Nomadic worlds, i.e., objects not gravitationally bound to any star(s), are
of great interest to planetary science and astrobiology. They have garnered
attention recently due to constraints derived from microlensing surveys and the
recent discovery of interstellar planetesimals. In this paper, we roughly
estimate the prevalence of nomadic worlds with radii of 100kmβ²Rβ²104km. The cumulative number density
n>β(>R) appears to follow a heuristic power law given by n>ββRβ3. Therefore, smaller objects are probably much more numerous
than larger rocky nomadic planets, and statistically more likely to have
members relatively close to the inner Solar system. Our results suggest that
tens to hundreds of planet-sized nomadic worlds might populate the spherical
volume centered on Earth and circumscribed by Proxima Centauri, and may thus
comprise closer interstellar targets than any planets bound to stars. For the
first time, we systematically analyze the feasibility of exploring these
unbounded objects via deep space missions. We investigate what near-future
propulsion systems could allow us to reach nomadic worlds of radius >R in a
50-year flight timescale. Objects with RβΌ100 km are within the purview
of multiple propulsion methods such as electric sails, laser electric
propulsion, and solar sails. In contrast, nomadic worlds with Rβ³1000
km are accessible by laser sails (and perhaps nuclear fusion), thereby
underscoring their vast potential for deep space exploration.Comment: 22 pages including "Highlights" page; accepted by Acta Astronautic