Binary planet system vs planet + moon?

Question

I know two bodies orbiting each other are considered planet + moon if the center of gravity is inside one of the objects, which becomes the planet. However, isn't this definition rather arbitrary? Is there a logical reason for saying the center of gravity has to be inside one of the orbiting bodies, or were astronomers saying, "Look, we have to draw the line somewhere, so this is where we'll draw it"?

Also, based on this definition, how close are the Earth and the Moon to being binary planets? Considering the size of the moon relative to the Earth, while the center of gravity may still be inside the Earth, it must be pretty close to the surface.

I'll give enuf the 10 points for the reference with the interesting graphic. As for the answers, none of them convince me this definition still isn't arbitrary - if the center of gravity is inside the larger object, the "planet" will still appear to wobble in a tight circle.

In the Earth/Moon system, it occurred to me that finding the center of gravity is possible if I know the weight of the moon. So, checking Wikipedia, I see the moon is .0123 earth masses. Factor in its 240,000 distance from the Earth, and the center of gravity of the Earth/Moon system is about 3,000 miles from the Earth's center, or about 1,000 miles from the Earth's surface. That ought to produce a measurable wobble to an observer on Mars with really good equipment. Also, I figure if the moon were about 325,000 miles away, Earth/Moon would now qualify as a binary planet system.

Answer 1

Two objects of near equal mass can also orbit around a common barycenter which is outside both (see animation at link below).

The definition of a planet includes the necessity of orbiting a star. Don't know what the IAU would do with two "planets" orbiting each other in the way described above... it would certainly mess with the new definition of a planet "clearing it's vicinity." I'm picturing two objects orbiting each other while the barycenter follows an orbit around the star.

We need some undergrad physics major to simulate the 3-body system... smile

Aloha

Answer 2

For the binary planet system to work the center of mass of the two objects must be between the two objects, that way both objects are orbiting a fix point somewhere in their system. A Planet + Moon system has the center of mass of the two objects almost at the core of the "planet" With the Earth/Moon system the center of mass is still within the inner core of our planet. Making it by definition a Planet/Moon system. If the Moon, givin its present size were further away from the earth it could be that the center of mass would move out of the center of the earth...but that would be have the distance to Mars for that to happen. The earth is much heavier than the moon. Remember we are mostly Nickel and Iron...the moon is made of lighter material than that.

Answer 3

I think you've pretty much answered your own question there! If an object orbits the sun (this could be a planet, dwarf planet or even asteroid) and another body orbits that object where the centre of gravity falls within the first object, this would be a planet (or dwarf planet or asteroid) and moon system.

Where two bodies orbit each other with the centre of gravity falls between the two, this would be a binary system.

Answer 4

I've heard that the tracks of both components of a binary planet through space, in the reference frame of the sun, is always concave toward the sun. Earth and Luna satisfy that requirement for double-planet status as well as the requirement that the COM is inside Earth.

Answer 5

this question's been asked like a billion time mostly for pluto-charon
i think this is an excellent definition if the center of grav is outside the planet, then the moon doesnt orbit the planet, it orbits a point in space,and therefore not a moon...

Answer 6

Its the first time I consdidered this.

Seems like an elegant way to define it. If the center of gravity is WITHIN the larger one, then the small one orbit around it, if not is doesn't. They then orbit around each other.