Wednesday, January 20, 2016

Rogue Planets

'Rogue planet' is the nickname for planets that do not orbit a star. They are not large enough to have any internal fusion to keep them warm, so after enough time for them to lose the heat of formation, they would be cold, as cold as the environment where they were located. This could be as low as a few degrees K.

They are very hard to see. During the cooling down period, there would be a very small infrared signature, but detecting a planet is very hard, even for ones which are lit up by a nearby star. With no lighting, they simply do not put out any photons. So, assessing how many there are is difficult, unless there are clues which can be determined from other means.

Rogue planets might be detected by a transit, if there was a star that was being watched, as in an exo-planetary hunt or to pull out spectroscopic data, and the starlight amount dropped. If the planet was near the star being monitored, the reduction would be small, 0.1% or of this order, maybe smaller if the planet was a small one. It would be larger if the planet was very near Earth, as then the inverse shadow it casts would be significantly larger on the star’s disk. If the rogue planet was halfway between Earth and the observed star, a factor of four pops in, so it would be, in this example, 0.4% different, which is also somewhat hard to notice unless specialized equipment was being used. Simple telescopes recording images would not necessarily notice this. If the relative motion was typical of stellar differential motion, 20 km/sec, it would take about a day to cross.

If the planet was only a tenth of the way from Earth to the observed star, it would be a reduction of 10% for an hour, which is possibly noticeable through direct observation. However, these numbers give the impression that, in the rare instance that there was a transit, and even if the star being transited was being watched by someone, it could easily happen without being detected. So, by and large, rogue planets are invisible.

Why would anyone care about rogue planets? When another star comes too close to a solar system, the gravitational effect of the star, or more specifically, the gradient of the gravitational effect of the star can alter the orbits of the planets in that solar system. If it is close enough, and passes by slow enough, it could change the ellipticity of the orbit, the orbital tilt, or even the orbital radius. This would affect any inhabitants of such an affected planet. Stellar encounters are one of the galactic perils.

A planet might be only 1/40,000 times has heavy as a star, and in this example, a planet would have to pass about 1/200 as close to the solar system it was disrupting as a star would, in order to have the same effect. This means that, if the density of rogue planets were the same as stars, there would be about 1/40,000 times the chance that a rogue planet would disrupt a solar system, as compared to the chance that a star would do it. So, it is hard to see that rogue planets present a galactic peril in this situation.

On the other hand, if rogue planets were much more populous, for example, if the total mass of material in rogue planets were 10% of that of the material in stars, then the probability of solar system disruption is the same percentage. That might constitute a galactic peril. Thus, the question is, how many of them are there? They are nigh on invisible and even a 10% mass contribution might be overlooked.

Perhaps considering how they form can give some clues as to whether there could be large numbers of them floating in interstellar space. There are several possibilities.

In detecting stars, it is easiest to see O stars, the brightest ones. Going down the sequence to the end, red dwarfs, M stars, brings down the brightness to being almost undetectable, as stellar radiation is moving into the infrared. But it is just possible to detect the next smaller class, brown dwarfs, where fusion is barely happening, and the amount of radiation emitted is almost undetectable, save for brown dwarfs very close by. However, red dwarfs are about three quarters of all stars from O through M. They are the lightest stars by mass, but still constitute about 40% of the total mass of all stars, O to M.

No good estimates exist for brown dwarfs but they might have a total mass of the same magnitude as red dwarfs, or perhaps even more. It takes no imagination at all to extend this curve to rogue planets, which are the conceptual equivalent of a red dwarf or brown dwarf, as far as initial formation from gas clouds, and say there could be a large amount of them. For convenience, let’s call this formation process condensation. Thus, extrapolating the condensation process which makes stars to lower masses would indicate there could be very many rogue planets inhabiting our galaxy.

There are other possible sources, and one has already by mentioned in passing. Stellar encounters of the close kind could even eject planets from a solar system. This method of stripping planets from a solar system depends on the density of the stars, so it would be much more common in the galactic core than out here in the spiral arms, but it is certainly not impossible here. A stellar encounter might strip one planet from a solar system, or it might strip all of them. Our solar system has quite a few, and it is simply not known what the average number of planets is in other solar systems. At least we know that most stars have planets, which is more than we knew a decade or two ago.

As for solar system formation, we experience the tail end of the process, where all planets have very stable, long-lived orbits. It is not necessarily true that the other end of the process is a gentle and forbearing as we now observe. Perhaps it is common for a planet or two to be ejected when the stellar disk first forms into planetary masses. Two planets with radii that are close might lead to an ejection. This is something that has not been explored in detail in planetary formation simulations, but if it is true that planets are often lost, the number of rogue planets from this source could be of the order of stars. This is small compared to the numbers needed for a galactic peril, however.

There are certainly other, more violent methods of stripping planets from a solar system. Consider a nearby supernova. It might either directly eject a planet, or cause orbital changes so that the orbits are no longer stable, and lead indirectly, after a long interval, to the ejection of a planet. If a supernova star is part of a binary star system, or a multiple one, planets in the partner star’s solar system might be the victims of ejection. Perhaps even other stars could be affected, if they were close enough.

These violent means of forming rogue planets appear to be able to produce them in the numbers comparable to stellar populations. Only condensation seems a possibility for forming them in numbers far in excess of the number of stars, but still not taking up as much mass as the stars. Thus, rogue planets as a galactic peril are still in the running. With some luck, they could be one of the principal reasons an alien civilization would have to leave its home planet.

No comments:

Post a Comment