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Rogue Planets -- What One Is and Why You Should Care

A rogue planet is a planet that has been ripped from its home solar system and hurled into intergalactic space. The scenario varies, but essentially something from outside the solar system comes along and knocks it out of its orbit, while imparting incredible velocity. The result for the hapless inhabitants is that their familiar sun slowly dwindles in the black sky, Survival is burrowing into the interior of the planet. Heat is life, cold is death.

Rogue planets can wander for millennia through the galaxy without ever encountering another star.

What does that mean for the science fiction writer? Think of it as a natural, really big generation ship, wandering forever through the vastness of space. If you believe that particular theory, you could think of it as a microcosm of the end of the universe: a planet slowly freezing to death as the stars recede. Eventually the atmosphere lies frozen on the planet's surface, the outside temperature slightly above absolute zero.

The good news is that you may have a ringside seat to some really great starscapes, if you're lucky....

What about the captive inhabitants, and they are captive. We are probably looking at an ultrastable civilization, long lived and very conservative -- literally. Energy is life. Whatever transuranium elements remaining in the planet's crust would be mined to power nuclear reactors. Of course, eventually all the fissionable material will be used up. Waste storage will become vital; fortunately nuclear waste emits large quantities of heat, some isotopes for thousands of years. But eventually even the nuclear waste will go cold.

Mining the thermal energy from the center of the planet may at some point become the only energy source. We can imagine the last alien survivor in the hollowed-out core of the planet huddled next to the last bit of cooling iron. (Don't forget the alien is weightless, being at the center of the planet.)

Pretty dismal scenario.

But what if there is another scenario? Imagine a rogue planet with a moon. Now things are different. We have a gravitation-powered thermal machine -- with planetary heat generated by tides.

Nobody had seen this scenario in practice until the close flyby of Pluto and Charon by the New Horizons probe. Planetary scientists couldn't believe their eyes. Instead of two frozen worlds, they saw a dwarf planet with recent surface activity. I would say "geological activity," but that would be geocentric of me. We need another term. Instantly a lot of questions were answered. Astronomers repeatedly overestimated Pluto's size because they thought it was darker than it really was. Like it or not, over time Pluto just shrank below planet size. Not until they discovered Pluto's moon Charon did they have an accurate mass estimate. But I digress.

What does this have to do with rogue planets you ask? Now instead of the planet slowly cooling as the radioactive core decays, the tides from the accompanying moon continually generate heat from friction. Because energy is constantly removed from the system the moon approaches the planet over the centuries. Eventually, the moon reaches a stable orbit when the interior of the planet cools down enough that there are no more tides, or breaks apart under gravitational forces and forms a ring. Same outcome, but after an immeasurably longer time.

Why should you care? We always pull for the victim, the underdog, the one who, through no fault of their own, gets the short end of the stick. After all, that's what science fiction is about, isn't it?

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