Our solar system today probably seems like a pretty well ordered place – it runs like clockwork, is predictable, and it is benign overall. However, it was not always so, and may not always remain that way in the future. In those early chaotic when the proto-sun and the proto-planets were forming, the orbital dynamics were anything but well ordered. Collisions and close encounters between these proto-objects, and associated interacting gravitational forces at play meant that some bodies spiraled into the proto-sun; others in their gravitational dances were ejected, in a slingshot sort of way, from the proto-solar system altogether.
In the future, as our sun and solar system along with hundreds of thousands of other ‘nearby’ suns (stars) orbit the galactic center, distances between stars vary, sometime coming close enough for their respective gravitational forces to cause another dance and throw monkey wrenches into the peace and tranquility and well-ordered-ness of the respective stellar systems and debris (like planets orbiting their parent suns). As before, some objects could be perturbed and dislodged and fall into their sun, or be ejected from the gravitational control of that sun altogether and head out into space and a lonely, endless night, existence. In addition, the ejection process will be alive and well in binary (tug of war) star systems (and there are an awful lot of them about).
While there is nothing to be said for those unfortunate bodies (and life forms – if any) that get cataclysmically into their parent suns, it is not quite ‘The End’ for those now sunless orphans. There could be multi-thousands (or more) of orphaned planets (of all sizes and makeup) in deep space beyond the bright light and solar energy of a sun. There could be an orphan planet within a light year of us and we would never know or suspect.
Could orphan planets form all by themselves in the depths of lonely space? Well, if a large interstellar dust and gas cloud can gravitationally collapse to form a star(s) and associated stellar (solar) system, I fail to see why a smaller interstellar dust and gas cloud couldn’t collapse to form a planet sized object, probably a ‘failed star’ like a Jupiter, maybe with moons. From that of course it is easy to extrapolate and suggest such a Jovian ‘failed star’ might have smaller abodes (planets) form and orbit same – a nearly invisible solar system. Or perhaps it is just an orphan planet with associated moons. Either definition amounts to the same thing – a rose by any other name applies.
Regardless of initial origin, conventional wisdom would suggest that these orphans must be lifeless, even if before the event they had life.
When I was a high school biology student (1962-63), it was absolutely gospel (and no correspondence would be entered into contrary) that our sun was the be all and end all of the existence of terrestrial life. No sun; no life. All life ultimately depended on photosynthetic plants, which in turn could not exist without sunlight. Even then however I seem to recall speculation about the possibility of a non-photosynthetic based ecology in the atmosphere of Jupiter which gladdened my heart no end – however, it wasn‘t Jupiter that broke the photosynthetic mold, but good old Mother Earth herself. Therefore, gospel ain’t gospel any longer! Today we know about chemosynthesis (organisms that can produce organics from inorganic substances and derive energy from the process.)
A well known, if little understood example of chemosynthesis are the colonies of microbes (dubbed ‘rusticles’) that are eating the iron structure of the RMS Titanic, resting some four kilometers below the surface of the North Atlantic. Within another generation or two, the famous shipwreck will have been basically consumed by microbes, without any benefit bestowed by our sun.
However, an orphaned planet has severe problems quite apart from a lack of solar energy. What about heat? Sources of heat (apart from a parent sun) include gravitational contraction, radioactivity, chemical activity, friction, etc. Therefore, heat should not be too much of a problem for some planetary abodes. Rocky planets like Earth have radioactive elements that partly comprise their crusts and interiors, and radioactive decay gives off heat, and rock is a good insulator. I doubt if chemical activity or friction will contribute much, but for Jupiter-sized planets, gravitational contraction means that these types of planets (like Jupiter, Saturn, and Neptune) give off more heat energy than they receive from the sun.
Of course, friction could be a source of heat in some rare cases. One other heating scenario is plausible if an orphan planet had a satellite(s) of the right composition. The satellite(s) might be warmed by tidal frictional forces courtesy of their parent-orphaned planet akin to what we observe on Io and Europa. These satellites are heated by the effects of Jupiter’s tidal attractions on the interiors of these moons, which get flexed and stretched and compressed, ever alternating between extremes. The resulting friction results in heating. An alternative version could be two orphaned planets of roughly the same size, orbiting each other in relatively close proximity. Each would mutually tidally heat the other, but only for a while. You cannot produce heat energy out of nothing, and the price paid would be their orbital separation increasing until the gravitational bonds weaken so much that you would have – for all intents and purposes – two separate orphaned planets. This is akin to our own Moon which is retreating from Mother Earth, albeit ever so slowly, over time.
However, heat tends to be the final end waste product in any energy chain of events. Heat itself is not useful as an energy source for living things; albeit quite useful in contributing to the environmental friendliness in which organisms thrive, like keeping temperatures suitable for liquid water or for biochemical reactions. I mean an infrared lamp may feel very good, but it is not providing you with any calories!
Could one have an origin of life (biogenesis) event on an orphaned planet? Why not, providing you had the appropriate chemicals, all mixing it up in an appropriate liquid medium (water most likely), and an energy source(s), and lots of time.
Therefore, an orphan planet could have had a biogenesis event, coupled with suitable chemicals for chemosynthesis and heat. What more do you want! Well, are there any positives to be had?
Are there any obvious advantages to being a life form on an orphan planet? I think so. What if your initial parent star were a variable star, or a very massive star that is going to have a very short lifespan, maybe terminate in a supernova – advantage orphan. Maybe your initial orbit was so lopsided (elliptical) that you alternatively froze and fried – advantage orphan. Gravitational (tidal) locking – keeping one hemisphere always turned toward one point, say the surface of your parent star – causing temperature extremes, is now irrelevant. The tilt of your axis (which can also cause extremes in heating/cooling is now also irrelevant. Your weather, such as it is, would be relatively gentle without solar energy driving it. Then too, inhabiting deep space via-a-visa crowded debris filled solar system reduces drastically those nasty collision impacts. A thick Jovian (Jupiter) type atmosphere or thick ice cap is probably good shielding from radiation, though even a nearby supernova might be bad news. You are also insulated from all those nasty bug-eyed-monster alien types. They can easily find solar systems, but not orphans. In any event, your orphan planet isn’t very desirable real estate to alien invaders!
One day, in the not too distant future, it will be a fairly straightforward exercise to compare solar systems – what’s a typical solar system; what’s an atypical solar system. However, questions central to what a typical planet and an abode for life is will still be difficult to answer, as that population of orphan planets (and ages of same) will take a long time yet to resolve itself.