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I am pretty sure this is a widely acknowledged assumption among most scientists, considering our current understanding of the formation of planets, stars, and g

I am pretty sure this is a widely acknowledged assumption among most scientists, considering our current understanding of the formation of planets, stars, and galaxies.

ggreer 12y ago
These stars were formed from gas clouds in galaxies. Wouldn't a gravitational slingshot with ∆v greater than galactic escape velocity likely strip their planets? Unless you mean the ex-planets themselves would end up alone intergalactic space. Apparently, the vocabulary used for such objects is, "sub-brown dwarf."[1] 1. http://en.wikipedia.org/wiki/WISE_0855%E2%88%920714 http://en.wikipedia.org/wiki/WISE_0855%E2%88%920714
antognini 12y ago
The free-floating planets result is still controversial and there is a substantial fraction of the astronomical community that doesn't believe it. If it is true, it introduces problems in our understanding of planet formation because planet-planet scattering as we currently understand it is unable to produce free floating planets at the rate claimed. It is also possible that planets form in isolation (so really they're very small brown dwarfs), but given our understanding of star formation it is difficult to form isolated objects that small.
- fragsworth 12y ago
  > "given our understanding of star formation it is difficult to form isolated objects that small." No, that is demonstrably false. You can try this with any particle gravity simulator - Place a bunch of particles, and watch how many get ejected. Star systems behave similarly.
  - antognini 12y ago
    The problem is that you cannot treat star formation as a set of collisionless particles. In general you can estimate the scale of star formation through the Jeans mass, which is the mass of a gas cloud at which the dynamical time becomes equal to the sound crossing time. This is essentially the scale at which the cloud becomes unstable to perturbations --- pressure is no longer able to support the cloud against gravitational oscillations. The Jeans mass depends inversely on the square root of the density of the cloud. The main problem in forming very low mass objects is that you need very large densities. For a Jupiter mass object you need a density of ~5 x 10^4 / cm^3. Even in molecular clouds it is difficult to achieve densities that large. There are ways around it --- turbulence can produce small regions of very high density, for instance. But like I said, they're not well understood. In general it's much easier to create a low mass object in the vicinity of something bigger because then it can form by fragmenting out of the disk that surrounds the larger protostar.

I am pretty sure this is a widely acknowledged assumption among most scientists, considering our current understanding of the formation of planets, stars, and galaxies.

ggreer 12y ago

These stars were formed from gas clouds in galaxies. Wouldn't a gravitational slingshot with ∆v greater than galactic escape velocity likely strip their planets? Unless you mean the ex-planets themselves would end up alone intergalactic space. Apparently, the vocabulary used for such objects is, "sub-brown dwarf."[1] 1. http://en.wikipedia.org/wiki/WISE_0855%E2%88%920714 http://en.wikipedia.org/wiki/WISE_0855%E2%88%920714

antognini 12y ago

The free-floating planets result is still controversial and there is a substantial fraction of the astronomical community that doesn't believe it. If it is true, it introduces problems in our understanding of planet formation because planet-planet scattering as we currently understand it is unable to produce free floating planets at the rate claimed. It is also possible that planets form in isolation (so really they're very small brown dwarfs), but given our understanding of star formation it is difficult to form isolated objects that small.

fragsworth 12y ago
> "given our understanding of star formation it is difficult to form isolated objects that small." No, that is demonstrably false. You can try this with any particle gravity simulator - Place a bunch of particles, and watch how many get ejected. Star systems behave similarly.
- antognini 12y ago
  The problem is that you cannot treat star formation as a set of collisionless particles. In general you can estimate the scale of star formation through the Jeans mass, which is the mass of a gas cloud at which the dynamical time becomes equal to the sound crossing time. This is essentially the scale at which the cloud becomes unstable to perturbations --- pressure is no longer able to support the cloud against gravitational oscillations. The Jeans mass depends inversely on the square root of the density of the cloud. The main problem in forming very low mass objects is that you need very large densities. For a Jupiter mass object you need a density of ~5 x 10^4 / cm^3. Even in molecular clouds it is difficult to achieve densities that large. There are ways around it --- turbulence can produce small regions of very high density, for instance. But like I said, they're not well understood. In general it's much easier to create a low mass object in the vicinity of something bigger because then it can form by fragmenting out of the disk that surrounds the larger protostar.