Astronomers Determine the Mass of a Solitary Rogue Planet Drifting in the Galaxy

A world the size of Saturn is careening through the vastness of space without a parent star, with astronomers having determined its mass for the first occasion. The planet’s mass is approximately 22 percent that of Jupiter and is located about 3,000 parsecs from the Milky Way’s core. Up until now, researchers had only sporadically observed these cosmic outcasts via a gravitational phenomenon known as microlensing, but had not managed to ascertain the true weight of these objects or their distance.

The advancement came from observing the same transient event from two distinct viewpoints. Ground-based telescopes in Chile monitored a far-off star as it illuminated, amplified by the gravitational influence of the rogue planet situated in front of it. Simultaneously, the Gaia spacecraft, stationed 1.5 million kilometers away at the L2 point, registered the same brightness surge about 1.9 hours later. This slight interval is crucial. It enabled scientists to compute the microlens parallax and eliminate the conventional mass-distance ambiguity that has hindered rogue planet investigations for years.

Bridging the Einstein Desert

This specific mass resides firmly within what astronomers refer to as the Einstein desert, an enigmatic void where few microlensing entities have appeared. Some researchers speculated that larger planets were less likely to be expelled from their original systems or that the smallest failed stars could not emerge at such minimal masses. The recent discovery, announced on January 1 in Science by Subo Dong and his team, demonstrates the existence of Saturn-sized rogue planets in numbers previously unnoticed.

“The recorded mass of KMT-2024-BLG-0792/OGLE-2024-BLG-0516 offers direct proof that the FFP events detected by microlensing surveys are due to extrasolar planetary-mass bodies,” explains Subo Dong.

Microlensing functions like a dynamic magnifying glass. When a massive entity passes in front of a background star, its gravitational field distorts light and temporarily enhances the star’s luminosity, even if the lens itself is completely shadowed. The challenge is that a solitary observation from Earth cannot determine if you are viewing a distant massive object or a lighter one that is much nearer. Gaia’s additional perspective made all the difference.

The dual observation was somewhat fortuitous. Achieving such an alignment necessitated the appropriate object to transit in front of the suitable star precisely when both Earth-based and Gaia observations were occurring. Dong’s team utilized data from the Korea Microlensing Telescope Network and the OGLE survey to model the star’s light curve, then integrated Gaia’s timing offset to accurately define the planet’s physical characteristics.

A Turbulent History

The planet’s mass is too low to align with theories that suggest such bodies form directly from collapsing gas clouds, as brown dwarfs do. Instead, the researchers propose that this planet likely originated within a protoplanetary disk around a star before being explosively expelled. Encounters with other planetary bodies or a stellar partner likely launched it into the abyss.

Each rogue planet serves as a quiet testament to tumult. Planetary systems may be significantly more volatile than previously believed, capable of ejecting even substantial worlds into interstellar isolation. Grasping the origins of these objects aids astronomers in understanding how solar systems develop and, at times, disintegrate.

As upcoming missions like the Nancy Grace Roman Space Telescope initiate systematic microlensing surveys, the number of recognized rogue planets is anticipated to rise sharply. Focused observations will probably discover even smaller wanderers, potentially Earth-sized orphans meandering in the darkness. For now, this Saturn-mass world serves as evidence that the galaxy contains far more solitary planets than are visible to the naked eye.

Science: 10.1126/science.adv9266