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Most Planets In the Universe Are Homeless
StartsWithABang writes: We like to think of our Solar System as typical: a central star with a number of planets — some gas giants and some rocky worlds — in orbit around it. Yes, there's some variety, with binary or trinary star systems and huge variance in the masses of the central star being common ones, but from a planetary point of view, our Solar System is a rarity. Even though there are hundreds of billions of stars in our galaxy for planets to orbit, there are most likely around a quadrillion planets in our galaxy, total, with only a few trillion of them orbiting stars at most. Now that we've finally detected the first of these, we have an excellent idea that this picture is the correct one: most planets in the Universe are homeless. Now, thank your lucky star!"
Not a physicist, but there's a few reasons. First and foremost, I believe there simply aren't enough wandering planets to explain it. Dark matter accounts for something like 90% of the gravitational effects that we see. If wandering planets were to blame for that much mass, they would definitely be much, much more noticeable even without giving off light like stars. Secondly, wandering planets simply don't fit the bill for what we're seeing in regards to gravity - if it were all planets, we would be seeing much different galactic formations.
As I understand it, getting "captured in a gravity well" is actually pretty tricky. Unless you form in orbit around a larger body, you're most likely by far to just do a hyperbolic single-pass encounter. To be captured, you need to impact the larger body (a very rare occurrence), or dissipate your momentum in its atmosphere (almost as rare), or have some sort of multi-body interaction (probably rarer still).
This is all approximate -- technically, I guess everything orbits everything within its historical light-cone. Almost none of those orbits are anything close to periodic, though.
There are "planets". 8 of them.
Then, there are a bunch of "dwarf planets" - Pluto, Ceres, Eris, etc.
"Minor planets" - there are thousands, millions, I'm not sure, but a lot of these.
"Exoplanets", let's divide these into two categories - system exoplanets, that orbit a star like our planets, dwarf planets, and minor planets, and systemless exoplanets that do not orbit a star.
These are all different kinds of planet. In astronomical terminology, the word "planet" by itself is reserved for the Big Eight, but all these other things are a kind of planet.
We've seen that with Jupiter, if it were not for being in orbit around Sol, Jupiter and its moons would effectively be their own dark solar system.
Read up on MACHOs vs WIMPS, two alternate theories of Dark Matter. "Your" idea is MACHO
http://en.wikipedia.org/wiki/M...
http://en.wikipedia.org/wiki/W...