They’d have to be awfully close to each other for that to be possible, probably too close to maintain a stable orbit. Otherwise one would have to be significantly bigger than the other

Yes. The Earth and the Moon are tidally locked, so if the Moon was the size of the Earth we would have what you posit.

Yea, that would be a double planet. Depending on the exact sizes, distance from the star, and distance from each other, it might not be stable in the long term. But in the right circumstances, it could exist for a very long time.

Edit: don't read what i posted. I'm probably wrong i didn't understand tidal lock definition. Left it here because for shiggles.

Sadly no. The definition of a planet: It must orbit a star (in our cosmic neighborhood, the Sun). It must be big enough to have enough gravity to force it into a spherical shape. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun. The gravity requirement would make both objects to massive to be tidaly locked and in the goldilocks zone I believe.

If you mean like the Earth and Moon pair, that could work. If you mean two planets in the same orbit, that would be unstable.

And I assume this is due to earth's size/mass and 2 such bodies would compromise each other's orbit, whereas Pluto & Charon do their thing fine (not "planets").

It wouldn't be that simple. You have to take the the inverse square law Into consideration

Not likely locked just slow rotation (check out "Eve Times Four" by Poul Anderson)

Even if they were in a perfectly perpendicular orbit around each other compared to their orbit around the star?

What's the lower (upper?) bound on "similar size"? The Moon is pretty big, as moons go: is there something about similar masses that would prevent them from simultaneously tidally locking?

Based on this post, I believe you are smarter than you think you are.

Huh maybe, gota stop commenting without my glasses. Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. I'm probably wrong as idk wtf I thought tidal lock ment.

I mean planets can definitely be tidally locked to a star, that's probably what you were thinking of. We have one (or two?) Of those in our solar system

[removed]

I don't get it. How exactly does the inverse square law tie into this specific case?

They are different sizes but Pluto and Charon are mutually tidally locked. They rotate around each other instead of Charon orbiting Pluto. They're really close together so they even share atmospheric gases every six days.

I think it's possible. Robert L. Forward was a physicist and wrote a novel called Rocheworld which depicted such a world.

There is actually a science fiction book about just such a pair. One is a water world and the other dry. The human explorers arrive just as the water is about to switch planets in a once a millennium event. Fun read. I think it was Rocheworld? I think I only read the second one of the series.

There is also an animated movie featuring this concept voiced by kirsten Dunst called Kaena: the Prophecy. And the movie Upside Down, also starring Kirsten Dunst, that has some ideas on the socio-economic repercussions of such a setup.

Ultimately the math probably doesn't work out for any of these concepts without some fantasy style materials being involved. In order for two planets to be close enough to share an atmosphere would require orbits of such extremes that it would rip the planets apart.

The moon is tidally locked to the Earth. The Earth is not tidally locked to the moon. If it was, the moon would only ever be on one side of the earth.

I don't think the "clear the orbit" criterion refers to a moon or double planet situation. It just means there isn't another big thing orbiting the star at the same distance. I don't know what shiggles and whatshingles are.

I mean I did edit it saying I'm wrong? Shits and giggles. I'd rather leave my correction that I'm wrong than just delete. And my glasses are not on lol don't get old can't see shit.

Tidal locking happens when extreme gravity differences cause the objects to slow down. The steeper the gravity quotient, the faster the tudal locking occurs. The moon is relatively small compared to the Earth so it stopped first (relatively) but as it recedes, the earth slows too. Meaning our days are getting longer. By about .25 seconds per century.

I think so, i've even simulated it a few times, but I think it's more stable if one of the bodies is significantly smaller

Similar question, if there was another Earth sized planet in our (therefore each other's) langrange point, would that be stable?

Planets yes. Earth like worlds no. Our own ELW is fine tuned to a fault. It follows that other ELWs would have a similar moon, gravitational forces, axial tilt etc.

I knew that they were tidally locked to each other but I had no idea that they can sometimes share an atmosphere.

>By about .25 seconds per century.

By about 0.002 seconds per century.

I misplaced the decimal by orders of magnitude. Pathetic.

The inverse square law proposed by Newton suggests that the force of gravity acting between any two objects is inversely proportional to the square of the separation distance between the object's centers.

So it is not as simple as the moon is tidally locked to earth, so, therefore, two earth sized planet could be tidally locked

I just didn't understand your edited comments.

This may help clear it up a little for you.

https://astronomy.stackexchange.com/questions/22638/a-tidally-locked-double-planet

2 planets in the same orbit would collide with each other in some amount of time depending on the other objects in the system, their masses and distances from the pair.

These other planets pull on the pair in differing strengths and cause one to speed up and the other to slow down. Eventually they collide and become one body.

