Mysterious Rogue Planets Could Outnumber Stars

Lonely “rogue planets” without a host star float around the void of space in surprising numbers.

What is a planet? The debate around what is and isn’t considered a planet has been so hotly contested that nearly two decades later the public is still split on the decision to reclassify Pluto from planet to dwarf planet.

The word planet came from the Greek word for “wanderer.” Before the advent of telescopes, the only visible difference between a planet and a star was that planets appeared to move across the sky at a different rate than the rest of the stars. Now we understand these objects are entirely different from stars, and that many other objects are taking up space as well.

The discovery of exoplanets and isolated Planetary-Mass objects (iPMOs) let us know that our solar system is not unique in its ability to host planets. While explaining exoplanets is fairly simple, discovering iPMOs, aka rogue planets, adds a new piece to the planetary puzzle. According to the IAU definition of a planet, an object must orbit a host star to be called a planet. Planet-sized objects free-floating around in space meet the common sense criteria of a planet (a large, round object that is not a star, brown dwarf, asteroid, or something else), just without the host…