“It’s possible that the Milky Way is partially settled, or intermittently so; maybe explorers visited us in the past, but we don’t remember, and they died out,” says Jonathan Carroll-Nellenback, an astronomer at the University of Rochester and his collaborators in a new study that suggests it wouldn’t take as long as thought for a space-faring civilization to planet-hop across the galaxy, because the orbits of stars can help distribute life, offering a new solution to the Fermi paradox. “The solar system may well be amid other settled systems; it’s just been unvisited for millions of years.”
Or, as Seth Shostak, an astronomer at the SETI Institute thinks the Fermi Paradox is likely to be explained by something more complex than distance and time, that is, perception: “The click beetles in my backyard don’t notice that they’re surrounded by intelligent beings — namely my neighbors and me, but we’re here, nonetheless.”
“The sun has been around the center of the Milky Way 50 times,” said Carroll-Nellenback and his collaborators Jason Wright at Penn State, Adam Frank of Rochester and Caleb Scharf of Columbia University. “Stellar motions alone would get you the spread of life on time scales much shorter than the age of the galaxy.”
According to research by Carroll-Nellenback and his colleagues, reports Rebecca Boyle in Quanta, their simulations show that natural variability will mean that some galaxies will be settled, but often not — solving Fermi’s quandary with a solution that avoids assumptions about the intent and motivation of any exo-civilization seeking to settle other planetary systems.
The team calculated the speed of an advancing settlement via probes of finite velocity and range to determine if the galaxy can become inhabited with space-faring civilizations on timescales shorter than its age. They included the effect of stellar motions on the long-term behavior of the settlement front which adds a diffusive component to its advance.
Their models showed that the Milky Way can be readily ‘filled-in’ with settled stellar systems under conservative assumptions about interstellar spacecraft velocities and launch rates. We then consider the question of the galactic steady-state achieved in terms of the fraction of settled planets. They calculated a range of parameters for which the galaxy supports a population of interstellar space-faring civilizations even though some potential star systems are uninhabited.
The results point to ways in which Earth might remain unvisited in the midst of an inhabited galaxy, breaking the
link between astrophysicist Michael Hart’s famous calculation that a single space-faring species could populate the galaxy within a few million years, and maybe even as quickly as 650,000 years and their absence, given the relative ease with which they should spread, which means they must not exist and that humans must, therefore, be the only technological civilization in the galaxy.