Binary planets?

The recent fly-by of Pluto by New Horizons got me to wondering: have any binary planets ever been discovered?

I know that Pluto and its moon Charon are quite close together and I wondered if anyone has ever measured to see if they’re orbiting each other in a binary system, rather than as a moon orbiting a planet. And whether or not it’s true of Pluto and Charon, is it possible for two planets to orbit each other?


What do you mean “in a binary system”? The center of mass of the Pluto-Charon system is in empty space, not within either object, if that’s what you mean. They’re also both tidally locked to each other.

The barycenter of Pluto-Charon (the center of mass of the system) lies well outside the radius of Pluto’s surface, and the other, much smaller moons in the system (Hydra, Kerebos, Nix, and Styx) orbit Pluto and Charon together. So yes, binary planetoids (doublets) exist and are quite possiblely common in other systems, though large gaseous planets in this configuration are really unlikely. For that matter, while not as extreme as the 9:1 mass ratio as Pluto-Charon, Earth and its very large moon corotate about a baryceneter that, while still well within the radius of Earth’s surface, is displaced from the center of the planet by 4600 km, so it is about 1600 km below the surface.

And, aprpos of nothing other than the continued annoyance of hearing newscasters constantly mangle the name, Charon, named after the mythological Greek ferryman, should be pronounced with a hard “ch” (“kAr-on”) not with a soft “ch” or “sh” sound (“ChAr-on” or “ShAy-ron”).


I read one article about New Horizons and Pluto that mentioned that, being tidally locked and the barycenter being above Pluto’s surface, Pluto and Charon constitute the only known binary system in our solar system.

I did wonder about the tidal lock as a requirement. No doubt that’s not a requirement for binary stars.

Are there any other known pairs in our solar system that could be called binary systems? Is there an accepted definition for binary system, among astronomers?


I’ve seen it argued that the Earth and our moon should be considered a double-planet system, since the moon is so large compared to the Earth.

It seems to me that any reasonable definition of “binary planet” requires a barycenter that lies outside of the actual surface of the primary.

Blame James Christie, not the newscasters. He didn’t name it for Charon, but for his wife, Charline, or Char.

Yeah, well if we’re going to get all Greek on this pronunciation, it s Χάρων which sounds more like karone (according to my missus).

Then it should be named Charline. However, it is called Charon, which is the latinized name of Kharon, the ferryman who caries the souls of the dead across the rivers Acheron and Styx to Hades, and should be pronounced with a hard consonant.


I though it was pronounced “ZEE-na”.

I would think to be a “binary planet”, both objects should be planets. If we further restrict the definition of planet to “originally formed as a planet”, then I think “binary planet” would not occur. Could two coalescing objects both orbit each other AND orbit a much much larger mass? Perhaps in theory it could happen, but then it’s just a theory looking for an observation to explain.

Earth and Luna are awfully darned close to being a binary planet. Under the current IAU definition of “planet”, a binary planet is impossible, but that’s just because the IAU definition has that silly bit about having “cleared its orbit” (which, by any reasonable standard, Earth has also failed to do). Take away that triviality, though, and I don’t see any reason why a true binary planet couldn’t form. The Earth-Luna pair is at least suggestive of how a binary pair of rockballs might form, and a binary pair of gasballs isn’t too different from a binary pair of stars, which occur in every conceivable configuration.

How do you pronounce Boston’s basketball team? :wink:

Luna is hypothesized (with a considerable amount of evidence and supporting simulation) via impact of the early Earth by a very large body, resulting in two masses in complementary orbit. Other doublets could form by being ejected together while orbiting a larger planet. (This is considered the most likely origin of Pluto-Charon.) However, it is really unlikely for two very large masses to be formed in the same orbit (one either tends to absorb the other or disrupt its formation via resonances between it and the central body) and two very large masses have enough orbital momentum that even small differences in orbital arguments make it impossible for the system to transfer momentum fast enough to come to equilibrium. I’m not saying it absolutely couldn’t happen, but current models of planetary formation suggest that doublets larger than a certain size (somewhat larger than Earth, if I recall correctly) are very unlikely. Then again, the formation of superjovian planets in very small orbits is also predicted to be nearly impossible, and we’ve found several instances of that already, so the current simulations may just not be well-informed.

The Celtics? With a hard “C” (kEl-tiks). But then, I don’t generally watch basketball, so it doesn’t bother me much when announcers get it wrong.


Excellent point, binary stars are common place. My concern is that within the coalescing gas cloud, getting two points of mass accretion to orbit each other would be a self-defeating effort if they’re spinning the same direction. But then again only six planets spin the same direction as the sun anyway. I don’t think there’s anything that prevents the levels of turbulence required for this, just a remote possibility in any given stellar system, like worse than one in eight.

[SELL- ticks] … California dialect

What’s odd to me about the basketball team name is that the word Celt or Celtic in any context other than basketball is pronounced correctly by damn near everybody as Kelt or Keltic.

So how/why did they screw up the basketball team name? It’s a mystery. As another devout anti-basketballer, it’s not a mystery I much give a hoot about, but it *is *a mystery.

How about Chiron?

And for what it’s worth, “K” isn’t quite the right sound for “Charon”, either (though it’s closer than English’s “Ch” or “Sh” sounds). It should be the same sound as in Scottish “Loch”, and sounds like you’re clearing your throat.

This is rather an awkward way of phrasing it, as the moon is in fact much smaller than Earth (about 1/81 or 1.2% of Earth’s mass).

That does indeed make it large in comparison to the typical moon/planet mass ratio in our solar system.

Why were they theorized to be nearly impossible, and what explanation has been given since the observations?

Perhaps they got it from the Scottish. Glasgow also has a football team called Celtic and that too is pronounced “Selltick” although in any other context the word is generally pronounced “Kelltick”.