Cha 110913-773444 scoffs at your puny attempts to define the boundary between a star and a planet.

Cha 110913-773444 compared to the Sun on the left and Jupiter on the right. Cha 110913-773444 is kind of a rogue planet, but also kind of a sub-brown dwarf. Cha 110913-773444 scoffs at your puny attempts to define the boundary between a star and a planet.

This simulator here is tons of fun. It's a pretty simple simulator of how gravity works between objects in a single solar system, simplifying matters in that it's 2-dimensional, so it doesn't take into account how objects orbit each other when they don't share the same orbital plane (which is the case with asteroids like Ceres and Vesta, plus Pluto and others), but it's still lots of fun to plug in a few numbers and see what happens. One person on Reddit has come up with the following:

whereby a moon spends some time orbiting its planet, is claimed by the sun in the centre, then eventually gets taken back by the planet after some time orbiting the sun alone, and eventually is thrown out of the solar system.

This simulation also shows quite well how easy it is for rogue planets to be formed in the beginning when orbits haven't quite stabilized yet and planets are still being bombarded by this and that and being affected by these shifts in gravity as other bodies pass by or grow in size. And as we know, there should be no reason why a rogue planet couldn't sustain life given the right conditions. And though a rogue planet is technically alone in space (unless it has moons of its own) it would still have stars in the sky as we do; it would just never have daytime. Many species of life in the deep ocean are completely blind (since it's so dark that eyes don't do them any good), so life on the surface of a rogue planet would actually receive more light than they do.

Here's what Wikipedia has on the possibility of life around these rogue planets:

In 1998, David J. Stevenson authored a paper entitled "Possibility of Life Sustaining Planets in Interstellar Space." In this paper, Stevenson theorizes that some wandering objects, that Stevenson refers to as "planets", drift in the vast expanses of cold interstellar space and could possibly sustain a thick atmosphere which would not freeze out due to radiative heat loss. He proposes that atmospheres are preserved by the pressure-induced far infrared radiation opacity of a thick hydrogen-containing atmosphere.

It is thought that during planetary system formation, several small protoplanetary bodies may be ejected from the forming system. With the reduced ultraviolet light associated with its increasing distance from the parent star, the planet's predominantly hydrogen and helium containing atmosphere would be easily confined even by an Earth-sized body's gravity.

It is calculated that for an Earth-sized object at a kilobar hydrogen atmospheric pressures in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will be sufficient to heat the surface to temperatures above the melting point of water. Thus, it is proposed that interstellar planetary bodies with extensive liquid water oceans may exist. It is further suggested that the bodies are likely to remain geologically active for long periods, providing a geodynamo-created protective magnetosphere and possible sea floor volcanism which could provide an energy source for life. The author admits these bodies will be difficult to detect due to the intrinsically weak thermal microwave radiation emissions emanating from the lower reaches of the atmosphere.

A study of simulated planet ejection scenarios has suggested that around five percent of Earth-sized planets with Moon-sized moons would retain their moons after ejection. A large moon would be a source of significant geological tidal heating.