Planetary systems without star could exist by trillions in our galaxy.

Everywhere in nature there are more small objects than big ones. In our solar system we have one star, many planets and a lot more satellites than planets. There are more asteroids than satellites, more rocks than asteroids, and more grains of space dust than rocks.

For every star, there should be many objects that didn't get big enough to ignite and make a star, and most of them could have satellites.

The number of these systems is not probably enough to account for the total of the missed matter in our galaxy (dark matter), but at least a part of it, and let’s face it, it has to be enormous. If there are 200 billions stars in our galaxy, and let’s say 10 non solar systems for every solar system, it can be easily about 2 trillions. Some estimations raise this number a lot.

Rogue planets or planemos is the term used for planets escaped of his star, but also applies to the ones naturally born in the interstellar space by the same process of aggregation of matter that formed the stars.

Sub brown dwarfs is the name actually under consideration by the IAU (international astronomical union).

13 times the mass of Jupiter is needed to start the thermonuclear reactions that make a star, so, the planets in these systems could be of any size below this limit.

Common sense not always applies in science, especially when we consider subjects like quantum mechanics or relativity, but in this case, in my opinion we should take a look at the whole picture and consider the possibility that they could exist in mind blowing numbers.

We have some evidence of their existence due to observation techniques based on gravitational microlensing, but current estimations of their numbers are still imprecise and varies from 2 sub brown dwarfs per star in our galaxy to an enormous amount.


Could life exist in such places?

Why not?, but life like the one we know would be very limited because of the lack of energy, nothing quite like what we have on earth, but something like the oases around hydrothermal vents we know where life thrive on heat and chemicals without any need of sun light, could be a very common thing.

link to A proposal for a habitable planet in interstellar space

There wouldn't be need for a strong magnetic field and an ozone layer as protection against lethal radiations from the star. Also, climates would be more stable.

The planets would not necessarily be frozen. With thermal energy coming from volcanic activity and gravitational stresses caused by a close orbit around a gas giant, let's think of Io, we could find all kinds of temperature ranges. 

Evolution wouldn't have been so hampered by a so long period of rock bombardment like the one planets around a big star have to undergo. Even if it is true that mass extinctions favour the diversity of species, it is not clear if as a whole could be considered as beneficial.

One of this places formed at the same time that a bigger system with star could have the orbits cleaned of collision objects 1000 millions years before and therefore events like the Cambrian explosion of life on earth could happen a lot sooner.

Another consideration would be unknown types of life. We can imagine that in absence of an abundant energy source, life could find its way taking advantage of the still mysterious low energy nuclear reactions (LENR) that we know may happens in biological processes, or at least there is some literature and reports about it (link), but this is still somewhat scifi.

And besides, we could consider that some of the planets we will discover in the future, and maybe some of the ones we already know could be captured planets not naturally born around a star. A big part of the stars in our galaxy are binary stars, which mean two stars orbiting each other. many of them could be captured stars.The number of captured planets should be a lot bigger than the number of captured stars.

Anyway, the tendency seems to point to a broader view of what a solar system is, and instead of the classical view of a star orbited by planets, more something like just objects orbiting each other no matter what they are, stars, planets, black holes or rocks.


A non solar system could be orbiting our sun every 137 million years:

A pattern of periodical mass extinctions have been found in our planet every 137 million years. The most likely cause of this phenomena is a non solar system orbiting our star.
Every time it crosses the Kuiper belt, a shower of rocks of many sizes is unleashed into the inner solar system with catastrophic consequences to life on earth as some of this objects will on time collide with our planet.

Today scientists are puzzled by the strange orbits and configurations of many systems recently discovered with planets orbiting in the opposite direction that they should have.

They still don't realize that those are captured planets coming from interstellar space, either ejected from other stars or systems without star, or just formed there.

                    Extrasolar art by masbt