Definition of a binary planet

[Aury]

Is something that unfortunately the IAU never really came up with/thought about much...

There are two nice definitions I have found:
1) The gravitational barycenter of a two-body system must be between the surfaces of the two bodies, not within the surface of any one body.
2) The less massive of the two bodies must be gravitationally controlled more by a mutual parent body (the parent Star(s)) than the more massive of the two
Furthermore, with the standard IAU definition of a planet, the planetary discriminant must be greater than 1 relative to its distance from the parent body (ie, it must have the gravitational power to 'clear the neighborhood', and thus be a proper planet in its own right)

Definition one is satisfied by the Pluto-Charon system, and will be satisfied by the Earth-Moon system in ~1-2 billion years due to the slow 'unwinding' of the Earth-moon system.

Definition two is, to some people's surprise, satisfied by the Earth-Moon system. - Sol exerts more than twice the gravitational influence on the Earth-Moon system than the Earth does! So while the moon may have the orbital characteristics of a satellite, the moon has the gravitational characteristics of a 0-degree co-orbital planet!

Furthermore, the Moon ALSO has the gravitational power to clear the neighborhood, were it an independent body, unlike either Pluto or Charon (which have less than 1/10th the gravitational power required; ironically, were Eris swapped with the moon, it too could qualify as a planet in a binary planet system, because it has just enough gravitational power for that distance from Sol; Pluto would not however)

My own definition that I use, I combine definitions 1 and 2 with an AND condition, as well as an AND condition for its gravitational dominance; so while the Earth-Moon system is a planet-satellite configuration for now, wait around a billion or so years, and there will be a 5th terrestrial planet in the Sol system: the Moon.

(Also, random fact about the Earth satellite system: it is predicted that at any given time, the Earth has a 1-meter companion temporary satellite that sticks around for a few months to a year before being ejected, and another taking its place; there is at least one stable 5 meter quasi-satellite that follows the same Apollo orbits as the Saturn V 3rd stages; the 1 meter bodies are much smaller and harder to detect however.)

[matix]

Very interesting, Never really thought about what would define the rules for binary planets

[Vachtra]

I'm not sure the theories are quite thought through yet. Since the entire idea is based on speculation and limited observation, there is no reason to believe the trend to move away from the earth is continuous or that it won't get closer as well. The main influence on the moon is the sun as mentioned. The moon and earth both orbit the sun, neither of them curving outward but always toward the sun. The moon's orbit is a little more erratic but the sun is still the main influence.
In my looking into this the tides are cited as a cause of this distance widening of the moon. This is a little odd since there is no gravitational friction. Also the tides are a result of the moons gravity not the other way around.
Interestingly the movement of the moon away from the earth at such a rate does correlate somewhat with the rising sea level. If this is the reason for the movement then as the icecaps freeze and thaw then the moon shifts with the overall surface of the water, not a degrading or unwinding orbit but a symbiotic relationship with the earth.

(To be posted to one of my blogs)

[Atarlost]

There is tidal friction for non-locked bodies. Tides produce heat and that thermal energy comes from the kinetic energy of the bodies.

The moon may be tidally locked to the Earth, but the Earth is not tidally locked to the moon.

[Vachtra]

I'm sure there is friction on earth but it goes both ways as the tides interact with the continents. Like water in a bath tub sloshing around it, although it pushes one way it also pushes another. The water doesn't just go over continents pulling everything in it's path with it. If that were the case then we may eventually have the moon stand still in the sky but it would still be in orbit. Earth's center of gravity, not in relation to the earth moon combo, doesn't change.
Before any yelps come up, this does not include rockets launching, etc. merely a note relative to tidal activity.

[Vachtra]

This may be my own musings, and probably is, but should Jupiter be considered a planet since it hasn't really cleared out it's orbit. There are two asteroid clusters that orbit the sun with it, all keeping their discance from eachother. Sure it's the largest thing in the orbit by far but it hasn't finished cleaning it's orbit.
And.... Yes I do think Jupiter is a planet. Just throwing that out there.

[matix]

The moon doesnt just pull on water and such to create tides, it also pulls on continents and everything. So shouldnt we assume that the same could affect the moon?

[Vachtra]

We should definitely assume that the moon also has the same pulls on it. Unfortunately there isn't much that the moon has on it that will pull much, nearly no atmosphere to speak of. And while I'm a big fan of rounding as little as possible (20 significant dights is my fave) What we are really dealing with are two enclosed systems, earth and moon, which although rotate around eachother, aren't swapping any spit.
Shortly saying, the tides on the earth are not affecting the moon since the moon is being pulled by the center of gravity of the earth and the earth likewise with the moon. When tides shift on earth then the center of gravity isn't moving around, the planet is, and that just barely. In fact the center of this dance with the mood is somewhere around 1000 miles under the surface of the earth. the deepest part of any ocean doesn't even hit 7 miles. Numbers may be off a little but the point won't change. Water is a small part of the entire mass of the planet and it merely jiggles compared to the while picture. I drew a scale once of the layers of the planet and found that indeed if you look closely at a cue ball it's rougher to scale than earth.

