Slashdot Mirror
Can We Call Pluto and Charon a 'Binary Planet' Yet?
astroengine writes The debate as to whether Pluto is a planet or a dwarf planet rumbles on, but in a new animation of the small world, one can't help but imagine another definition for Pluto. As NASA's New Horizons spacecraft continues its epic journey into the outer solar system, its Kuiper Belt target is becoming brighter and more defined. Seen through the mission's Long Range Reconnaissance Imager (LORRI) camera, this new set of observations clearly shows Pluto and its biggest moon Charon locked in a tight orbital dance separated by only 11,200 miles. (Compared with the Earth-moon orbital separation of around 240,000 miles, you can see how compact the Pluto-Charon system really is.) Both bodies are shown to be orbiting a common point — the "barycenter" is located well above Pluto's surface prompting a new debate on whether or not Pluto and Charon should be redefined as a "binary planet".
You're just trying to troll Neil Tyson for the hilarity that ensues.
seriously....call Pluto what it is...ClickBait.
What's with this "dwarf" nonsense — and big planetarism? We demand equal gravity for all planets!
In Soviet Washington the swamp drains you.
The arguments for demoting Pluto from its planetary status still holds. And hardly anyone objects to Pluto and Charon together as a binary system. But this "new" insight does not promote Pluto/Charon to planetary status. Binary dwarf planet, binary kuiper belt object, binary plutoid. Absolutely. Binary planet? No.
- Henrik
- when the Shadows descend -
Pluto is a planet. The definition of a planet is arbitrary, and always will be.
Trying to forcefully change the definition after it's already in use is fucking retarded and does nothing but cause confusion.
For other instances of dipshits trying to hijack language and make it worse, see "non-flammable" and the dipshits who insist that a kilobyte is 1000 bytes.
Planet and Dwarf Planet are arbitrary labels defined by the IAU.
How can you "debate" about that?
sic transit gloria mundi
This mission will put a new spotlight on Pluto and its âoedwarf planetâ status, potentially highlighting its current classification as a woefully inadequate description of such a dynamic and interesting binary system.
Ok, so it's a "binary dwarf planet" - can we tone down the prose now?
sic transit gloria mundi
The barycenter of the Sun and Jupiter is above the surface of the sun, does that mean we'd have to reclassify our solar system as a binary star system now? http://spaceplace.nasa.gov/bar...
Unless Pluto (and Charon) shifted orbit into the planetary plane, nothing has changed and any desire to call it a planet is just sentimentality.
Pluto doesn't care what you call it. It's going to be around when you are dust in the wind. Oh and I don't care either.
Mean what you say...say what you mean.
the best question:
The moon is currently 239,000 miles away, and the barycenter is at 0.75 Earth radii from Earth's center. If the barycenter was at the earth's surface, how close would the moon be?
For the purpose of this calculation assume the earth is a uniform sphere with a mass of 5.97x10^24 kg and a diameter of the earth is 7,900 miles, and the moon is a uniform sphere with a mass of 0.0123 earths and a diameter of 2,160 miles.
what is that? Why not furlongs or Manhattans?
Defining it based on barycenter will lead to curious outcomes. What if the barycenter moves into and out from the planet (such as with multiple moons)?
And what if Pluto had a second moon, equal in mass and distance as Charon but always on the exact opposite side (L3)? The barycenter would be at the center of Pluto, but why does this change cause Pluto to become a "real" planet?
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.
320,000 miles
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.
BTW, I actually did the full calculations and accounted for the radius of earth/moon in the distance. But according to the equation for calculating barycentric coordinates, the distance of the barycenter from the center of the primary is linearly proportional to the distance of the centers of mass of the two bodies... so a pretty close estimate would have been (1 / 0.75) * 239,000 miles.
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.
Earth/Luna is a binary planet by the criteria.
It's not. The center of gravity is under Earth's surface.
Most significantly, Luna's orbit is never convex with respect to the Sun.
The Moon's orbit is convex.
I wouldn't mind having what's left over from "very nearly" a billion dollars.
No phone lines yet. Sorry.
No.
I suppor tthe right of the people of Pluto to decide their own destiny, and not be ruled by any arbitrary group of people on some other planet.
Freedom for the Plutocrats
Uh, folks, Pluto actually has FOUR moons... Charon: Discovered in 1978, this small moon is almost half the size of Pluto. ...
Nix and Hydra: These small moons were found in 2005 by a Hubble Space Telescope team studying the Pluto system.
Kerberos: Discovered in 2011, this tiny moon is located between the orbits of Nix and Hydra. ...and it just keeps on thumbing it nose at the dwarfists, and now, the binarists.
But tell me, how is a FIVE body system a BINARY system? Hmmmmm?
