Not all planets have the luxury of orbiting a star. Some wander the galaxy in solitude, untethered and invisible, until now... Astronomers have measured the mass and distance of one of these rogue planets, offering a rare glimpse into the life of a cosmic drifter.
The newfound planet is roughly a fifth the mass of Jupiter and sits nearly 10,000 light-years from Earth, toward the heart of the Milky Way. Its size hints at an origin in a planetary system, likely ejected into interstellar space after a chaotic gravitational shuffle with a sibling planet or passing star.
Detecting such lonely worlds is no easy feat. Without a star to illuminate them, rogue planets are practically invisible. Astronomers spot them indirectly, through gravitational microlensing: when a planet passes in front of a distant star, its gravity bends and magnifies the starlight, creating a fleeting brightening. Determining the mass of the lensing object requires knowing its distance, a notoriously tricky problem when the planet has no host star to provide context.
This time, luck was on their side. Ground-based telescopes in Chile, South Africa, and Australia first caught the event on May 3, 2024, while the Gaia Space Telescope observed it six times over 16 hours. Gaia's vantage point, 1.5 million kilometers from Earth, offered a slightly different perspective, allowing scientists to triangulate the planet's distance much like our two eyes gauge depth. With distance in hand, they could finally calculate its mass: about 22 percent of Jupiter's.
The discovery demonstrates the power of coordinated observations, and researchers say it opens the door to studying more rogue planets in detail. The upcoming Nancy Grace Roman Space Telescope, with its ability to scan the sky 1,000 times faster than Hubble, could uncover countless more of these cosmic wanderers. And of course, if you want to learn more details, the research was published in the journal Science.