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'Homeless' Planets May Be Common In Our Galaxy
sciencehabit writes "Our galaxy could be teeming with 'homeless' planets, wandering the cosmos far from the solar systems of their birth, astronomers have found. In a paper published online today in Nature, the researchers list 10 objects in our galaxy that are very likely to be free-floating planets. What's more, they claim that in our galaxy, free-floaters are probably so populous that they outnumber stars."
Ok, so I actually have an advanced degree in astrophysics. While you do describe the basic observations that lead us to believe dark matter exists, it's not true that galactic dark matter and large scale dark matter are different beasts. True, MAssive Compact Halo Objects (such as rogue black holes, neutron stars, brown dwarfs, etc) (note the correction) were a possible explanation, but we've done observational studies that look for them using microlensing, and although we did find a few, there wasn't nearly enough (i.e. several orders of magnitude less) to explain our galactic rotation curve. WIMPS (such as neutrinos) have been ruled out since they fail to explain the observed large scale galactic structure, and there aren't nearly high enough neutrino counts in neutrino observatories to make them a viable option.
It turns out that, for BOTH galactic rotation curves and large scale darkmatter, you need about 10 times more mass than what we can associate with stars, so the two problems actually do have the same constraints. Therefore, it's very likely that there is some form of matter which only interacts gravitationally (and not electromagnetically: i.e. with light) with normal baryonic matter which has so far been unobserved. (Not surprising, since a lot of our matter detection techniques rely on interactions with light, and besides the required density of this stuff would make it very rare on Solar System scales - it only becomes significant on galactic scale interactions).
On the original article - It's not too surprising that there are lot of free roaming planets, it just indicates that there was a higher degree of fragmentation in molecular clouds than was once thought. However, it would require an insanely HUGE number of them to explain dark matter observations - planets are generally much less massive than stars, somewhere around 10^6 to 10^8 times less massive, so there would have to be 10's of billions times more of them than stars to explain dark matter observations, something that the article does not assert is true.