

People used to argue about whether or not planets like the eight (or more) in our solar system were rare. Starting in the 1990s with the discovery of the first exoplanets, it became clear planets are common around other stars. What about planets without stars? Astronomers have identified a handful of such planets, but a new simulation developed at the University of Leiden suggests there could be as many as 50 billion rogue planets in the Milky Way.

In the immortal words of Douglas Adams: “Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.” Rogue planets, even the largest among them, are but tiny specs floating in the infinite cosmic void without a star to point the way. That we’ve spotted any of them is a minor miracle, but the technology doesn’t exist to conduct an accurate survey of rogue planets. Thus, the importance of the new simulation.

The team built a simulation of 1,500 stars in a region of space called Orion Trapezium. Of course, we don’t know how many planets really exist around these stars, but the model included between four and six planets in orbit around about 500 of those planets. That’s a total of 2,522 planets in the model.

Over the course of millions of simulated years, gravitational interactions between the stars kicked more than 350 of those planets out of their solar systems. That works out to approximately 14 percent of all the planets in the model becoming rogue planets.


We don’t know how many planets exist in the galaxy, but there are about 200 billion stars. Most of them are in clusters not unlike Orion Trapezium. Estimating even a modest number of planets on average, that could mean billions of rogue planets in the Milky Way. The team used a number of guesstimates to arrive at 50 billion. Some of those might even have come from our own solar system.

Most of the confirmed or suspected rogue planets we’ve spotted are under 100 light years away, and several of those are too faint to characterize beyond the most basic details. If there are anywhere close to 50 billion planets without a star, it’s likely astronomers will discover more of them that are close enough to study with instruments like the upcoming James Webb Space Telescope.

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