[Atarlost]

It doesn't really matter which body is suffering drag relative to the other in this case. Earth is tidally heated by the moon. Rock bends. That's why Io is hot enough to be volcanically active in spite of its small size. That thermal energy comes out of the kinetic energy of the Earth-Moon system.

[Vachtra]

You're comparing earth's tides to Io... Since no one has actually gotten a lot of data on what actually is happening on Io it's impossible to be certian what it is that makes it so active. It supposedly has a solid surface but it bulges 300 feet. that doesn't sound that solid. It seems more probable that Jupiter's high electic forces are literally cooking Io from the inside and what we see is the slag on top of a smelted moon.

I still don't buy that the earth is being heated in this manner. kenetic energy is potential energy released. If you release all the potential energy then where is future kenetic energy coming from.

Scenario: The gravitational pull of the moon affects the tides, again not the only source of tidal activity.

Result: The water shifts on top of the oceans causing waves that eventually make it to land and slosh around, sometimes violently eroding away land. The water then moves back the other direction as the tides shift. There is little change in deep ocean currents due to the tides. It's mostly a surface thing which includes rivers and such.

Effect: The tides convert the energy to work as they remove the rock. Heat is absorbed by the ocean, water is really good at this, and the ocean is kept moving. Heat produced from the pressure is also absorbed by the water and released as the pressure goes down at low tide. High pressure releases heat and low pressure absorbs heat. Excessive heat merely turns the water to vapor which turns into clouds mixes with the atmosphere and returns as rain. I'm sure the impact of each drop does something but that's getting a little picky.
In the end the energy of the enclosed system of earth hasn't changed. It merely moves between kenetic and potential (there's no such thing as thermal energy, if it's hot and the heat is being transfered then it's kenetic, if it's not being transfered then the heat is merely a property of the substance and is potential)

Side note the change in ocean pressure is small. Usual pressure can reach over 13,000 psi whereas the most tides will change it is just over 13 psi or 1/1,000 of normal for the deepest areas of the ocean. Granted the average is around 5,200 psi, 13 psi still isn't much.

Looking back at what prompted this I see that there seems to be a minsunderstanding of what was said. I mentioned that there was no gravitational friction. In other words I was saying that gravity doesn't cause friction. I wasn't saying that there was no friction anywhere. Gravity is one force and friction is another force. An object will always be affected by gravity, it's part of being an object, but it will not always be affected by friction. That depends on work being applied to the object and what there is around it to cause friction in opposition to the work. In space there is next to nothing to cause friction. The problem with satelites falling out of the sky is that they're in low orbits where there is some friction. Without occasional help they will be slowed down and fall.

[Atarlost]

Tides don't just warp fluids. No real material is perfectly rigid or perfectly elastic. This creates heat. Stresses on a material that is neither perfectly rigid nor perfectly elastic produce heat. By the law of conservation of energy that thermal energy comes from somewhere and the only other energy in the system is kinetic in the form of orbital velocity.

Conservation of energy is one of the best tested of all fundamental physical laws.

[Vachtra]

I know. Let's discuss what heat is.

Heat is the propincity of an object to release or absorb energy. The releasing and absorbing of such is kenetic energy. The state of being hot is static energy. Only when this heat is transfered does any work take place, the work being the transfer of energy, in this case heat. If an object is extremely hot and still isn't transfering any energy then the heat is merely a property of the object and can not be called hot or cold unless comparing it to the properties of another object.

The terms hot and cold are only relative. To liquid nitrogen an iceberg is hot since the contact would cause the liquid nitrogen to boil away. To molten glass 120 degree outside heat is cold since prolonged exposure can crack it due to rapid cooling.

Energy must be transfered for any work to take place.

A big misconception in your statements is that heat is being created. Heat is not created but is the transfer of energy from a source to another. The energy was already there it's just that the material wasn't willing to give it up yet. Read over the conservation of energy again. The orbiting of planets transfers no energy from one to the other. Thus all the energy on earth either comes from either the sun which is radiating energy, although earth is radiating about the same amount out, or from another part of earth.

You may be thinking about experiments where heat is a product of a chemical reaction. In such reactions you can determine the amount of heat being produces (merely the sum of the energy no longer needed) by finding the total energy of the chemicals to begin with and the total energy of the chemicals at the end and seenig what's left. There are other reactions though that absorb heat going on at the same time. The heat from the first reaction isn't lost. It will find a place within another reaction that needs it.

The temperature inside the earth is mostly static energy in that it more or less just sits there without a need for an outside cause. Again if there is no transfer of heat there is no work. In this case I am using the core as a whole for this statement. Heat may well be moving within the core here and there but if it stays in the core then as a whole there is no transfer. Where it originally came from can only be speculation.

