What are Rogue planets?

When we look out into the night sky, we see a cosmos filled with stars. The distances between the stars is extraordinarily vast, which maybe makes you wonder what, if anything, exists between the stars. Amazingly, there are some interesting things that can be found in what looks like empty space. Perhaps the most interesting are rogue planets. 

Rogue planets are planets that do not orbit a star, but rather, find themselves drifting through the emptiness of interstellar space. How does a planet find itself in this position? During the formation of a solar system, a large number of planets will form in orbit around the star. Either through collisions with other planets or by a gravitational slingshot, a number of planets will be flung outside the solar system and into interstellar space. With no star, rogue planets are dark, frozen worlds existing in a state of eternal night. However, some may eventually find their way to another solar system, wherein they may be caught by the gravity of another star and find a new home in another solar system.

Not all rogue planets may actually be frozen. In fact, some models suggest that liquid water could even form on the surface of a sunless planet. How is that even possible? As it turns out, without a star, it’s more likely that a planet will maintain a thick atmosphere as there is no solar radiation to strip away lighter elements. A thick atmosphere combined with internal heat could supply the surface with enough heat that surface temperatures stay above the freezing point of water. If the rogue planet also happens to be geologically active, it is entirely possible that life itself could form. Perhaps any life forms on a rogue planet would be similar to organisms we find at the bottom of our oceans, where the lack of sunlight has led to the evolution of bioluminescence. If life has found a foothold on these dark worlds, it may just be the only form of light you could find. Image credit: NASA/ESA, Hubble

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