The Rogue Planets That Wander the Galaxy Alone

Astronomers are searching for mysterious, free-floating worlds across the Milky Way. The Milky Way is home to hundreds of billions of stars, and many more planets. Some come in sets, as in our own solar system. But not every planet orbits a star.

Some planets actually wander the galaxy alone, untethered. They have no days or nights, and they exist in perpetual darkness. In a kitschy NASA collection of travel posters for destinations beyond Earth, one of these cold worlds is advertised with the motto: “Visit the planet with no star, where the nightlife never ends.”

Astronomers call these worlds free-floating, or rogue, planets. They are mysterious objects, and a small group of researchers around the world is dedicated to studying them. Of the thousands of planets that scientists have detected beyond our solar system so far, only about a dozen are sunless and coasting on their own, somewhere between us and the center of the Milky Way. At least, astronomers think they are. “We are sure that these objects are planets,” Przemek Mroz, an astronomer at Caltech, told me. “We are not fully sure whether these objects are freefloating or not.”

Mroz has spent perhaps as much time thinking about these strange objects as anyone on Earth. He and his team just announced another finding—the smallest known rogue planet— today. The object is between the masses of Earth and Mars, a blip in interstellar space so relatively tiny that it might seem insignificant. But according to scientists’ best theories about the way planetary systems arise all across the universe, rogue worlds should exist.

The term rogue planet suggests that these objects desert their stars on purpose, striking out on their own to carve a new path through the Milky Way. In reality, rogue planets are usually kicked out of their star system, banished to a solitary existence circling the center of the galaxy.

The beginnings of a planetary system, including our own, are thought to be quite messy. As planets swirl into shape out of the cosmic fog surrounding a newborn star, they jostle one another around. The gravitational game of pool can shove planets toward the edges of a system, and even eject them altogether. Nearby stars can scramble planets too. Most stars are not born alone, but in clusters of dozens to thousands, and in such a crowded environment, a passing star with its own entourage of planets could whisk away a planet from another, keeping it for itself or casting it out into space.