Well, to be honest, all satellites and their planets orbit each other. In most cases the center of mass of the system is inside the planet because it’s very much more massive than the moon. In the case of Pluto and Charon, the difference in mass isn’t that great so you can tell that they’re orbiting around a point that isn’t entered on the planet’s center.

No Lagrange points only work because the mass is negligible at them

Edit Lagrange are stable because they don’t impact forces on the other two bodies when you account for rounding errors

No, the planets would probably end up orbiting each other or colliding. Lagrange points are stable for things like satellites because their masses are insignificant compared to the Sun and Earth. If an Earth-sized planet was at one of the Lagrange points(mostly L1 and L2, I’m not sure about L3-5), the planets would be attracted to each other much more than to the sun.

That's not necessarily possible though. The Moon is tidally locked to the Earth because its smaller by a large margin. The Earth would eventually tidally lock to the Moon as well, but our Sun will destroy both the Earth and Moon long before that ever happens.

Two similarly sized bodies would take longer to lock to one another than the Moon did to the Earth, but less time than it'd take the Earth to lock to the Moon. I've no idea how long that'd actually take though, or if it'd be within the lifespan of the average star similar to the Sun.

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

|Fewer Letters|More Letters| |-------|---------|---| |JWST|James Webb infra-red Space Telescope| |L1|Lagrange Point 1 of a two-body system, between the bodies| |L2|Lagrange Point 2 (Sixty Symbols video explanation)| | |Paywalled section of the NasaSpaceFlight forum| |L3|Lagrange Point 3 of a two-body system, opposite L2| |L4|"Trojan" Lagrange Point 4 of a two-body system, 60 degrees ahead of the smaller body| |L5|"Trojan" Lagrange Point 5 of a two-body system, 60 degrees behind the smaller body|

^(6 acronyms in this thread; )^(the most compressed thread commented on today)^( has 11 acronyms.)
^([Thread #8450 for this sub, first seen 19th Jan 2023, 06:15]) ^[FAQ] ^([Full list]) ^[Contact] ^([Source code])

Like the Pluto-Charon system

The universe is essentially infinite, but I'd argue that's not enough for the chances of two identical mass and size bodies to form right beside each other and have a stable binary gravitational influence WHILE orbiting a star.

They would most likely collide(possibly like how proto earth collided with a similar proto planet which ejected the material that became the moon).

The problem is as they neutrally orbit each other, one would be closer to the star and feel a stronger gravitational tug, while the other planet would be pulling against(which would be in constant flux due to their changing position to each other relative to the star) and quickly lose stability

In theory, yes. We have discovered from binary asteroids to binary stars so binary planets are physically possible. However; if the separation between them is too close tidal friction could become an issue, making them lose momentum over time resulting collision between the two.

Other planets in the solar system would likely perturb their orbits, too.

Upside Down was soooo good 🐝

[removed]

Yes but they would know each others existence through gravitational influences. For example a comet hurtling through the solar system would be influenced by its mass, and we would know it’s there, then of course space age we could just check it out

Oh for sure they would, but it would just slowly influence it a tiny bit over huge time spans(unless it is jupiter size and closer to earth than mars or venus). The star is going to be a much more powerful pull than even that would be.

Hahaha maybe one of the planets woulf eventually find a home as a moon of it though

[removed]

Isn't Theia hypothesized to either have formed or migrated into Earth-Sun L4 or L5 point and then had to be gravitationally nudged out of there by Jupiter and / or Venus?

Yes, but you may run into problems related to tidal heating

Yep, Named after the Roche limit.

If they are tidally locked, there won't be any tidal friction, would there?

Could a super Earth be orbited by an Earth like moon with an atmosphere? Thus answering the question above

Yes, it is possible. It's highly unlikely that they'd be the same, like most women's boobs. One is always a little bigger than the other.

Here's the thing that no one else here has mentioned -- The orbits of two similarly sized objects around each other (such as Venus and Earth), while also being in orbit around a much larger object (the sun) is much more stable if the planet's orbits around each other is in the opposite direction as their orbit around the sun. (clockwise and counter-clockwise, for example.)

That's how the math works, but weirdly, there are no examples of counter-rotating natural satellites in our solar system, even though they should be more common.

There is still much we do not understand in the universe.

And btw anything living on a planet system like that wouldn't be to happy given the crazy geological activity.

Remember friend, you can never have enough zeros.

[deleted]

Which was also the name of a pretty cool comic book back in 2014 or 2015

100% yes.

What scientists and computer modeling has discovered (theorized) is that binary systems in general are far more common than solitary ones. So binary stars, binary planets, binary moons (binary here being objects which orbit around one another and their center of mass orbits around another body).

