2 Planets Can Share the Same Orbit, In 3 Different Ways 73
StartsWithABang writes: One of the most important characteristics of a planet, at least according to the IAU definition, is that it clear its orbit of all other bodies. But if we allowed for a special caveat — the possibility of two similarly-sized objects sharing the same orbit — could we have a stable configuration where that occurred? Surprisingly, not only is the answer yes, but there are three ways to do it: to have one at the L4/L5 Lagrange point of the other, to have a close-orbiting binary planet, or to have orbit-swapping worlds, where they periodically change spots with one another. Unbelievably, our Solar System has a history of all three!
2 planets (Score:2, Funny)
One Orbit
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Only if you don't spring for the two-person space suit.
And I think its gonna be a long long time (Score:2)
It's lonely out in space.
On such a timeless flight.
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If you don't like the definition of planets because it is too fuzzy --- wait until you learn the distinction between stars and planets :)
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No, that's pretty much always been the way it was.
Re:Problem with the definition of a planet (Score:4, Insightful)
Re:Problem with the definition of a planet (Score:5, Interesting)
They'll say, "oh, it's okay, there's enough of a size difference between those bodies that they don't count". But the thing is that there's no way that most of the current "8 planets" would have cleared their orbits without help from the giants. It's pretty much accepted science in astronomy that Jupiter, and to a lesser extent Saturn, scattered most of the bodies in our solar system. Mars has a Stern-Levison parameter (rating of the ability of a body to scatter small bodies) two orders of magnitude less than Neptune, and Neptune has multiple Pluto-scale bodies in its orbit. Pluto may be small compared to Neptune, but it's not so small in comparison to Mars, yet Mars has two orders magnitude less ability to scatter them. Mars didn't scatter these things away - Jupiter did. Heck, a number of the models show that the planets didn't even form in their current locations.
There's all this misuse of the Stern-Levison parameter out there to say things that it doesn't. The parameter is based around a probabilistic simulation of the body and a bunch of "small bodies" with a mass distribution and orbital distribution similar to our asteroid belt. But of course, that tells you very little - our asteroid belt only has the size and mass distribution that it does today because of the influence of other planets - and when I say "other planets", I really mean overwhelmingly Jupiter (only a tiny fraction of asteroids are in Mars resonances). Jupiter has stopped these bodies from coalescing into larger bodies and scattered the vast majority of its mass elsewhere. That's not the situation that the solar system was in during formation. There were numerous large "planetissimals" scattered around. The Stern-Levison parameter says absolutely nothing about the ability of a body to scatter large planetissimals. And even concerning scattering asteroids, it doesn't state that the scatters are enough to "clear the orbit", only that their angle changes on a pass by more than a given number of degrees.
Basic point: a standard based around the "8 planets" having cleared their orbit is a lie. The science says that most of them aren't responsible for clearing their own orbits.
And while we're at it: what sort of stupid standard puts Mars and Jupiter in the same group but in a different group than Pluto and Ceres? There was a perfectly reasonable standard under discussion at the IAU conference shortly before they switched what they were voting on: a definition built around hydrostatic equlibrium. A lot of the planetary scientists left thinking that this was the version that was going to be voted on, and being happy with either "no definition" or an "equilibrium definition", saw no need to stick around for the final vote. Hydrostatic equilibrium actually is valid science, and it's very meaningful. A body not in hydrostatic equilibrium is generally made of primordial minerals. It's the sort of place you'd go to research, for example, properties of how the solar system formed. A body in hydrostatic equilibrium has undergone mass conversion of its primordial minerals to new forms. It's undergone massive releases of energy (which may still be present, depending), associated action of fluids, etc, and are the sorts of places you would go to study mineralization processes, internal processes or search for life. They're very different bodies, and there's a very simple dividing line - one that's much easier to calculate/measure than a pseudoscience "cleared the neighborhood" standard.
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Slashdot supports <b> and <i> tags. I don't know what kind of primordial ooze you just crawled out of, but this is civilization, son, and you'd better act like you're part of it while you're here.
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There's a lot of good stuff in your post. In other star systems, there are almost certain to be Earth-sized bodies that don't have giant planets to clear their orbits. They wouldn't be classified as planets under this definition, no matter how Earth-like they otherwise are.
I agree that hydrostatic equilibrium is a better basis for the definition. I'd also propose that a system of two bodies in hydrostatic equilibrium that orbit a star, with a barycenter outside of either body, be classified as a double plan
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One complication - hydrostatic equilibrium also is somewhat dependent on local conditions, i.e. composition of the body. A hypothetical planet made entirely of Mercury, for instance, might be/have been in HE when only 5 feet in diameter (before it froze solid, or orbiting close enough to a heat source to remain liquid). That's of course an absurd extreme, so there's probably an example somewhere out there in the universe! :)
So IMHO, HE is a necessary criterion, but may not be sufficient. I read an articl
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They'll say, "oh, it's okay, there's enough of a size difference between those bodies that they don't count".
No, they'll just point out that while the orbits of the two planets appear to cross when looking at a 2D top-down view of the solar system, in 3D space they come nowhere near each other. The closest point in their orbits is 2 AU apart. Unless you want to say that Neptune's orbital zone its supposed to be clearing is twice the distance between the Earth and the Sun, Pluto is irrelevant.
For someone who seems to care a lot about Pluto you seem to have forgotten how absurdly tilted its orbit is.
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And while we're at it: what sort of stupid standard puts Mars and Jupiter in the same group but in a different group than Pluto and Ceres?
One that looks at this picture of orbits [wikipedia.org], notices the striking difference, realizes that this clearly puts Pluto and Ceres in a completely different class of objects, with completely different origins etc, and feels like acknowledging that obvious fact?
