Hubble spots a rogue planet in the Milky Way using Einstein's theory

Rogue planets are free-floating planets that do not orbit a star and instead travel through space. (Image: NASA)

Stargazing occasionally provides serendipitous findings in the dark of night. That’s what happened when astronomers spotted a hidden planet using old Hubble data. It is the sole planet orbiting its lonely universe, which is billions of light-years away from any star. It was found with help from Albert Einstein’s theory of general relativity.

What Is a Rogue Planet and How Was It Discovered?
The lonely world, called a rogue planet, was not orbiting a star. It was found through a brief event recorded in 2023. The Optical Gravitational Lensing Experiment and Korea Microlensing Telescope Network first spotted it. Hubble then helped confirm the find.

The planet caused a microlensing effect, predicted by Einstein in 1915. When the object passed in front of a far-off star, its gravity warped and magnified the starlight. The whole process took a mere eight hours, making it one of the briefest ever recorded.

Einstein's Theory Uncovered the Rogue Planet
Einstein's theory posits that mass distorts space. This distortion can bend light from distant objects. Astronomers use this effect to find dark objects in space. Because rogue planets give off no light, they’re usually invisible. But microlensing makes them briefly visible by brightening background stars.

Researchers say the lensing object is likely a Neptune-mass planet. It could lie 15,000 light-years away in the galactic disk. Another theory suggests a larger Saturn-mass object further out. Both are possible, based on the data collected.

Why This Planet Matters and What Comes Next
Scientists are still checking if the planet is truly alone. If it had a nearby star, another microlensing signal would have appeared. But there was no second signal. This hints the planet may not have a host star nearby.

The team checked Hubble images from 1997 for more clues. They found no bright star near the object’s path. That means the planet, if not completely alone, may orbit a very dim star.

Even though the planet is tiny in the sky, future telescopes may separate it from the background star. But that will take years, as the two objects move slowly apart.

The microlensing event was found by chance, said Professor Przemek Mroz of the University of Warsaw. His colleague Mateusz Kapusta spotted it while scanning old data. The team believes many such hidden events remain in Hubble’s archives.

Their research is now available on arXiv, a site for early scientific papers. This discovery shows how long-term space observations can uncover distant, hidden worlds, even years after the data was first collected.