Astronomers discover smallest rogue planet to date


According to, a new study has found the smallest rogue planet ever recorded. 

Rogue planets are planets that have been ejected from their solar system by a more massive planet and roam the galaxy untethered to any star. Most of the rogue planets that have been discovered are anywhere from two to forty times the mass of Jupiter, Jupiter being approximately three hundred times the mass of Earth.

The discovery, reported in the Astrophysical Journal Letters on October 29th, could help prove a long-standing cosmic theory. According to the study authors, this could be evidence that rogue Earth-sized planets could be the most common objects in the galaxy. A postdoctoral scholar at the California Institute of Technology, and the Lead study author, Przemek Mroz, has said, “The odds of detecting such a low-mass object are extremely low, either we were very lucky, or such objects are very common in the Milky Way. They may be as common as stars.”

Rogue planets, by definition, have no star to illuminate them. This makes it hard to detect using traditional methods like tracking how much the sun wobbles due to the gravitational pull of the planet or watching the planet cross in front of their star. Detecting rogue planets involves a facet of Einstein’s general theory of relativity known as gravitational lensing. Through this, an object that travels in front of a light source will act as a lens and focus the light more intensely. 

“If a massive object passes between an Earth-based observer and a distant source of light, its gravity may deflect and focus light from the source, the observer will measure a short brightening of the source star.” The smaller the light-bending object is, the briefer the stars perceived brightening will be. A planet several times the mass of Jupiter might create a brightening effect that lasts a few days, while a planet roughly the size of Earth would only brighten the source star for a few hours, maybe less, according to the researchers of the study. 

The star used for this study, located roughly 27,000 light-years away in the densest part of the galaxy, brightened for a record time of just 42 minutes. Depending on how far away the rogue planet is from the source star, being impossible to calculate with current technology, is likely between one half and one Earth mass. In either case, it would be the lowest mass rogue planet ever detected. As technology improves so will the measurements, as the equipment used to detect this microlensing event is nearly 30 years old. 

If orphan planets of roughly Earth’s mass are indeed some of the most common objects in the galaxy, it shouldn’t be long before many more are discovered.



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