Thoughts on rogue planets and life

Planets could be ejected as the solar system formed

There's an article from 1999 from BBC on rogue planets here, that explains about how rogue planets may turn out to be a large source of life in the universe.

A rogue planet is a planet that doesn't orbit a star; it's a planet that was thrown out of its solar system a long time ago and wanders the galaxy forever. It's impossible at present to tell how many rogue planets there are, and we've yet to discover one simply for the fact that it's almost impossible. Most planets we discover now are found due to the pull they exert on the stars as they orbit them, or as they pass in front of their stars, creating an ever so slight dimming of the light that reaches us. Rogue planets don't have this; they don't create their own light, are small in size, and so far impossible to find.

In theory though, as the article states, there should be no reason why life couldn't exist on the right planet:

Mr Stevenson, of the California Institute of Technology, says the ejected planets would possibly carry with them an atmosphere of hydrogen. This could act to trap any internal heat. It may even exert enough pressure to maintain oceans of liquid water on the planet's surface.

With ammonia and methane gases in the atmosphere, and volcanoes bubbling heat and molten rock to the surface, these lonely planets would look remarkably similar to the Earth when life began here some four billion years ago.

Alternative energy

All these planets would lack is the energy supplied by a sun.

Writing in Nature, Mr Stevenson says: "If life can develop and be sustained without sunlight (but with other energy sources, plausibly volcanism or lightning in this instance), these bodies may provide a long-lived, stable environment for life (albeit one where the temperatures slowly decline on a billion-year timescale).

"The complexity and biomass may be low because the energy source will be small, but it is conceivable that these are the most common sites of life in the universe."

It's also good to remember that though these planets are completely dark from our point of view, they would see pretty much the same sky we do, just minus a sun and moon. It's easy to forget when you live in the city that the sky is actually full of stars, and that on a really dark night it can even look like this:

Click here to see the full-sized image.

So perhaps they wouldn't be all that lonely. In fact, we may seem to be a bit lonely out here on the edge of the Milky Way, so it's all a matter of comparison.

One big difference I could see though is in the propensity for a civilization to begin traveling in space. I don't think I could see a civilization making forays into space like we do except for LEO if there was no solar system to explore. For a rogue planet it's most likely that the nearest star would be some light years away, a journey of tens of thousands of years for a civilization at our level. It's possible that a rogue planet could have a moon or two of course, and that would spur space exploration.

Lastly, on the effects of a moon: I think in general planets with civilizations with moons would be quicker to commence space travel than those without, simply for the fact that it presents an interesting object in the sky all the time, and an easily achievable destination. In my opinion the best size and distance would be about 500 km in diameter, and a distance perhaps a third of ours, 100,000 km. A moon like Phobos or Deimos would be even easier to reach, but because they are so close and so small, their gravity is such that a person (any person, mind you, so any visitor to the moon could do this) standing on their surface could toss a rock up into the air that would break orbit and head toward the planet, in effect becoming a missile. Having a space race to the moon for us was a matter of national prestige, but if our moon happened to be a potential site for launching weapons below, who knows what sorts of problems that could have created.

In the meantime there are enough solar systems with possible Earth-like planets to discover, but I'm also really excited over the idea of finding the first rogue planet. Don't forget that for all we know there could even be one somewhere between us and Alpha Centauri.