Moon around Kuiper Belt Object

UncleJosh writes "Today's NY Times (free reg rq'd) has a story about the first Kuiper Belt Object (KBO) with a moon, 1998 WW31. The hubble telescope has been used to get information about the size and orbit of the moon. Seems lots of things have moons. Coming more than 20 years after the discovery of Pluto's moon Charon the discovery of a KBO with a moon also follows the discovery of asteroid Ida's moon Dactyl and other moons of asteriods."

Comments

Ob Star Wars Ref

[Anonymous Coward]

That's no moon!

Read Kim Stanly Robinson

[Noodlenose]

In the "Mars" Trilogy by Kim Stanley Robinson he envisions a time when mankind will actually settle on Kuiper Belt Objects to mine them: Nice if you like low gravity. And now you can even see a moon rise :-)

Too bad this isn't in the main section

[ramoth4]

I'd like to see what others think about this, instead of a measly 2 replys.

This poses major questions though, about what exactly is a planet. Is Jupieter a planet? It puts out more heat than it takes in. What about Pluto? It's not in a regular orbit. What about that whole "Nemesis" theory? What's that got to do with this?

Re:Too bad this isn't in the main section

[seann]

apparently, they've ruled out pluto as a planet. I have the reference, but it's in a big book with no cover under my bed holding it up on it's edge.
I supose they consider jupiter a planet, because it is large. "Gas Giant"

Re:Too bad this isn't in the main section

[Anonymous Coward]

I think the story of Pluto is that the people that originally "discovered" it thought it was bigger than it actually is. They discovered the presence of a massive object by its gravitational effects on Neptune, and thought it was large enough to be considered a planet. It was later discovered that it was smaller than originally thought. We still call it a planet today because we've been calling it one all along. It's the largest (discovered) Kuiper Belt object, but if we had just discovered it recently, it's likely that it would not be considered a planet.

Re:Too bad this isn't in the main section

[wnknisely]

Couple of points:

Jupiter is a planet. The fact that it puts out more heat than it takes in is a consequence of its size - the gravitational well is causing a massive heating of the material, and it's radiating that heat in the Microwave and Radio. A "star" is generally an object creating heat through nuclear fusion, not throught gravitational force.

What I find most interesting about this article is the preponderance of moons in the solar system. It's pretty hard to just "drop" something into a stable orbit around another object - and even harder when the central object is small, and in a place where there are other objects to perturb the "moon's" orbit. Apparently there something about orbits and gravity that makes reasonably stable orbits more common than most would expect.

Perhaps it has something to do with the sum total of all the gravitating objects sorting things into these kinds of orbits (sort of like the Jupiter and Saturn systems.) At any rate, it would be an interesting simulation to run on some super computer...

Re:Too bad this isn't in the main section

[Cy Guy]

[Pluto]'s the largest (discovered) Kuiper Belt object

Which would therefore make Pluto the first discovered Kuiper Belt object to have a moon, making a total of Eight with the seven referenced in the article.

It was later discovered that it was smaller than originally thought. We still call it a planet today because we've been calling it one all along.

Given that there are other Kuiper Belt objects on the same order of magnitude in diameter as Pluto, and that Kuiper Belt objects with moons seem common, isn't there even greater reason to reclassify Pluto? With a mass of just 4% of the next smallest planet (only 1/8th the most massive moon in the Solar System), why should it continue to be singled out from the other KB objects? Isn't science about taking new information and changing our assumptions and definitions to comform with new facts as they discovered?

Re:Too bad this isn't in the main section

[cmowire]

It's more of an evolutionary process. Even if the ratios are such that for every 1 object in a stable orbit around another, 10000 objects either were flung into a different orbit, impacted the body, or went out of the solar system, there will still likely be a lot of moons out there, becuase there's a LOT of objects out there.

Plus, as long as the object encountering another one doesn't impact or get flung out of the system, it will be in a new orbit and has a chance to encounter another object.

Our solar system works like clockwork. It got that way because everything that didn't look like clockwork was sliced off.

Re:Too bad this isn't in the main section

[argStyopa]

I don't know that this is much of a nomenclature problem for the professionals, only for us ignorant civilians that don't have the hard & fast definitions to hand (or a media that's sloppy using those definitions perhaps).

As a karma-whore, here are the def's from http://nedwww.ipac.caltech.edu/level5/Glossary/fra mes.html:
PLANET: An object that formed in the disk surrounding a star. To be called a planet, an object must be more massive than Pluto (1/500 the Earth's mass) and less massive than ten times Jupiter's mass. Unlike stars, planets do not produce light of their own but merely reflect that of the star(s) they orbit.
STAR:A celestial object that generates energy by means of nuclear fusion at its core. To do this it must have more than about 0.08 the sun's mass. If, for instance, the planet Jupiter were some fifty to one hundred times more massive than it is, fusion reactions would transpire in its core and it would be a star.

Re:Too bad this isn't in the main section

[p3d0]

I think the current state of affairs is that Jupiter is a planet because it does not do any fusion, and Pluto is a planet because everyone likes it too much to cast it back into the Cuiper belt.

