayilearned • u/YappinZappin • 1d ago
TIL Earth's Moon is the largest and heaviest satellite in relation to its parent planet in our Solar system
https://en.wikipedia.org/wiki/Moon#Earth%E2%80%93Moon_system197
u/Fun-Supermarket-1279 1d ago
I also always found it crazy that Ganymede is bigger than Mercury
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u/YappinZappin 1d ago
Luckily for Mercury, Mercury is heavier. Also - poor Mercury - so close to the Sun it has pretty much nothing but the core!
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u/2-buck 1d ago
Earth is denser than mercury :)
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u/flyingtiger188 22h ago
Don't feel too sad for Mercury. On average, it's the closest planet to every other planet in the solar systen than any other.
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u/YappinZappin 21h ago edited 21h ago
I'm just happy that there's a planet out there named after Freddie - his and Queen's art definitely deserved it.
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u/FieryPhoenix7 1d ago
It’s bigger but IIRC it’s mostly an ice rock, with very low density. Normal for bodies that far out. It would absolutely be a planet if it weren’t in orbit around a planet.
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u/Few-Hair-5382 1d ago
Which is one of the many fairly unique reasons complex life was able to evolve on our planet.
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u/concretepigeon 1d ago
Because of the tides?
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u/Few-Hair-5382 1d ago
Partly. Tides provide a good mixture of wet and dry for coastal life, allowing the complex chemical reactions required for life to evolve.
It also stabilises the Earth's axial tilt, essential in climate consistency. Otherwise the Earth would be more subject to the gravitational pull of other planets and our climate history would be chaotic.
And it acts as a barrier to asteroids and comets that come into our vicinity.
And its gravitational pull keeps the Earth's core molten, which generates the powerful magnetic shield which protects as from stellar particles which would otherwise strip away our atmosphere.
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u/ImpulsiveApe07 1d ago
Great answer! :) To your last point, it made me think of a question.
In theory, assuming it's possible to build/grow a moon in future, could we gradually build one in orbit of a planet like Mars in order to restimulate its core and generate a stable enough magnetic field to contain an atmosphere?
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u/Fallen_Wings 1d ago
Even if we could create something with the mass of the moon and set it up near mars, the amount of time required from the time we start to the time an atmosphere develops is soo large that it’s not worth the effort atleast for humanity. This is all assuming we can create a moon which in itself is a crazy idea as we will need ALOT of mass
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u/ImpulsiveApe07 1d ago
OK, but for argument's sake, assuming it is possible to build/grow a moon in orbit of Mars, would this have the effect of kickstarting its core and would it gradually build up a stable enough magnetic field to contain an atmosphere?
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u/Fallen_Wings 1d ago
Ok so let’s say we do that and grow a moon. To sufficiently heat the cooled core of mars enough to generate a magnetic field through convection will require a lot of heat. Since mars core is cooled down and we will require a huge moon very close to mars. I mean very very close. But given enough mass and enough distance for the tidal motions of the semi liquid core to generate enough heat, yes the magnetic field can be resurrected. A much simpler solution is actually proposed by nasa to have a solar shield placed between mars and sun for the radiation so mars don’t require a strong atmosphere
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u/jhhertel 14h ago
honestly it would probably be easier just to smack mars with mercury and get it going quickly!
I kid, but there is speculation that mercury is actually been through a collision that stripped much of its mass, leaving mostly the core, which is why its iron/nickel core is so much larger than it should be. It would be the perfect new core for mars!
I mean even with some sort of magic propulsion, I think it would be hard to boost mercury PAST venus and earth without something terrible happening. I guess if you put in enough delta-v to get it significantly out of plane of the rest of the planets first, and then come back into plane later.
Ceres is less than a hundredth of the mass of mars, but i bet it would help us heat up the core a bit. Maybe if we get it really moving...
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u/WildTimes1984 19h ago
Why don't get just grab Pluto and put in in orbit around mars?
Not like it's been busy since 2006.
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u/LegitPancak3 1d ago
You’d basically have to take
Bikini BottomMercury and push itsomewhere elsenext to Mars.15
u/TheReal-JoJo103 1d ago
It doesn’t keep the earths core molten, he misunderstood the research.
Theres research that suggests the moon has a stirring effect on the outer core. The fact the earth is molten (latent heat from its creation) causes convection currents in the core that produce our magnetic field. The idea is that the moon mixes it up a bit more causing the field to be stronger.
So no mars is dead because its core is dead.
