The Milky Way could be full of nomads

Artist's conception of a Jupiter-size rogue planet with the edge on disk of a spiral galaxy in the background. Image via the Wiki

In ancient times planets were easily distinguished from stars because they’d wander from one constellation to another. It was that feature that eventually led Tycho Brahe and Johannes Kepler to record exact positions — without the aide of telescopic lenses — giving birth to modern planetary astronomy. Now a new class of wandering planets has been proposed, and Kepler’s NASA namesake might play a role in learning about these mysterious objects too. Call them Nomads:

(– For now, characteristics of these foreign objects are still unknown; they could be icy bodies, similar to other objects found in the outer solar system, rocky like asteroids, or gas giants similar to the most massive planets in our solar system. Over the past several decades, astronomers have keenly hunted for planets outside our solar system. So far, the search has turned up more than 700 of these exoplanets. Almost all of these newfound worlds orbit stars, but last year, scientists found about a dozen planets with no discernible host star.

Planets could become nomads in one of two highly plausible ways: rogues could start out in traditional solar systems and be ejected through gravitational interaction between other planets — or in the case of multiple star systems, interaction with companion stars — or they could form directly, the way new stars and brown dwarfs are thought to accrete out of the interstellar gas and dust. The likelihood of the latter is unknown, but given our current understanding, that virtually every star system forms planets which interact with each other in chaotic ways, especially in the early stages, rogues are a statistical certainty.

What might they be like? Big, small, hot, icy, gaseous or solid? All of the above and more. Planets like Mars or large moons like Ganymede wouldn’t be changed much if they were ejected. If Mars was wrenched out of our solar system and sent sailing through the void, what little atmosphere it has would freeze out, but other than a light frosting it would appear much the same. Gas and ice giants like Jupiter or Uranus wouldn’t be much changed either. These planets generate considerable internal heat through contraction, rotational shear, and radioactive decay in their rocky-metal cores. They would remain in that state for a long, long time, longer than the universe is old in some cases, any retinue of moons they carried with them would remain much the same as well.

But planets like Earth or Venus, or super earths like those detected around nearby stars, would embark on a very different lifestyle as nomads. The thick atmospheres would freeze out. They’d be ice-covered balls with layers of frozen gases like CO2, nitrogen, oxygen,or methane etc., many meters thick piled on top. Perhaps a few pools of helium or neon rolling about languidly on the surface here and there when briefly stirred to life by dim starlight or distant hypernova. What’s interesting about the moons of nomadic giants or the frozen over terrestrial earth-like planets is there might be enough internal heat or tidal flexing to keep water liquid in pockets or layers beneath the insulating ice. And like the earlier post about Titan, where there’s water and energy, there could be life.

Imagine life on such a planet, floating untethered through interstellar space. If it ever evolved to complexity and intelligence those creatures would have their work cut out for them — if they ever wanted to become a space faring species anyway. First, they’d have to figure out their environment, understand that there was a huge universe beyond the icy roof overhead, then develop technology and fight their way out of miles of ice to put probes or “terronauts” on their own alien surface. Then they’d have to develop telescopes operating in what was for them a deadly surface environment, as well as invent space travel and achieve orbit. Where would they go from there? If they evolved on a world like Europa orbiting a giant with other moons, or a double planet like the earth and Luna, there might be useful resources and interesting places to visit well within reach. But for a moonless nomad, the nearest major body would be on average as far away as the nearest star. In some cases nomads would even be ejected from their galaxy, the nearest star could be millions of light-years away; talk about being alone in a vast emptiness!

But who knows? If such creatures ever did come to be, they might have other natural advantages to take up the slack. Long life spans for example, or the innate ability to hibernate for long periods. The latter is common enough here on earth across many taxa, it would be a huge boon for humans eyeing interplanetary jaunts and even interstellar voyages. Maybe there are creatures out there, wanderers by planetary fate, looking up at their crystal clear skies at distant stars and feeling sorry for any fellow life forms trapped in a single violent solar system, with no ability to peacefully slumber away the eons as alien spacecraft drifted between local star systems and other nomadic worlds.

Simulations of our early solar system and variants on it suggest that dozens of planets of varying size could have been ejected in the first few hundred millions years. There could be lots of these wandering worlds, more than all the traditional planets orbiting all the stars in our galaxy combined. The big problem with figuring out how common nomads might be or how they formed is, by definition, they’re extraordinarily dim. We’re just now barely able to detect earth-sized exosolar planets when they cause their local sun to wobble, or pass in front of it. Telescopes with the resolution to directly “see” distant planets orbiting other stars are on the drawing board. Nomads as large and hot as Jupiter might be detected by a godawful sensitive infra-red telescope. But for smaller, cool nomads the only way to conceivably see them would be if they crossed conveniently in front of distant stars or glowing gas, or the way we have detected signs of the dozen or so cited in the article, for mere moments when they pass through the gravitational focus of a distant star. Such events are rare indeed.

For now we have only our imagination, guided by planetary science and the dreams of science fiction writers, to explore the endless varieties of possible nomadic planets that could be swarming invisibly throughout the Milky Way, or maybe even just beyond the influence of our own sun.


  1. scotlyn says

    There’s nothing here to argue about, so I usually don’t comment, but I just want to let you know that I’m enjoying your ‘space’ posts very much.

  2. Aliasalpha says

    Then they’d have to develop telescopes operating in what was for them a deadly surface environment, as well as invent space travel and achieve orbit. Where would they go from there?

    With my studies of 1950s scifi B movies, my leading hypothesis is that they’d come directly to earth to steal our women!

    One interesting thought for the space travel ambitions of a species on a rogue planet would be propulsion, they’d need something powerful enough to outpace the travel speed of the planet for it to be worthwhile and they’d pretty much have to explore in a fairly limited area ahead or they’d spend most of their time trying to catch up to their home

  3. slc1 says

    Over at the Panda’s Thumb, a Swedish commenter named Larsson has suggested the possibility of Jupiter/Europa type systems of such unattached planets where a moon revolving around such a planet has a liquid water ocean under an ice layer, which is kept liquid due to the internal friction in the moon caused by the gravitational interaction with its planet. Depending on what is found on Europa when we get around to investigating it, this might well open up the possibilities of life far beyond star based planetary systems, since it appears that the number of such rogue planets may equal or exceed the number of stars in the universe.


Leave a Reply