Planets are like bacteria — they fill every ecological niche we can think of. They orbit big stars and little stars, young stars and old stars. Some even orbit dead stars. And some don’t orbit a star at all — they inhabit the cold and darkness of interstellar space.

Such nomads probably were born just like other planets — from disks of debris around young stars. But the environment around a young star is busy and crowded. Planets may pass so close to each other that their gravity whips both of them around. A planet may be hurled into the star — or into deep space.

Or a planet in a distant orbit may be kicked out by the gravity of a passing star. And yet another possibility is that planets may collide, knocking one or both of them out of the system like billiard balls.

Only a few of these “rogue” planets have been observed. So there’s no agreement on just how many there are.

A study released this year looked at “microlensing” events, in which the brightness of a star changes as it passes behind another object; the intervening object’s gravity acts like a lens, bending and amplifying the starlight. Based on the number of events likely caused by planets, the study concluded that billions of nomads could be scattered throughout the galaxy.

With no star to warm them, such worlds probably are lifeless. But it’s possible that internal heat sources could make some of them habitable — life-bearing worlds hiding in the darkness.

Script by Damond Benningfield