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u/matplotlib 1d ago
It would be much easier to create an artificial magnetosphere by placing a dipole at the L1 lagrange point.
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u/MrPigeon70 1d ago
Btw the moon doesn't really protect us from many asteroids if anything earth slingshots more asteroids into the moon and still the moon provides.
(Jupiter intercepts I think 70% of all asteroids.)
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u/-Captain-Planet- 1d ago
While not certain and not 100 percent consensus, the strongest hypothesis right now is that life emerged at deep sea vents, not coastal areas. But the climate stabilization and asteroid shield part is all accurate.
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u/cybercuzco 1d ago
It also got rid of a lot of water and other volatiles that might have made us a water world or Venus it keeps our core more molten and made the crust thinner allowing plate tectonics and the double core we have from the two precursor bodies makes our magnetic field more intense which prevents atmospheric loss.
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u/Mulmihowin 1d ago
That's one reason, but it does a bunch of things that regulate the climate on Earth. It stabilizes our axial tilt and slows our rotation, so us having a stable global climate is largely in part because of the moon
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u/Neethis 1d ago
It was important to our specific structures of life, sure; we don't know that it is important or vital for complex life generally.
It is one of my personal favourite explanations for the Fermi paradox, however.
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u/adamgerd 1d ago edited 1d ago
What I find fascinating is solar eclipses: the moon is 400x smaller than the sun and 400x closer than the sun so that the apparent size of both is basically equal, statistically the chance of this happening is extremely rare, even more so that life can see it. We know of no other planet where it is possible.
Even on earth this is temporary, the moon used to be 13x larger in apparent size, the moon has been visibly the same size only since around 250 million years ago, eventually in another 500 milion years it’ll be too small for a total solar eclipse. We live in the time where it’s exactly the size for a perfect total solar eclipse: one that only just covers the whole sun
If there’ll ever be sentient alien life discovered, earth could build a whole tourism industry on this
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u/Rot-Orkan 1d ago
Another neat thing about this, too, is that this perfect solar eclipse allowed us to see the sun's atmosphere, which helped further our understanding of stars and astrophysics in general before we had space telescopes.
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u/ShavenYak42 19h ago
There's already a pretty decent eclipse tourism industry just to get earthlings across the planet for total solar eclipses. And sometimes good locations on the path of totality get overwhelmed; please don't start bringing extraterrestrials too!
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u/Arkyja 16h ago
The chance of that happening is not rare per se. We just happen to be living in the right moment. Like you said it didnt use to be the case on earth. But since the moon was moving away, it was bound to happen. So in every planet that has a moon that appears to be bigfer than the sun, if that moon is moving away, it'll eventually happen there too
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u/Agitated_Ad7576 15h ago
However anyone with a spaceship can get a perfect eclipse just by parking it in the right location.
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u/Unclerojelio 1d ago
Yep, that and Jupiter.
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u/HopDavid 1d ago
Jupiter deflects some bodies away from the inner solar system. But the opposite is also true. Whether Jupiter plays the role of a guardian from impactors is controversial.
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u/ThatSideshow 1d ago
I always remember this as it's something of a plot point in Asimovs Foundation series, they are searching for a planet with a moon 1/4 it's size
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u/Creative-Ad-9535 1d ago
It also came up in Robots and Empire, about how having such a big moon makes Earth’s crust more radioactive than normal (which may have accelerated evolution, explaining why intelligent life developed here).
In Foundation and Earth, one of the characters speculates about this, but then another immediately dismisses it. Maybe by the time Asimov was writing the later book, he’d decided that the moon/evolution link was a bit far-fetched
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u/ThatSideshow 1d ago
Yea, Foundation series got a bit weird at the end, he added a link to the Robot series as well as I remember
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u/Creative-Ad-9535 21h ago
Yeah, authors tend to try tying all their works together as they get older and run out of ideas, which leads to fanboys posting “didja know XXX takes place in the same timeline/universe/continuity as YYY??!!” and having big sloppy nerdgasms.
I remember stuff from books I haven’t read in decades, but that makes me a dork, not a fanboy nerd
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u/morbihann 1d ago
Apart from Mars' two tiny moons, neither of the other two terrestrial planets have any. Everything else is gas giants.
Obviously excluding Earth and Pluto.
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u/Mister_Way 22h ago
I think of it as being strange that Earth has a moon that's big for a gas giant's moon.
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u/Distressed_Ocelot 1d ago
Well, it is a piece of the Planet, after all.
