The galaxy – and presumably, if we’re in a normal-enough galaxy, the rest of the universe – has a bit less empty space than we thought, according to a study by the Kavli Institute for Particle Astrophysics (KIPAC).

The research suggests the Milky Way could hold many millions of “nomad planets” – many thousands existing for each main-sequence star in the galaxy.

KIPAC – a joint institute of Stanford University and America’s SLAC National Accelerator Laboratory – is suggesting that nomad planets could outnumber stars in the Milky Way, by as much as 100,000:1.

Over recent years, gravitational microlensing (the refocusing of a star’s light by planets passing “in front” of them) has provided new insights into planets that don’t appear to be bound to stars.

Louis Strigari, leader of the team that reported the results to the Royal Astronomical Society, suggests that some of the planetary orphans could even retain enough heat for microbial life – if they have enough tectonic and radioactive processes to generate heat and enough atmosphere to retain it.

Most of the 500-odd exoplanets discovered in the past two decades orbit stars, but last year, astronomers turned up a dozen nomad planets that don’t orbit stars. Some may have been ejected from the solar systems in which they formed; others may need to be explained with new theories of planetary formation.

However, Strigari believes known values like the mass of the galaxy (estimated from its gravitational pull) mean it could be “teeming” with nomad planets. As new instruments like the ground-based Large Synoptic Survey Telescope and the space-based Wide-Field Infrared Survey Telescope, both of which will come into service during the 2020s, will provide the kind of observations that will drive new estimates of the number of nomads, Strigari’s paper states.

Strigari also speculates that nomad planets, if they are able to harbor life, could also provide a vector for scattering it through the galaxy. He is, however, clear that such ideas are speculative. ®