A few years ago, Polish astronomers at the University of Warsaw identified the first “rogue” planets in the Milky Way. These planets don’t orbit around any particular star and are unbound by gravity. The existence of these rogue planets is in accordance with theories of planetary formation, which predict that some very small planets will be ejected from their parent planetary system. Now, an international team led by the Polish astronomers has described the smallest rogue planet found to date.

Because they don’t orbit around a host star, rogue planets can’t be identified using conventional astronomical methods. To find this one, the researchers used a method called gravitational microlensing. When a large object like a planet passes close to a light source like a star, the planet acts as a “lens,” and its gravity can deflect and magnify the light. As a result, the brightness of the star will increase while they are in alignment. By studying the duration of the event and the shape of the light curve, researchers can determine the mass of the planet. This new rogue planet was the shortest microlensing event ever observed, with a time scale of only 42 minutes. The models developed by the scientists predict that this planet is even smaller than Earth, and close in size to Mars. 

Microlensing events are extremely rare, since they require exact alignment of a light source, a lens, and the telescope. The Optical Gravitational Lensing Experiment (OGLE) has been running for over 28 years, and involves tracking the brightness of hundreds of millions of stars in the Milky Way center. This new discovery is the first evidence that low-mass rogue planets can be identified using this technique. Studying these free-floating planets can help us understand how planetary systems evolved, and how they might change in the future. 

Lead author Przemek Mroz is a postdoctoral scholar at the California Institute of Technology. Dr. Radoslaw Poleski, a co-author, is a scientist at the University of Warsaw Astronomical Observatory. Professor Andrzej Udalski, also of the University of Warsaw, is the P.I. of the OGLE project. This work was complemented by additional observations by the Korea Microlensing Telescope Network. 

Managing Correspondent: Jaclyn Long

Original Article: A Terrestrial-mass Rogue Planet Candidate Detected in the Shortest-timescale Microlensing Event

Additional Press Articles: 

An Earth-sized rogue planet discovered in the Milky Way

Tiny rogue planet is the smallest free-floating exoplanet candidate yet

Astronomers find smallest ‘rogue planet’ in the Milky Way

Image Credit: © Jan Skowron/Astronomical Observatory, University of Warsaw

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