But there are some worlds that wander the Milky Way as solar exiles, catapulted from their native systems by interloping objects or cataclysmic events. Others are born in the interstellar medium without a parent star. These planets, with no sun of their own, go by many names—rogues, nomads, orphans—and there are estimated to be billions of them adrift in our Milky Way, sparking the imaginations of scientists and science fiction fans alike .

We live in an era when it is not unusual for exoplanets—worlds beyond our solar system—to be discovered in batches of hundreds . The vast majority of these planets are exposed to astronomers by their host stars, either when the planets transit in front of them, temporarily blocking stellar light, or when they exert an observable gravitational force on their stars.

Such free-floating places are shrouded in mystery relative to their star-anchored peers, because they lurk in the shadows of the galaxy, and leave few traces of their presence. But they are not entirely undetectable, as demonstrated by new research published on Monday in Nature.

Scientists led by Przemek Mróz, a PhD student at Warsaw University Observatory, analyzed the light curves of nearly 50 million stars observed between 2010 and 2015 by the Optical Gravitational Lensing Experiment (OGLE) at Las Campanas Observatory in Chile. The team flagged signs of microlensing events, which occur when the gravitational field of an object, such as a rogue planet, distorts and brightens a background light source as it passes in front of it, illustrated in the animation below.

Mróz and his colleagues winnowed down these five years of observation to 2,617 high-quality microlensing events, a significantly larger sample size than the same instrument's previous haul, published in 2011, which compiled 474 events.

There is likely a huge population of Earth-scale nomad worlds in the Milky Way

The duration of these microlensing events correlates to the mass of the object, with lensing of Earths and super-Earths lasting for only a few hours, Jupiter-mass worlds lasting one to two days, and stars lasting several days. Mróz's team statistically analyzed the distribution of these timescales, and found that six of the events were shorter than half a day, indicating they are likely planets that are roughly as massive as Earth. (Some of these suspected objects may be "wide-orbit planets," meaning that they are gravitationally bound to a star, but orbit at such an extreme distance that they appear indistinguishable from rogue objects.)