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Astronomers believe they've detected a rogue world like no other: A tiny, free-floating planet, roughly the mass of Earth, gallivanting through the gut of the Milky Way. This discovery, reported in the Astrophysical Journal Letters, may mark the smallest rogue planet ever detected, and it could help prove a long-standing cosmic theory.

According to the study authors, this little world could be the first real evidence that free-floating, Earth-sized planets may be some of the most common objects in the galaxy, Brandon Specktor wrote for LiveScience. "The odds of detecting such a low-mass object are extremely low," lead study author Przemek Mroz, a postdoctoral scholar at the California Institute of Technology, told LiveScience in an email. "Either we were very lucky, or such objects are very common in the Milky Way. They may be as common as stars."

Most exoplanets in our galaxy are visible only because of their host stars. In a literal sense, stars provide the light that allows astronomers to directly observe alien worlds. When a planet is too small or too distant to be seen directly, scientists can still detect it from the slight gravitational pull it exerts on its host star (called the radial velocity method) or by the flickering that occurs when a planet passes in front of the star's Earth-facing side (the transit method).

In June 2016, they witnessed the shortest microlensing event ever seen using equipment at the University of Warsaw. The star in question, located roughly 27,000 light-years away in the densest part of the galaxy, brightened for just 42 minutes. Calculations showed that the offending object was not bound to any star within 8 astronomical units (AU, or eight times the average distance from Earth to the sun), suggesting it was almost certainly a tiny planet on the run, ejected from its home solar system after a brush with a much more massive object. Depending on how far away the planet is from the source star (it's impossible to tell with current technology), the rogue world is likely between one-half and one Earth mass. In either case, this roaming world would be the lowest-mass rogue planet ever detected. According to Mroz, that's a "huge milestone" for the science of planet formation.

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