Could a Planet Exist Without a Host Star? Astronomers Say Rogue Worlds May Roam Freely

It is usually thought that planets are loyal companions of stars, held in place by gravity in fixed orbits, the way it is in our solar system. But astronomers say that planets can be on their own, not even tethered to a parent star, travelling through interstellar space in unusual and fantastic fashions. Some of these alien worlds take shape due to violent evictions or may orbit lifeless stars; the worlds could even be propelled to dizzying speeds by black holes.

On some of these rogue worlds, the idea goes, there might be oceans beneath icy crusts, where life, at least life at a microbe level, could exist.

Roaming Space at Extreme Speeds

As per a Harvard–Smithsonian report, so-called runaway planets can hit speeds of nearly 30 million miles per hour, becoming hypervelocity bodies akin to stars sent racing out of the galaxy from black hole encounters. In so doing, black holes could conceivably kick planets into the black chill of space.
Close-in gas giants can get the boot when other planets are ejected.

In a new study, researchers at the University of Chicago have discovered in simulations that a small body under half the size of the Earth would be able to cool a moon's subsurface ocean for more than a billion years without the help of the Sun.

The existence of an entire population of such rogue worlds makes a mess — but reveals a diversity among planetary systems and a normality of such systems also.Research suggests certain rogue planets could maintain hidden oceans beneath icy crusts for billions of years, offering potential habitats and challenging traditional views of planetary systems.