A team of astronomers has identified 70 to 170 new rogue planets in decades of telescope archives, effectively doubling the cataloged free-floating worlds in our galaxy and hinting at billions more drifting through space.
Detecting exoplanets is challenging. Unlike stars, planets don't emit light, so astronomers rely on indirect methods like radial velocity or transits. These have confirmed over 4,000 exoplanets, but all orbit stars.
Rogue planets, also called orphans or wanderers, drift alone in interstellar space—ejected from their original systems or formed independently from molecular clouds. These starless worlds have been known since the 1990s, yet mysteries persist about their formation, sizes, compositions, and galactic abundance.
Spotting them typically involves gravitational microlensing, where a planet's gravity bends light from a distant background star, briefly brightening it as seen from Earth. These events are one-off occurrences, requiring precise alignment of the planet, star, and observer—limiting repeat observations.
This new study overcomes that hurdle.
The researchers hypothesized that young rogue planets, mere millions of years old, remain hot enough for direct infrared detection.
Published in Nature, the study analyzed nearly 20 years of data from the European Southern Observatory, Canada-France-Hawaii Telescope, Subaru Telescope, and ESA's Gaia—80,000 images totaling 100 terabytes across 170 square degrees of sky, far larger than prior surveys.
Focusing on optical and near-infrared wavelengths, they pinpointed 3,455 candidates, with 70 to 170 Jupiter-sized objects likely true rogues, most within 400 light-years of Earth.
Precise counts are elusive due to uncertainties in ages and masses.
“We derived individual masses by matching luminosities to models. Young planets glow brightly but fade quickly,” the authors explain. “Without precise ages—as in our sample—distinguishing an old massive planet from a younger lighter one is tough.”
Follow-up spectroscopy will refine ages, masses, temperatures, and compositions.
The findings exceed predictions for planets formed solely from isolated cloud collapse, implying many were ejected from systems. Combined mechanisms suggest rogues abound—a prior study pegged their galactic total at over 16 billion.