You can see this in models yourself too. For instance, the game Elite Dangerous simulated the entire galaxy by using their stellar forge algorithm. Obviously its a game, so take that as you will, but they used a lot of real physics in creating their algorithm for the Stellar forge. And you will find many star systems have binaries all over the place.

Relevant xkcd

Yes, I was going to cite that example too. Charon is fractionally largest "moon" (12%) tied to a "planet", so that could encourage tidal locking. (Earths Moon is second at 2%.)

I think the "missing planet" is on the other side of the sun and we can't see it. Maybe.

U can say its already the state for the earth and the moon

You could have 2 planets in a relatively stable orbit for a very long time, like pluto and charon. However, fun fact, you cannot have 3.

Yeah it is absolutely possible, in terms of celestial mechanics.

In terms of the process of planet formation... it might be a bit improbable. But all that means is that you'd have to look for a while longer through the vast universe before you found it, right?

You could even paint a pretty clear picture by calculating the Earth-Earth Roche limit. Earth is not a rigid body, it is prone to deformation, but we could probably still use the rigid body equation if we give some extra padding. The Earth-Earth rigid body Roche limit is about 8000km so if you say it's ... I don't know... 2 or 3 times that you could probably avoid deforming your Earths too badly as they orbit each other.

In fact if you stick them at about 30 thousand km apart you could (I think...) preserve your 24 hour day, while also having a massive Earth in the sky of each Earth. But only from one part of each planet of course.

For people of this binary world, would be easier to establish a permanent presence in space than for us. But I think it would be harder to get to their Moon. (Assuming you keep our current Moon as it is, with maybe a slightly more wobbly orbit because of the proximity to the binary Earths around which it orbits.)

And of course exploring "the other Earth" would have been a major preoccupation throughout history. With all kinds of speculation and wild tales of what must lie above.

Really cool!

This kind of boggles my mind cause I knew our moon was unusually large as far as such bodies go relative to their planets, but the fact it's only 2% while exerting 17ish% the gravity on its surface as we have on Earth seems counterintuitive. I know that proximity to the center of the mass influences how strong the gravitational attraction is, but damn, that's way more skewed than I had imagined.

Why can't you have two planets in each other's Lagrange points? That should be stable

[removed]

Or it could be living off that heat!

The Lagrange point sets the mass of the third body to zero before solving the differential equation.

Sure, you could also orbit a Jupiter.

I don’t have enough expertise to give you a definitive answer, but I think you don’t fully understand how Lagrange points work. Mind you, I’m not sure I understand how they work, either, so what I’m about to say may be worthless. So keep some salt handy.

A Lagrange point is a place relative to the orbit of the satellite object where the gravity of the satellite and its primary cancel out. Any third object that somehow wanders into one of these points tends to stay there because of the gravitational interaction between it, the satellite and the primary.

So there are Lagrange points relative to the Earth and the Sun where the gravity of both cancel out. The JWST is stationed at one of these points now. Saves JSWT a lot of fuel for station keeping.

There are Lagrange points relative to the Moon’s orbit where the gravity of the Earth and the Moon cancel out as well. We’ve placed nothing there yet but that could change one day.

The deal with Lagrange points is that any third object that somehow makes it’s way to the location of one of the Lagrange points tends to stay there. This happens with small objects such as spacecraft or asteroids floating around the solar system because they don’t have much mass relative to the Earth or the Moon to affect the location of the Lagrange Point.

Now let’s consider a planet.

Planets are much bigger objects. Everything in a planetary disk starts out orbiting the star. Objects collide with each other frequently. The thing is these objects are not initially in any Lagrange points. They approach and each attracts the other. The Lagrange points aren’t likely to be along the vector of approach and there’s a lot of force pulling them together. As they get nearer, the location of the Lagrange points change because of the large amounts of mass approaching each other. If they collide, after all, there would be a lot more mass where the original planet was, changing where the Lagrange points are; the same thing happens when the 2 planets are closing in on each other.

So planets, because they’re so big, will orbit each other or collide. I don’t see how they could be in each other’s Lagrange points.

I think it would be statistically more likely to find a gas giant with 2 habitable moons

That’s an interesting question. A super earth would likely wreak havoc on an earth due to gravitational forces. Oceans sloshing, enormous heat generation with super volcanic activity. Poisonous atmosphere.

True, they can't be in each other's Lagrange points, simply because don't exist for a single object. They exist between two relatively massive objects orbiting each other, for relatively light objects.

If a planet of the exact same size as earth existed, in the same orbit/speed as earth, but 12 hrs (6 months) off, would we ever know?

