Sunday, July 31, 2016

Planetary Formation

Now that planets are being discovered on a daily basis, it isn't quite as interesting to try and figure out from scratch how they formed and how many there might be. Instead, why not just wait and see how many are formed? Planetary discovery has become such a popular topic in the news, that scientific institutions are all jumping on the bandwagon and planning and building new telescopes and clever instruments to find them. For just a little bit, let's swim counter to the flow.

Planets form because of angular momentum in the gas cloud that condenses to form a solar system. If you had a nice spherical gas cloud, not rotating in the slightest, it would simply stay spherically symmetric and collapse down to a single star, with no planets. However, we live in the Milky Way galaxy, and the galaxy is rotating, meaning every bit of the galaxy shares in the angular momentum it has. The sharing is done differently in the two principal parts of the galaxy. The central bulge of our galaxy, and other galaxies, rotates as a sort of solid object, meaning it all has the same rotational rate, and it rotates together. This means that as a blob of gas in the central bulge collapses, it is rotating originally at the rotational rate of the bulge, meaning it has some angular momentum.

Have you ever played that children's game of sitting on a swivel chair with some weights in your outstretched hands, and had somebody turn you around while you pulled in the weights? You would remember that you sped up in your spinning a lot. That is nothing compared to what a gas blob does. A gas blob starts out of the order of a light year in size, within factor of ten or so. It is going to collapse to solar system size, a hundred AU or so. This is about five thousand times smaller, and that means that a small bit of galactic angular momentum in the original gas cloud turns into some serious rotation when the cloud finishes its condensation. The collapse of the central part of the gas cloud into a star is even more dramatic. The galaxy rotates once in a hundred million years. A star rotates maybe once a day. That's a serious increase in rotation rate!

Even this grabbing of angular momentum by the star itself isn't enough to use up all the angular momentum gifted to it by the galactic rotation. Planets have to take up the large majority. As the cloud forms into a rotating disk, because nothing is keeping one dimension expanded and angular momentum is keeping the other two out there, the centrifugal force eventually comes into balance with the gravitational pull of the newborn star, or the central blob of gas that will become the star. No more collapsing, and the time clock on planetary condensation starts.

Keep this simple process in mind as you consider the galactic disk, the other significant part of the galaxy. The galaxy itself condensed from a gas blob, and went through the same condensation process, and formed a disk for the same reasons. But the disk isn't rotating as a solid body. It is shearing with a angular velocity curve that is much more mild than solid body rotation. And that means that the gas blobs in the disk are not rotating with the same angular rate as the those in the galactic bulge, but much less. When they condense to make solar systems, the solar systems will be smaller, with less angular momentum to keep the planets way out from the star.

Perhaps it would have been better to start with the disk, because that is where we have taken almost all our planetary search data, and where we live. But for ease of explanation, we started with the bulge. So, inverting the order, we would expect that solar systems in the central bulge to form much large and further out than in the galactic disk. We have some idea of how large they are out here, and we can judge from that information that those in the bulge are larger, meaning further extended.

This has an implication. Recall that the stars in the galactic bulge are much closer together and moving randomly, like molecules in a gas. This means encounters are more frequent and with shorter separation distances, and this means that planetary systems are more likely to be disrupted by stellar encounters. Put this in the pot with larger solar systems, and the conclusion is inescapable. There are less surviving solar systems in the galactic bulge than in the galactic disk. Inside the bulge, more solar systems are stripped of planets, or at least the outer ones. And therefore, there are even more rogue planets in the galaxy that we would otherwise have expected.

When a stellar encounter happens, planetary orbits are disrupted, and even if the stellar encounter itself does not fling some planets into the void, planetary interaction will. Planets form in a stable arrangement, with planets in resonance situations where they do not transfer angular momentum in one direction only, but instead, trade it back and forth, generally keeping the same average orbital radius over long periods of time, meaning, large numbers of orbital periods. Those that are not in stable orbits disassociate, with a few planets diving close enough to the star to be tidally captured and eaten, but mostly just condemned to the void. The same thing happens when planets that form in stable orbits are pushed out of them by a stellar encounter. A few are eaten and the rest are banished from the solar system. Some planets would necessarily remain, but by and large, the solar systems are stripped of planets.

This is just one more additional reason why looking for aliens in the galactic bulge is not the best choice. Even if their planet hangs onto the star it was born around, the orbital radius would be changed by stellar encounters. Since solar systems are born larger in the bulge, stellar encounters are both more frequent than with smaller ones, and more devastating. It would be easier for a stellar encounter to rip off a far-out planet, and easier for planetary interactions to push it beyond the brink of stellar control, out where gravity from the bulge in general is perturbing its orbit and juggling it into being a rogue planet.

Thus, once again we find a reason to prefer planetary hunting in the galactic disk.

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