I believe the rule of thumb is binary planets’ barycentre is external to either body. This is the case with Pluto/Charon, I think it’s also the case with Earth/Moon.
Other way around, the further apart the objects are the less likely the barycentre is to be inside one of them, you can picture it as a rubber band with a dot drawn on it, the more you stretch it the further the dot gets from both ends even if it gets further from one end faster.
That’s a good rule of thumb… but it’s probably not enough; no reasonable definition would call Jupiter a star, or even a brown dwarf, or the Solar System a binary system, yet the Sol - Jupiter barycentre is outside the sun… (the whole system’s barycentre is sometimes inside the sun, but that’s due to Saturn’s, Uranus’, and Neptune’s pulls cancelling Jupiter’s).
I’d call the barycentre thing a necessary but not sufficient requirement; a proper definition of double planet should probably also take into account other factors like the relative mass and density of the bodies, and their minimum and maximum distance.
I believe the rule of thumb is binary planets’ barycentre is external to either body. This is the case with Pluto/Charon,
I think it’s also the case with Earth/Moon.It is not the case with the earth and the moon. It would be if the moon was 40% more massive
Yeah, I went and checked after posting.
My hunch is that if the moon was closer it would ‘drag’ the barycentre closer to the moon.
Which, given the moon is slowly receeding, means it was probably a binary early on in the formation of the solar system.
Other way around, the further apart the objects are the less likely the barycentre is to be inside one of them, you can picture it as a rubber band with a dot drawn on it, the more you stretch it the further the dot gets from both ends even if it gets further from one end faster.
Nice analogy.
TYVM!
That’s a good rule of thumb… but it’s probably not enough; no reasonable definition would call Jupiter a star, or even a brown dwarf, or the Solar System a binary system, yet the Sol - Jupiter barycentre is outside the sun… (the whole system’s barycentre is sometimes inside the sun, but that’s due to Saturn’s, Uranus’, and Neptune’s pulls cancelling Jupiter’s).
I’d call the barycentre thing a necessary but not sufficient requirement; a proper definition of double planet should probably also take into account other factors like the relative mass and density of the bodies, and their minimum and maximum distance.