Space Is Full Of Planets, And Most Of Them Don’t Even Have Stars
“When we look at our Universe, where our own galaxy contains some 400 billion stars and there are some two trillion galaxies in the Universe, the realization that there are around ten planets for every star is mind-boggling. But if we look outside of solar systems, there are between 100 and 100,000 planets wandering through space for every single star that we can see. While a small percentage of them were ejected from solar systems of their own, the overwhelming majority have never known the warmth of a star at all. Many are gas giants, but still more are likely to be rocky and icy, with many of them containing all the ingredients needed for life. Perhaps, someday, they’ll get their chance. Until then, they’ll continue to travel, throughout the galaxy and throughout the Universe, vastly outnumbering the dizzying array of lights illuminating the cosmos.”
According to the International Astronomical Union, planets need to have enough mass to pull themselves into hydrostatic equilibrium, they need to orbit a star and not any other object, and they need to clear their orbits in a certain amount of cosmic time. But what do you call an object that would have been a planet, if only it were in orbit around a star, but instead wanders through the heavens alone, unbound to any larger masses? These rogue planets are surprisingly ubiquitous in our galaxy and beyond, and we expect that they’ll far outnumber not only the stars, but even the planets that are found orbiting stars. Where do these rogue worlds come from? A percentage of them are orphans, having been ejected from the solar system that they formed in, but the overwhelming majority ought to have never been part of a star system at all.