Orbiting doesn't transfer energy it's merely the change in position of two objects in a stable pattern. There is static energy in that if something changes that orbit then the two can be pushed together and the attraction, gravity, can then cause all kinds of havoc when the objects are close enough for energy transfer, like Jupiter is with Io.
This is why I have been calling earth a closed system.

Side note: I find it disturbing that people really think they can know what's going to happen in billions of years from at best a few hundred years of observation. It's like watching a horse race without knowing the horses and only watchin the horses leave the gate on the first race and then saying who will win the next fifty races. Not only is there a great lack of data but also the presumption that we even know what data we need to remotely understand what's going on.

[Atarlost]

All that's true for perfectly rigid objects and point masses. Except for the stuff about hot and cold which is a rather insulting straw man. Temperature doesn't matter, only the change in thermal energy due to tidal heating.

There are no perfectly rigid objects.

Any body subject to gravity is distorted from a sphere by that gravity. Even airless rocky bodies. The tidal bulge attracts in return.

If the body is not tidally locked the body orbiting with it moves relative to it. The bulge draws the other body towards it, slowing or speeding its orbit and contrarily speeding or slowing the rotation of the second body. The warping of the body to move the bulge with the other body creates heat as warping any real material does. And, yes, on a very small scale that heat is kinetic energy. That kinetic energy comes out of the orbital energy of the two body system. An astrophysicist can probably show you the math of why, but I expect the viscosity and elasticity of the bodies comes into it via the rate at which the tidal bulge responds to the change in gravity caused by the relative motion of the other body.

[Vachtra]

Not trying to be insulting, I just don't think you understand the issue and making sure that anyone reading can follow the line of reasoning with relative ease. You are talking about gravity creating heat and the moon heating the earth, that is to say making it hotter than it would have been otherwise by transference of energy which is the only way for one object to add energy to another and, if you think about it, to loose energy in the process.

Lets look at the airless rock you mentioned. You're right, the exact same thing is happening there in a micro scale. Energy will move about in the object even if it's in a distant orbit and rotating relative to the the thing it's orbiting or even if it's orbit isn't perfectly circular and it isn't rotating at all.

In this case the change in pressure is nearly instintaneous unlike the oceans. The pull will cause the pressure in a straight line relative to the object it's orbiting to decrease while the sides being tangent to the orbit will increase in pressure. The decreased pressure will absorb energy being released from the sides who have pressure increasing and thus releasing energy. Since there is no way for the energy to leave the body (I doubt this would cause electromagnetic disturbances to any real degree although it does have its own little micro field) the bulk of this energy is merely shifting in the enclosed system, from higher pressure to lower pressure, that is a rock. The total energy does not change. Again this does not take into account the sun heating it but merely orbital influence. Throw radiant (kenetic) energy into it and you have a whole new discussion.

The bulge mentioned is in response to the thing it is attracted to. It is not what attracts the thing. The attraction is between the centers of gravity no matter if it's a marble or a galaxy as long as it is a system. this is how earth and moon orbit the sun together and how the solar system orbits the galaxy. There is a single point where the center of earth and everything on it is centered and no matter what the tides do that point does not move, everything else does. the same goes with the sun, moon, galaxy, and yes the lowly marble. The ocean doesn't have some special moon attractors.

The earth is merely a rock with differing parts and points of influence making the change in pressure, which is actually where frictional heat comes from, so complex as to be wonderful. Again orbiting is not energy but the balancing of gravitational attraction with velocity. Unless they come into relative contact no energy is exchanged.

Edit: After thinking about it for a spell it would make sense for the tides to do exactly what they're doing and in the process releiving the presure shift caused from the moon's orbit. Instead of the rocks decreasing pressure the water merely fills in and takes the pressure off. Likewise the sides on low tide. when the pressure incerases then water flows to a lower pressure until it's more even. This would actually relieve the stress inside the earth and lessen the influence of pressure on the dynamics of earth's internal energy.

Edit 2: i guess if you really wanted to break down why an orbiting mass affects the object being orbited you would have to break down the object being affected into it's coponents and track the individual orbits of every particle and the interactions of these particles with their close neighbors. The pushing and pulling of individual atoms in a masive cluster canceling out the other individual atoms effect to the overall orbit but creating countless exchanges of energy as they all try to orbit in their own particular way. Throw in some strong and weak nuclear forces and some magnetism and you have an equation only a mother could love.

[Atarlost]

The earth is merely a rock with differing parts and points of influence making the change in pressure, which is actually where frictional heat comes from, so complex as to be wonderful. Again orbiting is not energy but the balancing of gravitational attraction with velocity. Unless they come into relative contact no energy is exchanged.

If two bodies are not in contact they are not orbiting There's nothing magical about the strong and electroweak forces that they can exchange energy while gravity cannot.