There was a perfectly reasonable standard under discussion at the IAU conference shortly before they switched what they were voting on: a definition built around hydrostatic equilibrium(sp).
Hydrostatic equilibrium [wikipedia.org] is in fact used by the IAU to designate the difference between "dwarf planets" and "small Solar System bodies". If that were made the baseline for proper planets, we'd have to add somewhere between 53 and 200 new planets. You are tal
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Size Differential (Score:3)
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You might want to read the whole article, including the Mathematical Details
... given two massive bodies in orbits around their common barycenter, there are five positions in space where a third body, of comparatively negligible mass, could be placed so as to maintain its position relative to the two massive bodies.
--- snip ---
--- snip ---
Admittedly, the German version, which I read first, mentions this already in the summary.
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Astronomers don't want you to know this neat trick (Score:5, Funny)
How many different ways are there to rehash shit that everybody knew since forever into crappy clickbait articles?
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This. Seriously, I'm getting sick of having Ethan's blog everywhere I go. Guy's good at manipulating social media, but there's nothing on any of his post that isn't just a rehash of things that Slashdot and other tech/science blogs have already linked to years ago.
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My interpretation has been that the French helped with the Revolution to keep a large fraction of British forces occupied 'over there', so the French wouldn't have to fight them at home.
Also, it's a little-recognized point, but it's quite arguable that George Washington, as a fairly new officer in the British Army, accidentally started what we call the French and Indian war. He was tasked with building a fort at the confluence of the Allegheny and Monongehela rivers, which join to become the Ohio, in what
Re:Astronomers don't want you to know this neat tr (Score:4, Insightful)
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Speak for yourself. I read the whole article, and enjoyed it. Not everyone knows all the intricate details of planetary orbits.
third of three? (Score:5, Insightful)
One of the most important characteristics of a planet...
There's three characteristics, and this is probably the least important. But if you consider all three to be "the most" important, then I suppose it's one of them. That seems like a pretty silly way to express yourself, though. Personally, I'd probably just say "one characteristic".
Also note that we're talking about the IAU definition, which is not necessarily the only definition. Dictionaries still haven't accepted the IAU definition, and may never do so, because the IAU defines a planet as orbiting the sun, while science fiction writers continue merrily talking about planets around other stars, and show no signs of changing.
Editors... (Score:3)
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Forbes warning (Score:3)
Can we please have warnings on links to Forbes? Those interruptions for their (usually stupid) "thought of the day" are really annoying. Perhaps editors should look for alternative sources where possible.
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StartsWithABang isn't enough???
Oh, of course, everyone else has to change the way they do things, since you, delicate flower, are simply incapable of doing the same.
In that, why not accept all submissions, without any editorial oversight whatsoever. Or even have monkeys type them up...
Slashdot is getting more and more worthless by the day :-(
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Slashdot is getting more and more worthless by the day :-(
No, Slashdot is more and more going the way of Usenet [wikipedia.org] (before it became a warez distributor). It was a much more efficient, easily searchable "forum" with a centralized content hierarchy making it easy to find new topics. But no user-access control.
When it was a mostly used by university students and other techies, peer-pressure still worked somewhat keeping signal/noise high enough to be worthwhile. Then computers became more user-friendly and everyone with an agenda discovered it and waged its wars there
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Slashdot is getting more and more worthless by the day :-(
No, Slashdot is more and more going the way of Usenet [wikipedia.org] (before it became a warez distributor). It was a much more efficient, easily searchable "forum" with a centralized content hierarchy making it easy to find new topics. But no user-access control.
When it was a mostly used by university students and other techies, peer-pressure still worked somewhat keeping signal/noise high enough to be worthwhile. Then computers became more user-friendly and everyone with an agenda discovered it and waged its wars there (or spammed it). No user-access control meant no way to exclude them. Thus in the end most people moved to the much inferior web-forums which had user-access control (and the ability to embed kitten pictures ;-).
When slashdot was still a site mostly used by techies - known (or feared) by the rest mainly for slashdotting - editors, submitters, commenters and moderators mostly had the same agenda: Read interesting stuff within tech and science (and fiction). But nowadays "the rest" has learned in three ways:
So, learning from Usenet, if you want to have your nice old slashdot again, you have to make a new service which at least at first is not attractive to or to complicated to use for the non-techies. Otherwise just be happy in knowing that yes, capitalism works in bringing all the good things to the masses ..... reducing their quality on the way ;-)
bingo.
the history of all human organizations in a nutshell. First, exclusivity and quality; gradual dilution of both; finally, succumbing to parasites, decay, and fulminating infections. see also: religions, governments, businesses, and families
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Oh, of course, everyone else has to change the way they do things, since you, delicate flower, are simply incapable of doing the same.
Forbes is manipulative, deceptive shit, and anyone who links them is an idiot and a tool. Their occasional real article is there only to convince us that they are not shitbags.
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Since the moon orbit the sun, not the Earth, doesn't the definition of a planet not apply to the Earth ? ...
(the moon always curves toward the sun, never away, during it's 1 year journey. It does wobble on a montly basis
that allows the earh and it to swap arouind as being furthest from the sun.)
Yes, the Earth-Moon system is double planets sharing an orbit. But that doesn't mean they can't be planets. It's the common definition of the Moon that is wrong.
Stats with my Wang (Score:2)
And ends with me leaving Slashdot for a few months.
Enough of this SWAB crap!
Alternative Earth in science fiction (Score:2)
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Yes, it is unstable. Any discrepancy from precise balance will be exacerbated, and the presence of the Moon alone (ignoring effects, such as that due to Jupiter) is sufficient to perturb this system, leading to either a close encounter, or a Trojan- Greek relationship.
Your second consequence follows from the first.
obligatory nerd post (Score:2)
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He did the best that he was able!