Are the Earth and moon a double planet?

[Centaur]

And now folks are wondering if Pluto and Charon are a double planet...

Comments? What would be THE determining factor of either Earth/Moon or Pluto/Charon being a genuine double planet?

Of course Pluto is no longer regarded as a major planet. It's considered a dwarf planet or plutoid or trans-neptunian object or Kuiper Belt object, or most generally as an asteroid with the number 134340. Many asteroids have satellites. I imagine we could consider Pluto/Charon and perhaps others to be double or multiple asteroids.

Regarding the Earth and Moon, perhaps even more relevant than the mass ratio is the fact that the Moon's orbit relative to the Sun is always concave. The Moon is the only known planetary satellite for which this is always true. Perhaps that enhances the candidacy of the Earth-Moon system as a double planet. Ultimately it's a matter of semantic context. In one context the Earth and Moon could be considered adjoining planets that orbit the Sun in perpetually nearby concave heliocentric orbits while perturbing each other. In another context the Moon can be considered a satellite of the Earth, especially since their barycenter lies within the Earth.

[Hornblower]

Of course Pluto is no longer regarded as a major planet. It's considered a dwarf planet or plutoid or trans-neptunian object or Kuiper Belt object, or most generally as an asteroid with the number 134340. Many asteroids have satellites. I imagine we could consider Pluto/Charon and perhaps others to be double or multiple asteroids.

Regarding the Earth and Moon, perhaps even more relevant than the mass ratio is the fact that the Moon's orbit relative to the Sun is always concave. The Moon is the only known planetary satellite for which this is always true. Perhaps that enhances the candidacy of the Earth-Moon system as a double planet. Ultimately it's a matter of semantic context. In one context the Earth and Moon could be considered adjoining planets that orbit the Sun in perpetually nearby concave heliocentric orbits while perturbing each other. In another context the Moon can be considered a satellite of the Earth, especially since their barycenter lies within the Earth.

My bold. This looks like merely an artifact of this particular combination of orbital radius and period. Suppose we substitute a really tiny body, one that never could be considered a planet on its own, for the Moon. At the same orbital radius it would move in a very similar orbit and still be concave at all times. Would you consider such a system to be a double planet?

[transreality]

What about a small dwarf star orbitting a giant star, if the barycentre of the pair is inside the larger star, is it no longer a double star, but instead a primary with moon-star.

[Centaur]

My bold. This looks like merely an artifact of this particular combination of orbital radius and period. Suppose we substitute a really tiny body, one that never could be considered a planet on its own, for the Moon. At the same orbital radius it would move in a very similar orbit and still be concave at all times. Would you consider such a system to be a double planet?

That's a good point, Hornblower. However, my precise choice of words in the second sentence that you set to bold type still applies. An absolute was not stated. In reality the Moon's significant mass is indeed an important factor. Also please note my words that followed that allowed for different uses of terms depending on context.

[John Mendenhall]

What about a small dwarf star orbitting a giant star, if the barycentre of the pair is inside the larger star, is it no longer a double star, but instead a primary with moon-star.

Yes, the primary is called Frankenstein and the secondary is called Frankenstein's Moonstar.

Once again, I will leave my mainstreamer signature off this post.

Regards, John M.

[Centaur]

What about a small dwarf star orbitting a giant star, if the barycentre of the pair is inside the larger star, is it no longer a double star, but instead a primary with moon-star.

I'd call it a satellite-star. In any event, I dislike the modern application of the word moon to any object other than the Moon. It causes some people to occasionally initiate threads asking why the Moon has no name of its own.

[Centaur]

Yes, the primary is called Frankenstein and the secondary is called Frankenstein's Moonstar.

LOL. Very good, John.

[Ara Pacis]

Actually, the simple fact of the matter is that the IAU had not defined the term Double Planet, and even if it has, it is not part of the definition of planet as the concept of a double planet is incompatible with the definition of a planet, so a double planet could not be a planet. If the Moon were ever determined to be a planet, then the Earth would cease to be.

[Noclevername]

Actually, the simple fact of the matter is that the IAU had not defined the term Double Planet, and even if it has, it is not part of the definition of planet as the concept of a double planet is incompatible with the definition of a planet, so a double planet could not be a planet. If the Moon were ever determined to be a planet, then the Earth would cease to be.

And so Pluto finally has its revenge!

[swampyankee]

My bold. This looks like merely an artifact of this particular combination of orbital radius and period. Suppose we substitute a really tiny body, one that never could be considered a planet on its own, for the Moon. At the same orbital radius it would move in a very similar orbit and still be concave at all times. Would you consider such a system to be a double planet?