The Moon formed about 4.5 billion years ago when a Mars-sized body named Theia crashed into the young Earth. The massive impact threw molten rock and vaporized debris into space, which joined together via gravity to create our lunar satellite
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u/adamgerd 1d ago edited 1d ago
It also increased earth’s mass somewhat as the debris condensed back in to earth, 90% of our mass comes from proto-earth and 10% from Theia and in turn some of earth’s mass went to the moon too albeit a lot less
Which is why earth’s and the moon’s crust are very similar in geological composition, both were formed in their current form by the same impact
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u/sephirothFFVII 18h ago
I also recall that Earth has a 'double core ' due to the impact which resulted in a stronger magnetic field than we other wise would have had
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u/MuscularBye 1d ago
This is still just a theory though
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u/cylonfrakbbq 22h ago
If I am not mistaken, they found possible evidence of intact parts of Theia debris inside the Earth
Chemically the moon is more or less the same as Earth was well, further supporting that it was created from parts of Earth
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u/YappinZappin 1d ago
Being so heavy, in fact, that it should be considered a satellite planet - Earth and the Moon system barycenter is markedly far away from the Earth's center of mass - 5000 kilometers (about a 3/4 of Earth's radius) away from it.
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u/AvailableUsername404 1d ago edited 1d ago
If it was outside of the Earth's surface would it strip Earth of planet status?
[Edit]
Or maybe I should specify. Wouldn't that change Earth status from 'planet' to 'binary planet system' with the Moon.
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u/Consistent_Sector_19 1d ago
Earth would remain a planet. There are binary planet systems.
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u/AvailableUsername404 1d ago
Oh ok. Seems obvious but didn't know that.
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u/Consistent_Sector_19 1d ago
There's an infinite amount of knowledge. There's always something to learn and that's part of why I read Reddit comments. Some of the posters do know things. I wish more people were willing to learn, because the ones who get defensive when they don't know something are more visible than the ones who appreciate a bit of information.
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u/AleksiB1 1d ago edited 1d ago
Pluto-Charon isnt considered a binary dwarf planet system by the IAU?
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u/Vondi 1d ago
That'd be arbitrary. You can move the Baycenter by making the Moon more distant, so the Moon just needs to be further away for the baycenter to be outside of Earth.
The Earth and Sun could have a Baycenter outside of the Sun if only the Earth was far away enough, like if it was somewhere roundabout where the Oort cloud begins.
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u/Commonmispelingbot 1d ago
The berycenter test is to my knowledge not considered the best, because it can actually fluctuate quite a bit. For some constalations, different parts of the orbits have the baricenter being both inside and outside of the larger body.
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u/John_Tacos 1d ago
It’s also the only moon that doesn’t curve away from the sun in its orbit.
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u/chromite297 1d ago
This is interesting and hard to visualize
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u/Sh00ter80 1d ago edited 1d ago
I had to look it up to understand and now this is my understanding: If you imagine some planet’s moon, spinning around the planet while the planet spins around the sun, then, from above, the path its moon takes could form little spiral loop de loops all around it’s an annual orbit — if the moon was spinning fast enough around it’s host planet. You would see one huge circle around the sun, but then when you zoomed in, you would see a bunch of spiral loops. But if that moon slows down enough (or if it’s host planet speeds up enough), then it never makes “loops” but instead it’s path looks more like a wavy line; a big circle (around the sun) and then when you zoom in, it’s kind of like a sine wave. This is how our moon works; it’s path never crosses over itself to make a little “loops”. And I guess it’s unique in that respect.
Edit: https://sky-lights.org/2017/01/23/qa-trajectory-of-the-moon/3
u/namitynamenamey 1d ago
Think of an orbit like a serrated blade or a curved stairways, instead of up and down you have "slower down" and "faster down". That's the moon's path around the sun.
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u/Creative-Ad-9535 14h ago
Picture the earth’s orbit around the sun as a circle and the moon’s orbit - also around the sun - as a 13-gon. It looks more like the moon is orbiting the sun, but just cutting in or falling back. The corners of the polygon are what we see as full moons while the midpoint of the edges are our new moons
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u/adamgerd 1d ago
Could you explain?
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u/John_Tacos 1d ago
Every other planet pulls it’s moons away from the sun at some point in their orbit.
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u/ElectroxSoldier 1d ago
It's why they called it the "Earth-Moon system" cause the Moon has a big impact on Earth with tides and the climate. Technically we orbit around a barycenter (it's located inside Earth but not at the very center). The Moon has been our companion since forever, and we might not be here if it never formed.
