These lonely planets are believed to have originated in planetary systems similar to our Solar System but were later ejected for some reason. Contrary to the familiar image of planets orbiting around their host stars, a recent study suggests that these “orphaned” planetary worlds may outnumber stars in the galaxy at a ratio of 20:1. This indicates that the wandering planets in our galaxy are six times more numerous than planets orbiting their host stars.
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A Solitary Ice Planet in the Universe. Photo: NASA
“We estimate that our galaxy is home to 20 times more solitary planets than stars – which means there are potentially billions of wandering planets out there,” said David Bennett, a senior NASA scientist and the study’s author.
Essentially, exoplanets, which are planets outside our Solar System, are discovered through their impact on their host stars. However, solitary planets are located too far from their host stars, making them difficult to detect. One of the primary goals of NASA’s Nancy Grace Roman Space Telescope is to find these solitary worlds. Previous estimates indicated that the Roman Telescope, scheduled to become operational in May 2027, could detect around 50 Earth-sized solitary planets, but the latest figures show that this number may be much higher, possibly close to 400.
Since the discovery of the first exoplanet orbiting a Sun-like star in 1995, the number of exoplanets found has reached 5,000. Most of these planets are large objects orbiting their host stars.
Meanwhile, the research team revealed that solitary planets are often smaller or equal in size to Earth.
“We found that Earth-sized solitary planets are more common than larger planets. The difference in average mass between planets orbiting a star and those drifting freely is key to understanding the planet formation mechanism,” said Takahiro Sumi, a professor at Osaka University and the study’s author.
The chaotic nature of the planet formation process may explain how solitary planets begin their wandering journey in the galaxy. Smaller planets typically have weaker gravity to hold onto their host stars, making them more susceptible to being ejected and wandering alone in space.