Do binary planets exist?
According to our current definition of planets and satellites can we have a binary planet system? If we can where is the line drawn between a big moon and another planet?
Posted Sep 8, 2016, 8:02 PM UTC
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the line is vague at best and non-existent at worst. there are a number of classifications which the IAU enjoys ignoring for no good reason.
pluto and charon are technically binary dwarf planet system. their masses are so close in value that they both orbit gravitational barycenter located well outside of pluto's crust.
you may have already read this: https://en.wikipedia.org/wiki/Double_planet
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If I can defend the IAU a little bit, a binary planet can't exist based on the definition of a planet that they use. One of the criteria for a planet is that it's cleared its orbital path, which means a planet is the largest source of gravity in its orbit around the Sun. So, an object that shares its orbital path with an object of similar size wouldn't meet the criteria to be a planet. It would be a binary dwarf planet system like Pluto and Charon.
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that made sense at first until you said it would be a binary dwarf planet system. if neptune and uranus orbited a shared gravitational barycenter, i don't think anyone would call that a dwarf system.
also, the IAU's definition of a planet is of course only based on available data from our solar system, so it's incredibly limiting compared to the infinite possibilities for celestial bodies. extremely young stellar systems could have jupiter sized objects that haven't yet cleared their orbit of debris simply because they haven't had the time, yet the IAU would still call it a dwarf planet despite its size. actually, this makes me wonder how neptune is considered a planet since its orbit is littered. same question applies to all planets with trojans?
luckily new terminology is created all the time in astronomenclature!
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Yeah, the 'cleared its path around the Sun' part of the IAU definition is tricky. It doesn't mean that there is nothing else in the planet's orbit around the Sun, it just means that there is nothing else in the same orbit that comes close to the mass of that planet (i.e., the planet is the largest source of gravity in its orbit around the Sun). In the case of Neptune, you're right that there are TONS of objects in the same or close to the same orbit, but none come close to the mass of Neptune.
I agree, it's also weird to think about calling two gas giants in the same orbit around their star a dwarf binary system. They aren't small, so why would they be called dwarfs? The best answer I can give is we haven't observed anything like that (we thought we observed something close to this, but it turned out the periods were refined with new Kepler data and the planets weren't co-orbital: https://www.newscientist.com/article/dn20160-two-planets-found-sharing-one-orbit/), and in fact, physically that system would be unstable so it wouldn't last long. In fact, most of the time when we find these co-orbiting objects, they are small in mass and size, and dwarf seems more semantically acceptable.
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fair enough. can't wait for the JWST to pull back the curtain on the galaxy!
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[deleted]
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There's a LOT more out there than just our tiny solar system.
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Maybe. To be totally honest I don't think it's known. Finding exoplanets is still relatively new. The first one I believe was discovered in 96. We generally find them through measuring centers of gravity, or the transit method in which the planet is seen moving past the star. We have yet to find an exo moon (or exo planets orbiting other planets for that matter) . The closest we know to a binary planet is Pluto and Charon. But plutos a dwarf planet and Charon is a moon so it's still pretty typical in regards to our solar system. Hope this helps.