Astronomers discover the smallest rogue planet yet zipping through the Milky Way
Shane McGlaun - Oct 30, 2020

The term rogue planet is applied to planets that do not orbit a star. These planets fly unaccompanied through the galaxy, and astronomers have announced that they have spotted the smallest rogue planet candidate yet. The belief that the planet could be smaller than the Earth, with a mass somewhere between that of our world and Mars.

If the rogue planet can be confirmed, it would mark a milestone in the study of rogue planets believed to be common throughout the galaxy and beyond but challenging to detect. Andrzej Udalski, the co-author of the survey of the rogue planet, says that the discovery demonstrates low-mass free-floating planets can be detected and characterized via ground-based telescopes. So far, astronomers have discovered more than 4000 exoplanets, and most of them were found using the “transit method.”

To discover exoplanets, astronomers also use another technique called gravitational microlensing. In that technique, they watch foreground objects pass in front of a distant background star. When planets bend and magnify the starlight, it can reveal the foreground object’s mass along with other characteristics. The challenge of that technique is that the alignment of the source, lens, and observer has to be nearly perfect.

Since it’s extremely difficult to use gravitational microlensing looking at one star, astronomers observe vast swaths of the sky using an instrument called OGLE and a 1.3-meter telescope at the Las Campanas Observatory in Chile to monitor millions of stars in the center of the Milky Way. Using that technique, researchers discovered an event called OGLE-2016-BLG-1928 that lasted 42 minutes. It was the shortest microlensing event ever detected, and the researcher says it was clear that a tiny object must’ve caused it.

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Calculations suggested the object was between the mass of Mars and the Earth and probably closer to the size of Mars. It was determined that the candidate was likely a rogue planet because if it were orbiting a star, researchers would detect its presence in the light curve of the event. Scientists were able to rule out the planet having a star within eight astronomical units. Researchers think this type of rogue planet is probably common and believe that