My mistake...Pluto has FIVE moons. Charon: Discovered in 1978, this small moon is almost half the size of Pluto. It is so big Pluto and Charon are sometimes referred to as a double planet system. Nix and Hydra: These small moons were found in 2005 by a Hubble Space Telescope team studying the Pluto system. Kerberos: Discovered in 2011, this tiny moon is located between the orbits of Nix and Hydra. Styx: Discovered in 2012, this little moon was found by a team of scientists search for potential hazards to the New Horizons spacecraft flyby in 2015. may the 'little planet that could' keep right on thumbing it nose at everybody!
huh I had assumed that the barycenter would move to the surface as the moon got closer, but thinking about it more I see that you're right it would need to move further away.
a couple Qs if you don't mind, because you obv know a lot about this. so I guess if the barycenter is not in the middle of earth, then the earth wobbles as the moon goes around. Is this what causes tides, it's essentially the sloshing of the ocean as the earth wobbles? I always knew that "the moon causes tides", but I never understood the mechanism.
I guess a second question would be, is there a certain distance at which the moon would escape earth's gravity? I wonder what it is, esp compared to the current distance away? would it be 2x, or 10% or 10x?
greatly appreciate your thoughts, I don't know much about this.
Well, Earth has an unmanned expeditionary mission that will take pictures of Charon in July 2015 =^-^=
"Screw Sun, cross-platform will never work. Let's move on and steal the Java language." - Visual J++ Product Manager
The Earth-Moon barycenter is very nearly outside of Earth itself (it's about 0.75 Earth radii from Earth's center), so let's not get too high on our horses...
And the Earth - Moon should be classified as a binary planet. They are in such an intimate dancing orbit with each other that neither one can be adequately described without refering to the other.
This is more than a semantic squabble. Any exoplanet that is likely to support life as we know it must not only be in the Goldilocks zone, it must also have a companion close enough to create tides (and tide pools, and generally act as a celestial stirring rod).
Will
a couple Qs if you don't mind, because you obv know a lot about this.
I'm just a guy who was interested enough to Google and throw together some calculations.
so I guess if the barycenter is not in the middle of earth, then the earth wobbles as the moon goes around. Is this what causes tides, it's essentially the sloshing of the ocean as the earth wobbles? I always knew that "the moon causes tides", but I never understood the mechanism.
The gravitational forces between the earth and moon are major components of tides. However the barycenter doesn't seem to contribute directly. Moving the earth and moon farther apart (and thus moving the barycenter further away from the center of the earth) actually causes the tides to become weaker. This actually happens regularly as the moon gets closer and then farther from the earth in its orbit (the moon's orbit is not perfectly circular, but slightly elliptical). When the moon is at its closest, the tides are barely higher, and at its furthest the tides are barely lower.
I guess a second question would be, is there a certain distance at which the moon would escape earth's gravity? I wonder what it is, esp compared to the current distance away? would it be 2x, or 10% or 10x?
Gravity accelerates two objects toward one another, based on their mass and their distance. It works the same whether the two objects are initially moving toward each other or away from each other (away from each other, we usually call "decelerating", but there's no difference in the math).
Escape technically occurs when the two objects are moving away from each other, but the deceleration due to gravity will never be enough to overcome their initial velocity at their initial distance from each other. Gravity diminishes as the objects move farther apart, which results in less deceleration over time. In the case of escape, the velocity will never reach zero.
The answer is "yes"... assuming the moon magically appeared at that new farther distance but traveling at the same velocity as it is currently. According to the Wikipedia link on escape velocity:
The escape velocity at a given height is (square root of 2) times the speed in a circular orbit at the same height
Also, the orbital velocity of an object decreases as its distance increases. So increasing the distance of the moon would decrease how fast it would need to be going to stay in orbit.
But remember that the orbital distance suddenly increased but the orbital velocity did not change...
Let's say the moon is orbiting at distance R with orbital velocity V. Thus, all we need to do is figure out at what new distance R2 the new orbital velocity V2 = V * (1 / square root of 2).
This page contains the formula we need. Solving for r, r is proportional to 1/(v^2). So R2 is proportional to 1 / (V2^2), and substituting the equation above we find that R2 is proportional to 2 / (V^2), which equals 2 * (1/V^2), which equals 2 * R. R2 = 2*R.
Thus the answer is "2 times the original orbital distance, 478,000 miles".
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.
pedant point: Javascript isn't a programming language, the clue is in the name: it's a SCRIPTING language.
Political debates have me rolling my eyes so much I think I got optical whiplash. I should sue. - Foamy The Squirrel
why yes, you can have a quarter of a penny, since that's all I have.
Political debates have me rolling my eyes so much I think I got optical whiplash. I should sue. - Foamy The Squirrel
The answer is simple - that Lagrange point is not stable, so the moon would not remain there. Each moon would be pulled from that point by the other's gravity, until they either collide or one or both items are thrown from their orbits.
