So planets are those lumpy things that orbit stars, right? Well, if all of the recent nonsense about definitions is to be taken seriously. That’s why there are ridiculous terms like “planetar” and “planemo” being thrown about.
As far as I’m concerned though, if it’s round and smaller than a star, it’s a planet. But planets get flung about quite wildly as they form. Statistically, a few must get thrown out of their parent star system altogether. You’d think they’d just end up lost in the voids of interstellar space, frozen, dead and alone… You’d think.
Dave Stevenson of the California Institute of Technology decided in 1998 that he wasn’t so sure. Submitting a paper to nature entitled “Life-sustaining planets in interstellar space?”. (If you don’t have a Nature subscription, you can download a copy of the original here — it was actually submitted on my birthday!)
With no stellar wind to ablate them all away, he hypothesised that these “rogue planets” could hold on to thick atmospheres of hydrogen gas. Due to the insulation provided by this hydrogen atmosphere, he even hypothesised that these planets could retain enough surface warmth to have liquid water. They could potentially remain hot for some time, courtesy of geothermal energy and radioactive isotopes decaying. In turn, should one of these planets retain a moon (and apparently, around 5% of ejected Earth-sized planets could), tidal heating could also be a contributory factor.
It sounds far fetched, but his theory looks pretty sound to me. Dare I say it, rogue planets have long been a mainstay of science fiction (by Arthur C Clarke amongst others), but it’s almost certain that there must be at least a few out there. By probability alone, I put it to you that the Milky Way is large enough that if it’s possible, it has probably happened. In turn, by basic celestial mechanics, conservation of momentum causes the smallest member of a three body system to be ejected readily. With the right trajectory, a gravitational slingshot could easily set such a planet on a course for interstellar space. Just like the Voyager probes.
It’s a lovely idea, if nothing else. As always, it’s enough to make you wonder what might be drifting silently out there waiting to be discovered…