Astronomers might have discovered the largest collection of rogue planets yet. These worlds are free-floating, thrown into interstellar space away from their stars. Combing through decades of observations has revealed at least 70 new Jupiter-sized objects – potentially up to 170, including candidate rogue planets.  

The planets, reported in Nature Astronomy, are located in the Upper Scorpius OB stellar association, a region of the Milky Way. The researchers looked through over 20 years of images of the association taken by telescopes all over the world. 

Rogue planets are spotted by microlensing. These worlds are usually too faint to be spotted directly – but when they pass in front of a distant star bending its light around them they become visible. This approach doesn’t allow follow-up observations, as it’s rare.  

In a stellar association such as Upper Scorpius OB, the planets are still young, so they are hot enough to glow and can be seen by our instruments. Over 80,000 observations were used by the team. They measured the light of all the members of the association in both optical and near-infrared wavelengths. They also combined that data with measurements of their motion. 

“We measured the tiny motions, the colors and luminosities of tens of millions of sources in a large area of the sky,” the study's first author, Núria Miret-Roig, from the University of Bordeaux in France, said in a statement. “These measurements allowed us to securely identify the faintest objects in this region.”

Finding this many worlds suggests that they didn't just form spontaneously, but were kicked out of their star systems by gravitational interactions with other planets. The team believes that this might be the tip of the iceberg when it comes to rogue planets in an association like this. It's easier for smaller worlds to become rogue planets than Jupiter-sized objects.

“The free-floating Jupiter-mass planets are the most difficult to eject, meaning that there might even be more free-floating Earth-mass planets wandering the galaxy,” explained Miret-Roig. 

Upcoming facilities, like the Vera C. Rubin Observatory and the Nancy Roman Space telescope, are expected to discover even more of these objects but this work underlines the role that established telescopes still play in new discoveries. 

“This project illustrates the incredible importance of providing access to archival data from different telescopes, not just throughout the US, but worldwide,” says Chris Davis, Program Officer at the National Science Foundation for NSF’s NOIRLab. “This is something NOIRLab and specifically the CSDC has been working hard to enable over a number of years, and continues to do so with support from NSF.”