After seeing the https://en.wikipedia.org/wiki/Supermoon in all the papers last week, I got to wondering; just how big could a moon actually physically get?
This, combined with vaguely remembered astronomic knowledge that the Moon doesn’t orbit Earth, rather both orbit a shared orbital centre that is located inside Earth and that in some cases (Sun/Jupiter I think) the point they both orbit is in open space.
So, to actually get to the point: Would it be possible to have two Earths orbiting each other (or a point between them) at a distance that each Earth would look as big to the other as the Moon does to us now and the orbital centre bit would be where current Earth is relative to the sun?
Would the +/- distance from the sun in each orbit cause ridiculous seasonal differences that would kill everyone?
Would tides be the same?
Could each Earth have it’s own Moon like we do now?
Could they share a moon that has a weird figure 8 orbit around both?
Our distance to the Sun varies about 10 times more during a year than the orbit of the moon. That distance is a minor influence on the seasons compared to axial tilt, so killing everyone is probably not a consequence even if the distance between the Earth’s would have to be larger.
I don’t think the gravity works out the same in that scenario, but I can’t be bothered to calculate it right now. But the tides would definitely not be the same. The orbital period would be much longer, for one thing.
Our own moon? My WAG is no. We definitely couldn’t have a figure 8 orbit moon. Way too unstable.
IIRC our solar orbit varies between 92Million and 94 million miles; so unless the orbital separation is more than tat by a substantial amount, no significant effect. Moon is 1/2 degree wide; earth at same view angle would be instead of 240,000 miles away, if my memory is correct, about 800,000 miles away.
Tides? Good question. I’m guessing they’d be a bit higher. Tides are a feature of differential gravity, i.e. difference between pull of gravity at near side and far side of planet. OTOH, gravity falls off as square of distance, 6 times the gravity, little over 3 times the distance 1/9th gravity, so maybe actually lesser tides.
There’s a distance from one earth where the force of gravity of the other earth would disrupt orbits - so close in, maybe. As far away as the current moon, that’s probably not a stable orbit. Similar to a figure eight… the short answer would likely be no… over millennia, slight perturbations would disturb the orbit, so not stable.
The Moon is almost exactly 1/4 the diameter of the Earth, so for the Earth to look the same size as the Moon it would need to be 4 times farther away, about 1 million miles.
Tidal force is proportional to the mass of the orbiting body, and inversely proportional to the CUBE, not the square, of the distance. So with 6 times the mass as the Moon but 4 times farther away, the tidal force would be 6/64, or less than one tenth of what it is now.
I’m not sure where how you figure “6 times the mass of the moon”
The Moon is 1/4 the linear size, making it 1/64 the volume of the Earth, So if the other earth were the same density as our moon, the tidal forces would be 64/64 just the same.
But the Moon also has a much lower density so it is only 1.2% as massive. Assuming the other earth had the same density as Earth, it would be 83 times as massive as the moon. So the tidal effects would be 83/64.
You might enjoy Robert Forward’s Rocheworld series.
If there were a twin planet as massive as Earth at the distance of the Moon, the two would probably both be tidally locked to each other the way the Moon is to the Earth. Once that configuration settled in, the only remaining tidal effects would be minor ones caused by orbital eccentricity (both would rotate at a practically constant speed, but revolve around each other at slightly different speeds if the orbit was at all elliptical rather than circular, causing a bit of “wobble”).
If they were four times further apart (so that they each looked Moon-sized from the other), probably not (but I’m not sure they’d remain in orbit around each other for the lifetime of the Solar System if they started that far apart; as it is, the Sun’s pull on the Moon is about twice Earth’s).
Oops, I was remembering that Earth’s surface gravity is 6 times the Moon’s. You’re right, the mass of the Moon is 7.3 x 10[sup]22[/sup] kg and Earth is 6.0 x 10[sup]24[/sup] kg, so Earth is 82 times the mass of the Moon. So the tidal force of the new super Moon would be about 30% stronger than today.
If the two were tidally locked, does that mean that the section of the ocean that was facing the other Earth would constantly be pulled up, so you’d have to sail uphill to get past it?
I thought it might not be stable since the Sun would be pulling one of the pair more than the other at any one time but maybe it would balance out…would it be more stable if the plane of the Earthses orbits of each other was perpendicular to their orbit around the Sun? Maybe some rogue Earth collided with regular Earth at a weird angle during Solar system formation to switch things round.
IMHO, if the center of mass is outside the diameter of the bigger planet, we have twin planets. Like Pluto and Charon.
This is NOT the case for the Earth–Moon system, where the barycenter is located on average 4,671 km (2,902 mi) from the Earth’s center, well within the planet’s radius of 6,378 km (3,963 mi).
But it’s close.
So the Moon could get a little bigger, sure. I suspect twice as much?
Anyone wanna do the math on the two body problem here?
The ocean (and the land/seafloor) would be pulled a few miles “upwards” relative to where they’d be without the other Earth there, but that surface would be “level” in the sense that the net force of gravity wouldn’t make any direction “uphill” or “downhill” – the equilibrium shape of each planet would be a bit stretched out into a prolate spheroid with its long axis pointing toward the other planet.
