The “trillions of rogue planets” story is intriguing, and it’s based on high-end astronomy. These are the very early days of this type of research, and it can be expected that theories will follow data until the research expands and refines.
The (very) general background to rogue planets
Some of these rogue planets are known to be very big, Jupiter-scale things. If so, it’s pretty safe to assume that there are also many smaller planets of this type. There are usually many more small stars than big stars, and more asteroids than planets, for example. Some people are also calling these things “brown dwarfs”, which is a bit wrong terminologically. A “brown dwarf” could also be a star.
The rogue planets don’t emit much light except in the infrared. They’re practically invisible. Some have been located using ingenious astronomical techniques, but not trillions, for that reason. Gravitational lensing is one of the methods. Something unseen but blotting out the stellar background as it transits is another.
Another thing to explain here – Rogue planets were almost pure science fiction until about 15 years ago. The idea took a while to catch on with serious astronomy. The need was for an explanation for them, and the current theory is planets kicked out of systems by gravitational forces, which is a pretty safe, well-understood theory.
There definitely are a lot of planetary systems in our galaxy, in fact, 4000 at the very least, and more being discovered all the time. Most of these systems were discovered very recently. First it was a few, then thousands. Statistically, if one in a hundred systems throws out a rogue planet, it’d still be a lot of rogue planets.
An argument or so
Statistically, we don’t have a lot to work with here. The trouble with “trillions” is that it automatically goes to the extreme numerical outcome. That probably isn’t right. There are up to about 400 billion stars in the Milky Way galaxy. Not all of them have planets, and some are binary or even trinary systems. Most of the stars are in the center of the galaxy, which obscures a lot of information about them and possible planetary systems.
You can see where the problems start with calculating numbers of possible rogue planets. Using the 1 in 400 figure, that’d be about 4 billion rogue planets. The Milky Way is about 200,000 light years in radius.
You can see where this sort of logic goes. An average number of rogue planets can only be a guess. Based on these numbers, you’d get about half a trillion rogue planets over that huge area. If there were trillions, you’d get X number of rogue planets per light year. Not yet too plausible.
That sort of logic also fails to factor in the obvious multiple issues for huge numbers of rogue planets roaming the galaxy:
*If the rogue planets are roaming in the center of the galaxy, they’re subjected to a lot of gravitational stresses. Any trajectory could change quite easily, and quickly.
*Big stars, multiple star systems and other rogue planets or big asteroids could destroy them.
*The plasma in interstellar space isn’t exactly a welcoming environment. Those energies, recently encountered by the Voyagers, could be quite destructive.
*Some planets could be captured by star systems, in fact that’s an unavoidable option.
*The rogue planets could even be kicked out of the galaxy, as one star recently spotted seems to be doing.
*Roaming black holes could break them up.
Population mechanics
Over the 13 billion years of the existence of the galaxy, a lot must have happened in that time to dictate exactly how many of rogue planets there are. The population could expand and contract. Supernovae could add more free planets as stars explode. The planets could go off on eccentric orbits, and then meet whatever fate is in store.
Overall, though, the galaxy is likely to be a tough place to survive for any sort of free-ranging object, planet, comet, star, grains of dust, or whatever. If there really are that many rogue planets around, how do they survive?
The bigger an object is, the tougher it is to avoid possible contacts with big, powerful forces. There probably is a survival mechanism, but there’s nowhere near enough information yet to see how it works.
The other big question is where these planets are likely to be. It’s fair to assume that higher populations of star systems would produce more rogue planets, for instance. Rogue planets would also be affected by the action of the galactic disk, too, and would have to follow it. Pinning down populations of rogue planets is likely to be a tough task.
I’d be a little thoughtful on this subject in the course of study. There’s no clear evidence that the Sun has ever encountered them in its 4.5 billion years of life or that they’ve impacted the solar system any time recently. We’re quite a distance from the center of the galaxy, too, so local populations of rogue planets should be lower.
