‘It was recently discovered that Pluto has two moons beyond the one already known (Charon, discovered in 1978). This was very cool news, but it begs the questions: where did they come from?
Quite a few observations have been made, enough to compile some interesting data: the moons are small, they orbit Pluto farther out than Charon does, and their orbits are fairly circular.
There are several mechanisms that can create a moon. One is to have it form along with the primary (the planet). This seems unlikely for Pluto, given that it’s way far out in the solar system, where one assumes matter in the early proto-solar system disk was spread pretty thinly. It’s hard to see how you can get more than one object at a time in one place from those meager pickings.
Another way is to capture objects that are in orbit around the Sun. Turns out that’s tough too. Most of the time, an object orbiting the Sun won’t get captured by a planet: it’ll swing on by, leaving the planet with as much velocity as it had coming in. If you already have a biggish moon, then capturing an object is easier, though still pretty tough. And it leaves the question of how the big moon got there in the first place*.
Looking at our own Moon may give you an idea for a third way. The Moon is covered with craters, which are from impacts with other, smaller bodies like an asteroid. When it hits, rock and rubble get ejected, raining down on the surrounding area. If the impact is big enough (and the gravity of the impacted body small enough), then the ejecta can be launched into orbit.
If Charon formed from some impact event early in Pluto’s history, then there would have been a lot stuff ejected with it. It’s possible the two small moons were ejected bodily from Pluto at the same time, or were formed from smaller pieces which merged. They would have formed close in to Pluto and Charon at first, and then over billions of years moved outward as tides from Pluto and Charon gave them more energy.
An interesting side-effect of this idea is that there may be lots of multiple systems out there in the cold dark beyond Pluto. A while ago, it was crazy to think that these kinds of objects might have moons, but we know collisions happen a lot. If moonlets can form from these collisions, then we expect to see lots of multiples out there. We also would expect that they would be heavily cratered from all the impacts (unless something smooths out the surface, like melting from the heat of impact). Since we’re sending a probe to Pluto — the launch window opens on January 17th — these predictions can be tested. In a few years we may have a much better idea of how these systems form. That’s pretty amazing, given that just a few years ago we didn’t even know they existed!
For those with some technical background, a science journal paper has been published which describes all this too.
* I should add that this capture idea works better very early in the solar system. Pluto could have captured something smaller because the proto-solar system disk itself can help slow down the incoming object enough. It’s complicated physics, with lots of intermediate steps, but you can think of it as the stuff in the disk dragging on the incoming body, draining away some of its energy of motion, allowing Pluto to hold onto it.‘
January 3rd, 2006 at 2:38 am
Is this formation theory of these moons proved and confirmed? What can be the other reasons of the moon formation? One thing which hinders our exploration is the great distance of this planet from our home planet.
January 3rd, 2006 at 4:59 am
Varun, what do you mean by “proved”?
The BA was using language indicative of uncertainty (several “if”s and at least one “it’s possible that…”). The short answer is: we have no idea which of the possible mechanisms is the right one for Pluto and its satellites. But, at present, one seems more likely than the others.
We know (beyond any reasonable doubt, given the available evidence) that our Moon was formed as the result of a collision early on in Earth’s history. It seems likely that Pluto’s satellites were formed in a similar way. The only way to know for sure is to send some instruments out to Pluto to find out more about the system. And that’s what New Horizons is for.
January 3rd, 2006 at 7:09 am
I meant to ask that what can be most probable reason of their formation?
January 3rd, 2006 at 7:14 am
So how much is this mission costing?
January 3rd, 2006 at 8:50 am
Don’t get me wrong: I love this site and this blog, and I think that the BA is a champion for truth and reason.
However, “but it begs the questions: where did they come from?” in the intro paragraph of this entry represents a misuse of the term “begging the question”. I know that this misuse is so common that many of us are inclined to accept it, but I love language as much as I love astronomy, and I feel like some of our more elegant linguistic constructions are worth defending.
While I view Wikipedia with skepticism (and all other resources, for that matter), here is a fair summary of the meaning of the phrase at issue: http://en.wikipedia.org/wiki/Begging_the_question
Ultimately, the common misinterpretation of the phrase makes sense, given of the standard usage of the word “beg”. I feel that in the context of this blog, though, the fact that so many antiscientists use the “begging the question” argument makes the original meaning of the phrase much more valuable, and all the more worth protecting.
Ad astra, etcetera!
January 3rd, 2006 at 11:56 am
To be sure, it’s not “begging the question”.
http://skepdic.com/begging.html
January 3rd, 2006 at 11:59 am
What about the involvement of a third body in a capture? Two bodies approach the planet and the three interact. The end result is that one body gains total energy (kinetic minus binding) with respect to center off mass, and the other loses enough to become negative (captured)?
January 3rd, 2006 at 2:22 pm
Yes, a 3-body interaction works, but you need to have enough bodies with enough mass around to make it happen. If the bodies are a lot smaller than Pluto, the whole thing has to happen in a fairly small volume of space. Also, the “captured” body is probably not going to be all that tightly bound to Pluto and so it stands a good chance of getting thrown out with the next 2-body interaction (which is much more likely that the 3-body one that started it all).
January 4th, 2006 at 6:48 am
Stick says “So how much is this mission costing?
The cost of the mission, including the launch vehicle and operations through the Pluto-Charon encounter, will be roughly $650 million. Divided among the population of the United States (according to the U.S. Census clock at http://www.census.gov/main/www/popclock.html) over the 10-year duration of the mission, this comes out to about 20 cents per person, per year.
January 4th, 2006 at 8:03 am
Same types of arguments for ejecta as for captured bodies.
Some sort of multi-body interaction would have to occur for impact ejecta from the surface to enter orbit around Pluto, as well. Anything ejected from the surface would either escape entirely or fall back to the surface, unless its velocity is somehow re-directed more horizontally. After all, rockets need to steer their way into orbit while thrusting, or somehow otherwise convert their vertical motion into horizontal motion. Maybe interactions among ejecta particles would bend the trajectories of some into orbit.
January 4th, 2006 at 9:34 am
Another way to look at $650 million for the mission is to compare it to the cost of making the movies, which Phil was ranting about in another post. This is the equivalent of making Titanic, King Kong, and probably one of the LOTR movies. The nice thing is that when it is all done and we get results we won’t be charged $10 a head to view the images!
January 4th, 2006 at 11:30 am
5 Random Links – Pluto, Charon
This site has been added to Skeptic Rant’s “5 Random Links” for January 4th.
January 4th, 2006 at 3:30 pm
…And to those who addressed the point “begging the question”, you might want to look back a bit in etymological history to find the reason for the term. It’s almost implicit in the term itself, but perhaps not obvious enough to grasp instantly. “Begging the question” was originally “beggaring the question” as in “leaving it wanting” like a beggar asking you for money…
January 6th, 2006 at 2:32 pm
Anything ejected from the surface would either escape entirely or fall back to the surface
I may be missing something, but why assume that ejecta would be ejected straight up?
May 26th, 2006 at 11:19 pm
what is the moons n plutos environment?
May 26th, 2006 at 11:20 pm
how much does it cost to go to the moon? and pluto?
December 8th, 2011 at 5:58 pm
If anyone stumbles on this again and reads through the comments to here then there’s abonus article on the Plutonian moons origin linked to my name here via space-dot-com complete with more details and an animation.
We also know know about four Plutonian moons with an extra one – designated S/2011 P1 or just P4 informally – discovered earlier this year! Cut’n'paste :
into the search box here for that item which was posted by the BA on July 20th, 2011 8:57 AM.