Known as orphaned planets, rogue planets, or planets without parent stars, these “outliers” might be the most common type of planet overall.
Here in the Solar System, we can watch our star’s eight planets orbit with confidence, knowing full well that we’ve discovered at least the majority of round, orbit-clearing worlds around our Sun. But there’s a 4.5 billion year history that we can’t fully know from our vantage point today. All we can be certain of are which planets have survived until now.
What about the worlds that were formed around our Sun early on, and then ejected by some violent gravitational process?
What about the worlds that would have been planets had they only formed around a star, rather than in the abyss of interstellar space?
Over the past few years, we’ve begun to find these orphan planets — sometimes called rogue planets — in the spaces between stars. Based on what we know of stars, gravity, and cosmic evolution, we can make a ballpark estimate of the total number of planets in the Universe, and it likely outnumbers our stars by anywhere from a factor of 10 to 100,000. Space is full of planets, and most of them don’t even have stars.