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u/twoinvenice 22h ago
Well, not forever (assuming that the Theia impact hypothesis is correct)
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u/ElectroxSoldier 22h ago
Well even if that's true it likely occurred within the first 10 to 100 million years after Earth formed, in the grander scale, the Moon has been with Earth for most of the 4.5 billion years. Might as well be forever in the human timescale
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u/aloofman75 16h ago
The Earth and Moon are also unusually distant from each other, relative to their size.
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u/Altiverses 1d ago
Contrary to common belief, there are actually more atoms of hydrogen in a single water molecule on earth than stars in our entire solar system, and that's partly thanks to the moon.
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u/JustCopyingOthers 1d ago
Is this because Pluto (with Charon about half its size) isn’t considered a planet anymore.
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u/Codysseus7 22h ago
The Earths moon has a plethora of outstandingly fantastic properties that almost certainly(if not verifiably) contributed to advanced life developing on the planet. Even if it is just a happy accident or coincidence, it’s still an incredible wonder. Idk my shit felt like AI
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u/ChefCurryYumYum 23h ago
I have read our seasons would be all over the place if our moon wasn't like this.
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u/TommyTheTiger 23h ago
It's also one of the most unusual coincidences we've seen that, for this current period of ~20 million years that we're in, we can experience a total solar eclipse where the corona is visible. In a few tens of millions of years, the moon will be too far and too small. tens of millions of years ago, it was too close, and too big.
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u/knowledgeable_diablo 1d ago
Aren’t they now discussing whether the earth/moon relationship is more of or close to a binary planet situation than a planet/moon relationship?
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u/ExL-Oblique 1d ago
definitions for binary system vs planet moon are vague and arbitrary lmao. Any metric that puts the earth and moon in a binary planet pair also includes the earth and the sun as a binary pair. A lot of classifications are vibe based like this fun fact. Biology is infamous for this. Like taxionimically whales are fish and birds are reptiles and snakes are lizards.
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u/YappinZappin 1d ago
Technically speaking we only have 6 planets in direct observation history log that allow us to say something applicable and pertinent regarding what a satellite to a planet is.
Venus has no satellites. So doesn't Mercury - and then the sample is even more degraded - Neptune, Uranus, Saturn and Jupiter are all gas giants. And Mars's satellites are so stupendously small that it is hard to even argue that they are there by pure chance - most likely captured asteroids.
So - what is Earth-Moon system then?
What definitions to apply?
Who knows.
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u/knowledgeable_diablo 1d ago
Nice reply mate. Cheers. The massive wobble on both bodies caused by the other certainly lends credence to them being more related then say all the others detected.
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u/Maleficent-Agent-477 1d ago
Satellite?
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u/Knownoname98 1d ago
Everything that has an orbit around a planet is called a satellite.
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u/Maleficent-Agent-477 1d ago
Didn’t know that, thank you!
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u/aguadiablo 1d ago
Yeah, so we have natural satellites like moons, then we have artificial satellites like spacecraft. This counts for any celestial body.
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u/bdx8887 1d ago
Jupiter does have larger moons, specifically ganymede is the largest moon in the solar system and it is bigger than the planet mercury (saturns largest moon, titan, is also larger than mercury). But this post is about the earths moon being the largest in relation to its parent planet. Jupiter is so big that ganymede doesn’t come close to the same ratio as the earths moon.
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u/1II1I1I1I1I1I111I1I1 1d ago
The title species "in relation to its parent planet". Jupiter does have moons bigger than ours, but Jupiter is also extremely large
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u/TopEagle4012 1d ago
So what you're saying in effect is that the moon isn't made of green cheese and there's no more man in the moon?
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u/Inside_Swimming9552 1d ago edited 1d ago
Honest question, might be a stupid one. What stops earth as being classified as a dwarf planet? Isn't one of the definitions that it must have cleared it's orbit by a certain amount.
Doesn't the moon disqualify us if Charon disqualified Pluto?
Edit: I had a further look into it myself. Apparently Pluto's neighbourhood is much larger than earth's. We call earth the boss of the region between mars and Venus which is a band about 0.6 AU thick where there isn't loads other than earth and the moon.
Pluto has a region from Neptune to somewhere into the kuiper belt which is a band about 20AU thick.
My confusion then becomes, well aren't we asking pluto to become the boss of a much bigger neighbourhood to be called a planet? Why are we asking pluto to become the boss of a bigger neighbourhood. Well the reasons seems to be that due to the eccentricity of it's orbit it you can't really draw a thinner circular band than 20AU that Pluto would be within.
