We tend to think of interstellar journeys as leaps into the void, leaving the security of one solar system to travel non-stop to another. But a number of alternatives exist, a fact that becomes clear when we ponder that our own cloud of comets — the Oort Cloud — is thought to extend a light year out and perhaps a good deal further. There may be ways, in other words, to take advantage of resources like comets and other icy objects for a good part of an interstellar trip. That scenario is not as dramatic as a starship journey, but it opens up possibilities.

Let’s say, for example, that we only manage to get up to about 1 percent of lightspeed (3000 kilometers per second) before we run into technical challenges that are at least temporarily insurmountable. Speeds like that take well over 400 years to get a payload to Centauri A and B, but they make movement between planets and out into the Kuiper Belt and Oort Cloud a straightforward proposition. A civilization content to create way-stations and take its time could establish habitats all along the way, its distant descendants reaching the next solar system.

The idea takes me back to the island-hopping of Polynesian cultures as they pushed ever deeper into the Pacific, which is sometimes invoked to describe a civilization expanding from star to star. But the ‘island-hopping’ may actually involve small, dark objects exploited step by step all the way across to the target star, a process that could take millennia. A space-faring culture at home in the dark outer regions emerges. All of this depends, of course, upon the resources available, but the Oort Cloud is thought to be vast, comprising perhaps trillions of icy and rocky objects, a supply of raw materials on which such a culture could thrive.

Nomads Between the Stars

Adam Crowl recently passed along a new paper that takes this idea to another level. Louis Strigari (Stanford University) and colleagues have been looking at unbound objects, free-floating planets formed either directly in the collapse of a molecular cloud or ejected due to gravitational interactions in a solar system. Right now we know little about such rogue planets — Strigari and team call them ‘nomads’ — but they are quite interesting from the interstellar expansion standpoint as they, too, could provide even more stepping stones to distant destinations. Moreover, they cannot be ruled out as worthwhile targets on their own, as the paper suggests:

The name “nomad” is invoked to include that allusion that there may be an accompanying “?ock,” either in the form of a system of moons (Debes & Sigurdsson 2007) or in its own ecosystem. Though an interstellar object might seem an especially inhospitable habitat, if one allows for internal radioactive or tectonic heating and the development of a thick atmosphere e?ective at trapping infrared heat (Stevenson 1999; Abbot & Switzer 2011), and recognizes that most life on Earth is bacterial and highly adaptive, then the idea that interstellar (and, given the prevalence of debris from major galaxy mergers, intergalactic) space is a vast ecosystem, exchanging mass through chips from rare direct collisions, is intriguing with obvious implications for the instigation of life on earth.

It’s a dizzying thought when you couple this with the paper’s estimates on the number of free-floating planetary objects. The authors estimate there may be up to 105 compact objects per main sequence star in the galaxy that are greater than the mass of Pluto. The mass function of the lowest-mass nomads is modeled from what we see in the Kuiper Belt and the distribution of diameters in KBOs, while at the higher end (corresponding to masses several times that of Jupiter), evidence exists that nomads in open clusters follow a smooth continuation of the brown dwarf mass function. Drawing in evidence from microlensing as well as direct imaging, the paper goes on to suggest a galaxy in which the space between the stars is well populated with objects of planetary mass, most relatively small but some larger than Jupiter.

The authors acknowledge that much uncertainty exists about the mass function as we move from larger to smaller nomads, which makes space-based observations critical for refining these estimates. One way to move forward is through a survey of the inner galaxy (the proposed Wide-Field Infrared Survey Telescope, or WFIRST, could be significant here), while large scale galaxy surveys like the Gaia mission and the Large Synoptic Survey Telescope (LSST) should be sensitive to nomads greater than Jupiter mass. Even Kepler may come into play, as any anomalous microlensing events it encounters could imply a high value for the number of nomads between the stars. From the paper:

…we note that an additional outcome of the observational approach discussed above, especially regarding the detection of short timescale microlensing events, is that upper limits may be set on the density of nomads. This could set very interesting constraints on the population of planetesimals in nascent planetary systems.

Indeed. If resources like these are available in quantity between the stars, then a pattern of slow expansion would make interstellar migration almost inevitable if humans (or their machine surrogates) can adapt to life in the outer Solar System and beyond. Propulsion is always a huge issue, but in this scenario we also focus on the ability to build and maintain habitats on distant objects, exploiting their raw materials and preparing for the next leap outwards. Long-haul technologies would surely arise from a culture capable of these things, but the possibility exists that interstellar travel will mean slow and steady outpost building before the target is reached.

The paper is Strigari et al., “Nomads of the Galaxy” (preprint).