The location of the barycenter doesn’t make sense for use in definitions of planethood. You could have the situation where, due to the elipticity of the orbit, the barycenter is sometimes within one of the planets and sometimes outside. Are those double planets or does their status change twice every orbit?
That situation will, in fact, happen for the Earth-Moon system sometime in the distant future, assuming it survives that long. And after it no longer pertains, the barycenter will always be outside the Earth. So they are not currently a double planet, but will be one half the time for a while, and then will be one permanently? This does not make sense.
A bunch of reasonable answers to the big questions are found up-thread once we got past the confusion about how big the Moon isn’t.
Ref the snip …
Figure 8 & other weird orbits flat don’t work. It *might *be possible for something to make an S-shaped pass between the two primaries once. But anything that tried to stably orbit in a figure-8 fashion would end up unstable and either depart the system or crash into one of the primaries within a relatively few orbits. Like maybe the first. Such an arrangement *might *last 1,000 years with extreme good luck; it sure won’t last 10 million or 4 billion as the real Earth / Moon system has.
Each Earth having a small close-in Moon that orbits it alone *is *possible if the two Earths are far enough apart that the Moon orbit is *much *lower than the halfway distance to the other Earth.
See Natural satellite - Wikipedia for a wee bit more on this. Note that the article says it won’t work, but they’re talking about something like an object orbiting the real Moon in the presence of the real Earth or orbiting the real Titan in the presence of the real Saturn. That’s a vastly less symmetrical and hence less stable situation than we’re positing here. There’s a lot of good info in the rest of that article that the OP might enjoy.
Two problems with that definition, though: First, Terra doesn’t qualify as a planet under it, because we haven’t cleared our orbit, sharing it with another large body. And second, Terra and Luna certainly, by any measure, have more in common with each other than either does with Jupiter, so it makes no sense to create a category which includes Terra and Jupiter but not Luna. That’s why I favor the “rockball-gasball-iceball” classification over the “planet” classification.
There have been some good answers so far, so I’ll just point out that the question as to whether the bodies of Earth and Htrae would be tidally locked depends on their initial distance and rotation rates, the mass distribution and relative tilt of each body, and other factors (internal kinetics and stability of the orbit). However, it is very difficult to postulate a scenario in which this configuration could occur naturally. It is noteworthy, although hardly definitive, that no other planet in our solar system has a moon with such a significant mass ratio to the primary body as Earth and Luna. (No, Pluto and Charon do not count, as I’ll address in a minute.) Two distinct bodies of this size could not readily engage in dynamic capture of one another, so they’d both have to settle into similar orbits and slowly perturb to falling into a couplet. While the resulting couplet may be a stable configuration, the interim configurations are decidedly not, and it is almost inevitable that one body will be thrown into a widely elliptical orbit in such a way that the other becomes more circular, and any harmonic coupling will tend to reinforce that formation. A more likely configuration is two comparable mass bodies falling into each others libration points, but this only works if the eccentricity of the orbit is very small and the mass of the central body (in this case, the Sun) is large enough to dampen out instabilities; this again is more likely to cause the two bodies to fall into resonance orbits similar to the Galilaen moons of Jupiter.
The widely accepted (although by no means proven or without significant question) hypothesis of the formation and capture of Luna is an impact by some protoplanetary body (Theia) which directly impacted, contributing mass to the Earth as well as creating a ring of debris that coelesced into one or two separate bodies which ultimately formed Earth’s Moon. This kind of event is unlikely (although on a cosmic scale anything however unlikely will recur countless times) and we certainly see nothing like it in our solar system, nor have we seen any exoplanets with similar configurations to date, although this may well be just due to the limitations of our observation technologies.
Moons of the outer gas and ice giants fall into two categories; those which formed as part of the protoplanetary disk and enjoy relatively stable orbital conditions for billions of years, and those which are wondering trans-Neptunian objects (TNOs) which were captured by chaotic momentum exchage events as they passed near the large planets repeatedly. Such events can also contribute to the formation of couplets, as TNOs slow down enough to couple to one another and then jointly sling themselves out of the sphere of influence of the planet in an oblique orbit. The copernicoids Pluto and Charon were very likely joined in such an event, and then picked up their collection of tiny moonlets through other exchanges or were expelled from those bodies during impact events.
By the way, while it is assumed that Earth-Luna couplings are the result of rare collisions and reformation (based upon the Theia hypothesis and extensive simulation), planetologists could well be mistaken, and as we discover more detail about exoplanets (as the James Webb Space Telescope will allow) we may have to revise our expectation of planetary formation just as current exoplanet surveys have already upended existing models of gas giant formation. But there is a school of thought within the xenobiology community that we may likely be better off searching for life on large moons of giant worlds rather than looking specifcally for “Earth-like” worlds, which may be generally turn out more like Venus or Mars than Earth, because conditions suited to the development of life are more likely and may be more stable over planetary lifetimes. Certainly moons like Titan, Europa, and Enceledus, and Ganymede have the precursors and tidal energy to provide a credible basis for the formation of extraterrestrial life.