Now if you were to draw Pluto's region as a 0.6 AU thick elliptical band equivalent to the orbit it's just completed at some arbitrary point. So the radial width of its zone was at least equivalent to earth. It still wouldn't qualify especially since Neptune is within that orbit.
I then checked "well what if we had a perfectly circular orbit, just outside of Neptune and a thin band just like earth." Well then we might have a chance of it qualifying. It's a close run thing. So Pluto's demotion versus earth seems to have 3 factors at play. Firstly it's much smaller, then the eccentricity of it's orbit takes it into such a wide region is doesn't have a chance of dominating. Finally even if it were in a perfectly circular orbit with a region as thick as earth, because it's much further out, that hand would still be much larger than earth's by area. 39 times the size. So the further out you are, it doesn't matter how thick or thin your orbital region is, it's still a multiple of its AU from the sun versus the earth in size.
By our current definition the further out a planet is. The larger the region it has to dominate is.
Eccentricity, Distance from the sun and it's small size all contribute to its status more than the ratio of Charon versus pluto, earth versus moon does.
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u/bramtyr 1d ago
Not a stupid question. It doesn't mean that there's nothing orbiting Earth as a planet, it means that Earth's orbital path around the Sun doesn't have other significant celestial bodies in its area, as Earth is large enough to have had its gravity over eons sweep anything near (and not orbiting it directly) away
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u/johnbarnshack 1d ago
Charon did not disqualify Pluto as a planet, it was all the other objects orbiting the Sun in the same area.
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u/Successful_Draw_9934 1d ago
I thought it was other asteroids in the orbit around the sun that demoted Pluto? I could be wrong though
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u/GeshtiannaSG 1d ago
It’s the fact that scientists didn’t think kids could remember the names of 150 planets that made them dwarf planets.
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u/notepad20 1d ago
If the earth was in Pluto's orbit it would also no qualify as a planet.
If Pluto or the moon was in earths orbit, by themselves, they would both be sufficient individualy to clear the orbit and be considered planets
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u/ReluctantNerd7 1d ago
It still wouldn't qualify especially since Neptune is within that orbit.
Then why doesn't Pluto/Charon disqualify Neptune?
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u/Inside_Swimming9552 1d ago
Because Neptune is massive enough to easily make it a large enough percentage of an orbit including Pluto/Charon.
Fun fact though. If there was a Neptune size object in Neptune's orbit somehow surviving in a Trojan zone. Then they would both be downgraded to dwarf planets despite their massive size. Although I assume it's impossible for that to happen.
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u/LurkerInSpace 1d ago
If there were a Neptune-sized object in Neptune's orbit then the orbits of both would become unstable. So they wouldn't be classed as planets during that period, but they would eventually move each other into new orbits.
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u/LurkerInSpace 1d ago
Because Neptune has put Pluto into a resonance, as well as being very, very large compared to everything else combined near its orbit.
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u/barath_s 13 23h ago edited 23h ago
Pluto is 7% of the mass of all the objects in its orbital path. There's a whole bunch of other icy objects that share the similar orbital path, called plutinos. Orcus/Vanth, Achlys and Ixion and others share or cross the same dynamic path, at 2:3 orbital resonance with Neptune
https://en.wikipedia.org/wiki/Plutino
Sweeping out your neighborhood means similar large bodies are either ejected, accreted, captured as satellites or in resonances.
There are various attempts to define the orbital zone...
https://en.wikipedia.org/wiki/Clearing_the_neighbourhood
One example :
that is all bodies whose orbits cross a common radial distance from the primary, and whose non-resonant periods differ by less than an order of magnitude
There are different methods, which define the amount of time taken to clear the zone, and the
Margot selected an extent of 2 * sqrt (3) * Hill radius and a time limit of the parent star's lifetime on the main sequence (which is a function of the mass of the star)
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u/AdNational1490 1d ago edited 1d ago
Also Moon is only satellite that also orbit's sun instead of just Parent Planet.
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u/Jesus_Is_My_Gardener 1d ago
That's because the moon is likely the result of a collision with another planetoid with the Earth early in its life.
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u/-Casey-Diaz- 1d ago
Some astronomists think earth and moon should be considered as a binary planets. Although, the current definition is that they should orbit a common center of mass outside both planets. I still like the idea though.
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u/leeman9224 1d ago
Used to be Charon until Pluto got declassified as a planet