I got to thinking about all the planets in the galaxy that we know about (or are about to discover). Are there weird ones that aren’t quite in the same category as the Earth-types, hot Jupiters, super Neptunes, etc?

Well, sure. There are rogue planets. They’re little worlds (usually smaller than Earth) that, end up wandering the galaxy instead of marching around stars.

For one reason or another, these “rogue planets” escaped their systems at some point during the formation process. It happens as worlds are jostled around in the disk of material from which they form. Most of the time, the jostling results in a reshuffling of worlds. This is likely what happened with our gas giant planets early in the solar system’s birth process. Gravitational interactions basically push things around. A planet born in one area moves to a more distant orbit. Or, in the case of a rogue planet, it is ejected from the system altogether. In some cases, they have never been formally gravitationally bound to a star. No matter their origins, these lonely rogues are doomed to wander the galaxy forever.

Finding Rogue Planets

How do these planets get detected? About 4,000 know rogues exist in the galaxy. But, they’re not easy to see. Most are thought to be smaller than Earth. Since they’re not around stars, they don’t reflect light. But, their presence is detectable when their paths take them between us and a star. If the alignment is just right, they block a little light from the star. That short-term dimming during such a transit can be detected. (Measuring these short-term brightness flickers is, in fact, what the Kepler Space Telescope did, before it went out of commission.)

A long-term survey of stars, called OGLE (which the Optical Gravitational Lensing Experiment), spots such transits for lensing events. Lensing happens when a massive object, such as a star passes between Earth and a more distant object. The light from the more distant one gets ‘lensed’ by the gravitational influence of the larger one. That changes the brightness of the more distant object a bit.

The lensing objects don’t have to be stars. A planetary system can be involved. So can a rogue planet. And OGLE has been studying thousands of stars to find these microlensing events. In some cases, rogue planets are involved.

In the future, other telescopes, such as the upcoming Nancy Grace Roman Space Telescope will be able to measure these things from space. On Earth, a few ground-based observatories can also do broad surveys to look for microlensing, and thus find more rogue planets.

Could These “Free-floating” Planets Support Life?

The hunt for any planets in the galaxy (and beyond) has, at its heart, the search for other places where life could exist. Most astrobiologists agree that the places with the BEST chances for life are on planets orbiting stars. In particular, they should be in or very near the habitable zone around the star. That’s where liquid water could exist on a planetary surface. You also need light and warmth (which the star provides), and sources of food.

I really doubt that a rogue planet is a very friendly habitat for life. For one thing, there’s no warmth, and may not be any atmosphere or liquid water on its surface. Plus, since it got flung out from its star early in its evolution, it probably didn’t have time to entertain the formation of life.

Astronomers estimate there could be billions of these lonely worlds, floating between stars. So, with such a large number, there could be variations on the rogue planet theme. That’s why I’d never say “never” to any life. It’s possible, I suppose, that some form of microbial life could have gotten started before the planet was kicked out. So, there might be evidence of that.

Other Uses for Rogue Planets

I could certainly think of plenty of sci-fi scenarios where rogue planets could be useful in a galactic empire sense. I mean,what a great place (if you had the technology to do so) to stash a rebel base. Or a secret hideout. The possibilities are endless. But, any techno-savvy group is going to have to provide power, technology, and livable spaces for these places. That’s going to cost a lot. And, I would have to wonder about the return on investment. Still, it’s an intriguing idea, and until we get more data on these worlds, it’s fun to imagine such alternate timeline stories about them.