I mean they proved that a donut shaped planet could exist so I’m positive this can exist.. space is essentially the infinite so if the physics are possible, it’s probably out there

They orbit a common point approximately 1000km above the surface of Pluto.

L4/5 isn't stable for larger object. If Theia collected enough mass it would nudge itself out without the help of other bodies of mass. Being about 10% Earth's mass in the Sun-Earth L4/5 points I believe.

They made a movie, everything was reversed like a mirror. Edit, 6 months off, somehow I think that's what you mean

It's a volume and density thing, when you're dealing with cubed radii it gets a bit non intuitive

[removed]

Youn right, 6 months. And thanks for the suggestion!

Sounds like an old movie Journey to the Far Side of the Sun.

If density is constant then surface gravity increases linearly with radius: an r^(3) mass divided by gravity's r^(2) falloff.

So 10x the gravity means 10x the diameter and 1000x the mass. Crazy!

Density skews things, but even ignoring it is good enough for a sanity check. (I'm pretty sure 2x the density = 2x the gravity at the same size)

12 hours? Definitely. That's only about half a degree away in our orbit around the sun. The impact wouldn't be long in coming.

6 months though? Then it would be exactly on the opposite side of the sun, in our L3 point.

It was actually once speculated that there might be a "Mirror Earth" there, but it wouldn't be a particularly stable location, and our probes have long since confirmed there's nothing significant there.

Keep in mind the moon is only ~1% the mass of the Earth, it's not doing much.

I would imagine the time to tidal locking decreases at *least* linearly with increasing gravity from the "lock-er" (e.g. twice the force = half the time, maybe less. That's normal for most systems), in which case if the moon were Earth-mass, 100x larger, Earth would lock to it 100x faster, and the 50 billion years until tidal locking (~55 with time served) would be closer to 550 million - almost before liquid water appeared on our surface.

I meant the L3 point, but I assume you are right and that it's not possible.

Why closer? Gravity is a measure of acceleration, not force - replace the moon with an Earth-sized planet and it would accelerate towards us at exactly the same speed.

Of course, we'd accelerate towards it about 50x faster than towards the moon, which would rapidly destabilize things, but as long as we gave Earth a good strong sideways kick at the same time so they didn't collide on the first pass, the two should orbit their mutual center of mass, just as we currently do with the moon - even if that center is currently 1700km below Earth's surface. And over time tidal energy transfer would circularize the orbits, and tidally lock us to each other. Assuming we didn't give just the right kick to start with.

Is there a minimum stable distance for tidally locked binary planets?

They can't escape without one getting a huge outward kick of energy, and they can't collide without a huge inward kick. Tidal energy transfer is no longer happening, and I think you have to be pushing spiraling neutron star densities and speeds before you can shed much energy through gravitational waves. And so long as the sun is many orders of magnitude more massive, you're not going to get any pesky chaotically complex three-body problems cropping up. Just don't try to add a moon.

I suppose if they cross each other's Roche limit, where tidal stress will tear moons apart into rings, things might get complicated... but a quick search says that's only ~11,000km for Earth - a twin planet would actually be touching the surface before its center of mass crossed the line, but I think even such a "dumbbell planet" would still be stable so long as it wasn't spinning fast enough to throw stuff off the surface at the outer tips (you *know* that's where the spaceports would be!)

So long as they're well within each other's Hill Spheres I don't think the sun should be an issue any more than it is with our moon. (The moon's orbit is about 1/4 of the way to Earth's Sphere) And Earth already orbits our combined center of mass with the moon, it's just that the size discrepancy (it's only ~1% of our mass) means that's still within Earth's volume, about 3/4 of the ways up from the core.

Forming would be a different question, but e.g. if Theia had been considerably bigger (or faster?) when it hit proto-Earth the "splash cloud" might have coalesced into two much more similarly-sized sister-planets.

Exactly the same size (to how many significant digits?) would indeed take crazy long odds. But within 10% or 20% is probably not too outlandish.

Yeah I messed up with the 12 hrs, probes make sense.

Well, of all the points I think L3 would be stable the longest since the attraction between planets is minimized. Over time the extra force might cause them to spiral inwards, but the effects of other planets would destabilize things more quickly.

Hello u/Thirdy-DOg, your submission "Are Two Tidally Locked Earth in One Solar System Possible?" has been removed from r/space because:

• Such questions should be asked in the "All space questions" thread stickied at the top of the sub.

Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please message the r/space moderators. Thank you.

[removed]

I don't think so, if I remember correctly only the orbital distance and eccentricity affects tidal friction. Synchronous orbit aka tidal locking doesn't change anything mentioned above.

Tidal friction is caused by the planet's rotation. The tidal force deform the planet, and the rotation drags the deformed shape to be misaligned with the moon.