But I have to say that when I first heard that a Cuiper belt object had a moon, my first thought was "hey, that's the second one after Pluto".

Re:Too bad this isn't in the main section

[CheshireCatCO]

Actually, Jupiter emits most of its radiation in the IR. It's just blackbody emission, after all. Most of the nifty radio emission is tied to the magnetosphere, which is an entirely different kettle of fish. The internal heating is probably due to continued contraction (to expand on what you said).

As for capturing moons... it's really, really, really tough. You're right. You can't just capture a moon by having a body sling by and get caught. You need a dissipation to remove energy from the system (well, get it out of the graviational/kinetic forms). If you're a gas giant or a very young terrestrial planet, gas drag near the planet might work. KBOs and asteroids probably never were able to manage that and aren't really old enough for their moons to have been formed at an early epoch anyway.

The theory I've heard bandied about over the past year or two is that these moons are the result of an impact with the main body. The impact can toss material up into orbit around the object, creating a small moon. I don't think I've seen this put forth in any papers yet. (A quick search on ADS didn't show any hits, either. But my search-engine luck is pretty low.)

Hubble space picture of 1998 WW31

[Anonymous Coward]

Posted on the Hubble site 7 days ago when this was news.

This is not surprising

[n-baxley]

Two boides are attrackted while spinning in orbit around another, larger body. They start to co-orbit. That's physics people. Why do we have to call the smaller of these two objects a moon? These are just two asteroids who are orbiting each other. That's it. Sheesh.

Re:This is not surprising

[rakerman]

Probably because "binary Kuiper Belt objects" isn't as easy for people to understand as "moon around a Kuiper Belt asteroid".

This IS surprising (physics)

[Spamalamadingdong]

Two boides are attrackted while spinning in orbit around another, larger body. They start to co-orbit. That's physics people.

Two bodies approach each other from "infinity", and somehow they lose enough energy and/or angular momentum in their encounter to wind up in mutual orbit. How's that? What's the mechanism for dissipating energy, or transferring angular momentum from motion of the bodies around their center of mass to spin of the bodies themselves (Earth and Luna are doing this in reverse, but very slowly; far too slowly to capture anything).

Since you've set yourself up as the physics expert, perhaps you'd like to explain that to all of us. You'll probably get a publishable paper out of it too, so it's not like it isn't worth the work.

Re:This IS surprising (physics)

[MegaGremlin]

I have a couple of questions regarding this.
My understanding of gravity was that the presence of mass in space causes a curvature of space. Wouldn't that mean that both objects are actually travelling in a straight line, but seem to curve around each other from an outside frame of reference?
Someone please clear this up for me, I never could wrap my brain around this...

Re:This IS surprising (physics)

[CheshireCatCO]

The mechanism held responsible for this isn't capture, it's more akin to the "giant impact" model of the formation of Earth's Moon and Pluto's Charon. Capturing requires dissipation of energy, you're right. And without an atmosphere, asteroids and KBOs probably can't manage that. But if you bounce something loose off the surface, you can actually get a moon.

First?

[CaptainCarrot]

Strictly speaking, Pluto is a Kuiper Belt object, and does not appear to be too dissimilar to other Kuiper Belt objects in terms of composition. So WW31 is only news if you insist that Pluto is a planet and not what it otherwise appears to be.

All right, EE major, use your coursework

[Spamalamadingdong]

If you're a double-E you must either have studied or be going to study differential equations. You also have to understand conservation equations (conservation of energy is a sine qua non in EE, and though you may not have cause to have studied conservation of angular momentum in EE it shouldn't be that foreign a concept to you).

Your equations are pretty simple: energy (0.5 * mass * speed) is conserved, and angular momentum (mass times the cross-product of the velocity vector and the radius vector from your point of reference, which is simplest if you make it the mutual center of mass) is conserved. If you work this out even without vector math you get a very simple quadratic equation that anyone with junior-high algebra should be able to solve.

So. You have two objects approaching each other from a very long distance, with any third body a much further distance away (reducing it to what is effectively a two-body problem). If nothing is changed by the encounter, the track of the two bodies going away from each other will look just like the track of the bodies approaching each other: a parabola or hyperbola. Something has to happen near the point of closest approach to alter the energy, the angular momentum, or both in order to change the solution of your equation from a parabola or hyperbola into an ellipse. I think the cleverest explanations involve collisions, because the inelasticity of the collision neatly explains how energy is lost. However, the probability of collisions may not be sufficient to explain the number of paired bodies out there (and that's a job for statisticians).

I'm a double-E myself, so I shouldn't have any big educational advantage in this regard. Either you can peg the mechanism for producing two bodies in mutual orbit, or you can't. If you can't even appreciate the question (which I've been trying to explain here), you don't really have any business dismissing the whole issue with hand-waving. You wouldn't accept a hand-waving explanation for the current flow in a transistor or the resonant frequency of an LC circuit, and you shouldn't accept one here either - especially not from yourself.