This NASA scheme explains the principles of gravitational microlensing by a black hole. A cluster of galaxies, of course, can serve as the lens that magnify light, too.

Image courtesy NASA

First, the Scary Stuff About Nomad Planets

Much of the world first learned of nomad planets in mid-2011, courtesy of a study authored by Takahiro Sumi, an astrophysicist at Osaka University in Japan. Sumi's team relied on gravitational microlensing, which is the same technique used to discover exoplanets across the galaxy. The technique relies on the light of a background source like a star being distorted as an object passes between the background star and Earth; in fact, that light is temporarily magnified. Sumi and his colleagues detected 10 planets with roughly Jupiter-sized mass that didn't seem to follow a recognizable orbit [source: Sumi]. The news lit up space junkies' Facebook walls and Twitter feeds faster than a "Next Generation" movie announcement.

Like we said, the theory is that these chronic meanderers may develop from the same clouds of gas and dust that stars are born from or solar systems may eject them from orbit. On a related note, solar systems could attract and trap nearby nomad planets into wide orbits. It's entirely possible that, at some time, our solar system had more planets than we currently know of. Perhaps shedding a few extraneous planets is what made it possible for Earth to find its Goldilocks position in relation to our sun.

As for the fear Earth could collide with a rogue planet, yes, there's a small chance that it could happen, but our galaxy is chock-full of smaller, more common and equally lethal threats (translation: asteroids) to worry about. Our chances of getting wiped out by a comet or an asteroid a few miles wide are much greater, although still relatively slim in the grand scheme of things [source: Mosher].

Sumi's original study estimated that there could be two or three nomad planets for every star in our galaxy. Another study less than a year later by the Kavli Institute at Stanford ratcheted the estimate way up, saying that there could be up to 100,000 nomad planets for every star in the Milky Way.

The key word in that last sentence is "could." The Kavli estimate was determined by factoring in things like the following [source: Freeberg]:

  • the known gravitational sway of the Milky Way
  • the amount of matter that's available to create nomad planets
  • the way that matter would arrange itself into nomad planets, which could be as small as Pluto or as large as Jupiter

If correct, that theory calls into question how planets are formed and what role nomad planets have in our galaxy. For now, let's work on what we do know and explore the possibilities of what might be.