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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!"
so much resources out there for the taking, no need to come to earth
Planet is Greek for wanderer. So, I think the name is even more appropriate.
I always wondered why wandering planets couldn't be used instead of dark matter to explain where all the missing mass is.
Actually these are not planets according to the new classification.
First, it must orbit the Sun.
Second, it must be big enough for gravity to squash it into a round ball.
And third, it must have cleared other objects out of the way in its orbital neighborhood.
http://missionscience.nasa.gov...
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.
(FYI)
There are a few reasons astrophysicists know that it is extremely unlikely that dark matter is baryonic. First of all if all the stars in a galaxy shine on an object it heats up, this heat causes the release of radiation, called thermal radiation, and every (baryonic) object above zero kelvin (or -273.14 deg celcius) emits this radiation. However, dark matter does not emit any radiation at all (hence the name dark!)
If dark matter were baryonic it would also mean that it could become light emitting. If we got a clump of baryonic matter* and put it in space it would gravitationally contract, and would eventually form a star or black hole** - both of which we would be able to see.
So, because of these reasons the dark matter in galaxies and in galaxy groups/clusters cannot be baryonic, and so cannot be planets, dead stars, asteroids, etc. It would definetely not be planets as there is no way 10-100 times the mass of the stars in a galaxy would be planets, as the mechanism for making planets relies on supernovae, and the number of supernovae needed for the that many planets would be far too high to match our observations. I hope that this answered your question!
*provided the clump of baryonic matter was large, and the amount there is in galaxies definitely is!
** we don't observe black holes directly, but can see radiation from their accretion disks.
Dark matter accounts for something like 90% of the gravitational effects that we see.
I've always suspected that "dark matter" very likely isn't matter at all. I suspect it is simply a gap in our model similar to how relativity filled in gaps for Newtonian mechanics. Dark matter (and dark energy) are basically placeholders for observations that do not match our model. That means one of two things. Either there is something we haven't observed yet OR there is something missing from our model. Both are quite possible but we seem fixated on that former when it could very easily be the later.
I actually do have some background in physics (college minor and worked in some research labs) and I've never have any "real" physicist give me a satisfactory explanation as to why invoking some mysterious matter/energy is a more likely answer than a gap in our models. We understand gravity probably the least of the four forces and we don't have a model that integrates it into our Standard Model. Seems to me that the place to look may very well be in the math rather than in the stars.
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.
Systems composed of multiple stars (binaries, etc.) are more common than singular stars, like our Sun. A binary system is a risky place to be -- there is a strong probability that the gravitational interaction between the paired stars would, given enough time, eject any planetary body which forms there -- the "stable" regions depend on the orbital parameters of the two (or more) stars and can be limited to very narrow bands. So, if planetary formation is a typical process around stars and binaries are more common, then it's likely that the galaxy has a large population of planets ejected from unstable orbits around binaries.
For what it's worth, conjecture is that the Sun formed in a cluster and was, itself, ejected. Nearby stars with identical spectra (implying they formed from the same source material) have been identified.