Traditional usage says it's not, and there does not seem to be any reason to add a new category of Solar System objects, especially one which will never have more than one member.

My opinion is that one sensible criterion for classifying an object pair as a double planet's would be to require both bodies to be within each other's Hill radius relative to Sun and both to qualify as "planets." The Earth's Hill radius is fairly large, so the Moon is comfortably within it. The Moon's is smaller and, if my arithmetic is correct, the Earth is outside the lunar-solar Hill radius. It's not far outside -- if my arithmetic is correct the Moon's Hill radius relative to the Sun is about 330,000 km, while the Earth-Moon distance is about 384,000 km -- but it is outside, so the Earth-Moon system fails this criterion.

[Hornblower]

Actually, the simple fact of the matter is that the IAU had not defined the term Double Planet, and even if it has, it is not part of the definition of planet as the concept of a double planet is incompatible with the definition of a planet, so a double planet could not be a planet. If the Moon were ever determined to be a planet, then the Earth would cease to be.

This post is excellent for lampooning the IAU's poor choice of words, which can lead to some absurd interpretations if taken too literally.

[Buttercup]

Wouldn't *the* definition of "double planet" be two celestial bodies which revolve around each other? Like certain stars. If so, then Earth/Moon aren't.

I'm still wondering about Pluto/Charon. ^_^ So exciting -- 5 moons! :-D

[Ara Pacis]

Traditional usage says it's not, and there does not seem to be any reason to add a new category of Solar System objects, especially one which will never have more than one member.

There is only one member of the object category in our solar system referred to as "star".

[Ara Pacis]

This post is excellent for lampooning the IAU's poor choice of words, which can lead to some absurd interpretations if taken too literally.

Remind me again what the criterion is for an object to be defined as a planet WRT clearing it's orbit?

[swampyankee]

There is only one member of the object category in our solar system referred to as "star".

It's not that there won't be another member of the "double planet" category in our particular stellar system; it's that there won't be one anywhere: the term "planet" is, by IAU definition, limited to objects orbiting the Sun^{1, 2} (unless they've changed it since 2006). The Sun, insofar as I'm aware, is still considered by the IAU to be a specific example of a star. While there's only one member of this category in the Solar System, there are many other objects which have been placed into the category.








1: "The IAU members gathered at the 2006 General Assembly agreed that a 'planet' is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit." ( http://www.iau.org/public_press/news/detail/iau0603/ )

2: I find the this part of the definition to be overly restrictive: requiring that an object orbits one particular star to be called "planet" seems to violate the Copernican principle of mediocrity.

[Hornblower]

My opinion stands. Those delegates, only about 4% of the entire IAU membership, did a dreadful job of choosing their words. Instead of "has cleared the neighborhood", I think they should have said something like "gravitationally dominates its neighborhood". Then we would not have the strange practice in which some people interpret that as excluding Jupiter and Neptune, which have Trojans and 3:2 KBOs in their "neighborhoods" respectively. If those people are doing that merely to lampoon the delegates, I would say more power to them.

I would cheerfully ask those delegates if they meant to exclude an Earthlike body orbiting another star from what they consider to be planets.

[swampyankee]

My opinion stands. Those delegates, only about 4% of the entire IAU membership, did a dreadful job of choosing their words. Instead of "has cleared the neighborhood", I think they should have said something like "gravitationally dominates its neighborhood". Then we would not have the strange practice in which some people interpret that as excluding Jupiter and Neptune, which have Trojans and 3:2 KBOs in their "neighborhoods" respectively. If those people are doing that merely to lampoon the delegates, I would say more power to them.

I would cheerfully ask those delegates if they meant to exclude an Earthlike body orbiting another star from what they consider to be planets.

I'd agree with the "poorly worded" part. For one thing, "cleared its orbit" is vague, and restricting the definition to objects orbiting the Sun excludes all the non-luminous thingies which are too small to have ever been stars orbiting other stars.

[Ara Pacis]

It's not that there won't be another member of the "double planet" category in our particular stellar system; it's that there won't be one anywhere: the term "planet" is, by IAU definition, limited to objects orbiting the Sun^{1, 2} (unless they've changed it since 2006). The Sun, insofar as I'm aware, is still considered by the IAU to be a specific example of a star. While there's only one member of this category in the Solar System, there are many other objects which have been placed into the category.

Okay, but that's not what you originally wrote. You wrote "solar system objects" not "planets". I know some people object to having categories with one instance in them because if offends their sense of aesthetics or something, so I didn't know what your objection was.