So as a planet cannot have two moons that orbit opposite each other, the concept of a binary planet with a definition based on the location of its barycenter is valid. But we'd first want to see one - Pluto/Charon is a poor example, as Pluto is considerably larger and heavier than Charon, so 'Planet/moon system' defines it better. If we start to find real binary planet systems outside of our solar system and stat characterizing them, then we will be able to know what sorts of systems happen and how they form, and maybe then we will find that Pluto/Charon belongs as an outlier there. But that's for a future time.
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
Interestingly, Jupiter is the only planet which has it's barycenter with the sun outside of the sun.
The definition of whether something orbits something else, or whether it is a binary system is pretty arbitrary. It would be nice and neat if we could say that if the barycenter is inside the larger body, the smaller body is orbiting the larger, but that would mean that Jupiter would not be orbiting the sun.
if the barycenter is not in the middle of earth, then the earth wobbles as the moon goes around.
The barycenter for any two objects is never the middle of either. It's always somewhere on a line directly between the two centres of mass. Every individual satellite that humanity has launched makes the earth wobble a little bit (albeit a miniscule amount).
"They invented a reason,
That's why it stings,
They don't think that you matter,
Because you don't have pretty rings."
-Jonathan Coulton
Diameter, in km
Mercury ... 4,880 ..... 12,104 ..... 12,756 ...... 6,794 ... 142,984 ... 120,536 ... 51,118 ... 49,532 ....... 2,222
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Pluto's diameter is half the diameter of Mercury. Should we also consider Mercury a dwarf planet? Can the Earth be considered to be in the same league as Jupiter/Saturn as they have 10 times the diameter of Earth?
thanks you're cool! I hope the moon never slows down to the point it crashes into earth. that would suck!
That's no moon! (Ask David Weber)
They are not considered to be in the same league. The classes are: Gas giants, terrestrial planets, dwarf planets.
Do you care about the security of your wireless mouse?
Agreed, that one is a bit far fetched. It's a many-body problem and all it takes is a bit of eccentricity or pull from other moons, planets, and the sun to destabilize.
But back to the first question, what if the barycenter moves in and out of the planet due to multiple moons? This would be akin to the solar system, where the barycenter moves in and out of the sun. I don't know if we could easily call it a ternary planet, quaternary planet, etc.
I think I prefer Isaac Asimov's tug-of-war definition of a binary planet. It would be considered a binary planet if the smaller body has a concave orbit around the sun; in other words, the two are both primarily orbiting the sun and just happen to be close to each other. This would, however, define the earth/moon system as a binary planet and Pluto/Charon would be a planet/moon.
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.
Another reason I don't care for the barycentric approach is because it depends so highly on the radius of the larger body. What if the barycenter of the moon were right above the surface but well within the atmosphere? What about a gas giant where the definition of the radius is a bit fuzzier?
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.
In retrospect I don't like how I phrased this:
Thus, all we need to do is figure out at what new distance R2 the new orbital velocity V2 = V * (1 / square root of 2).
Change it to the following:
Thus, all we need to do is figure out at what new distance R2 the original orbital velocity V = V2 * (square root of 2), where V2 is the velocity of a circular orbit at distance R2.
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.
Again, I would have to be convinced that a group containing more than two objects with sizes within an order of magnitude of each other would be stable. Myself, I can't see it. Two large moons would push each other into chaotic orbits which would, sooner rather than later, lead to either a collision or an ejection.
The only way I can see a system with two large moons is with a planet that is completely dominant, such as Saturn or Jupiter and it's moons. (I'd argue, for instance, that Earth could not have held on to two moons.)
But these questions can really only be answered when we have more binary-planet candidates to categorize.
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
Barycenter not closer to either planet than a chosen percentage of the distance between their centers would be better.
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
We've known for at least a decade now that Pluto/Charon's barycenter is outside the mass of Pluto. That was one of many arguments used to delist Pluto from the Solar System planets. Those same "Pluto is a planet" fossils probably would demand Ceres be restored to planetary status, if they lived two hundred years ago.
There is no America. There is no democracy. There is only IBM and AT&T and DuPont, Dow, General Electric, and Exxon
That definition would reduce the problem to the relationship of their masses.
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.
I was thinking of this thread again (the tug-of-war definition) and thought of another interesting thing to calculate: What if I swap Pluto and Charon in the equation? Would it indicate that Pluto is also more influenced by Charon than the sun at times?
The answer is yes, meaning Pluto does not have a concave orbit either. The tug-of-war value focusing on Pluto as the primary body is 337.3 at perihelion, but focusing on Charon as the primary body it is 39.3. Since both are greater than 1, this means Pluto and Charon cause each other to have an occasionally convex orbit.
So perhaps this definition would still find Pluto/Charon to be a binary planet, but in a different way than the Earth/Moon are. Compare the numbers above with Earth/Moon: Earth-centric has a value of 0.45, Moon-centric has a value of 0.006. Both being less than 1, Earth and Moon always have concave orbits around the sun.
All my liberal friends think I'm a conservative, all my conservative friends think I'm a liberal.