Astronomers have discovered a planetary free agent floating through the Milky Way, unbound to the gravity of any nearby stars. The discovery, detailed Thursday in Astrophysical Journal Letters, suggests the Milky Way may be teeming with rogue planets.

The new exoplanet, the smallest free-floating planet astronomers have found, is too small to be directly observed.

Researchers were able to spot the rogue world -- which is somewhere between the size of Mars and Earth -- with the assistance of a microlensing event.

"If a massive object -- a star or a planet -- passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source," lead study author Przemek Mroz, a postdoctoral scholar at the California Institute of Technology, said in a news release. "The observer will measure a short brightening of the source star."

"Chances of observing microlensing are extremely slim because three objects -- source, lens, and observer -- must be nearly perfectly aligned," Mroz said. "If we observed only one source star, we would have to wait almost a million years to see the source being microlensed."

To increase their odds of locating microlensing events, researchers rely on surveys. In this instance, astronomers utilized data collected by the OGLE survey, featuring a Chilean telescope and led by a team of astronomers at Warsaw University in Poland.

Every night, the OGLE survey's 1.3-meter Warsaw Telescope scans the Milky Way center, home to hundreds of millions of stars, looking for changes in stellar brightness.

Thankfully, microlensing detection doesn't depend on the lensing object's brightness. It's the lensing object's mass that matters. The more massive the lensing object, the longer the microlensing event lasts. The microlensing event caused by the recently discovered rogue planet lasted just a few hours.

Researchers were able to calculate the mass of the rogue planet by measuring the light curve and duration of the microlensing event.

"When we first spotted this event, it was clear that it must have been caused by an extremely tiny object," said study co-author Radoslaw Poleski, researcher at the Astronomical Observatory of the University of Warsaw.

Researchers were able to determine that the lensing object was likely less massive than Earth and about the same size as Mars. Astronomers were also able to determine that the object was without a host star.

"If the lens were orbiting a star, we would detect its presence in the light curve of the event," said Poleski. "We can rule out the planet having a star within about 8 astronomical units -- the astronomical unit is the distance between the Earth and the sun."

Since OGLE found the first rogue exoplanet several years ago, the survey has discovered several planetary free agents floating through the Milky Way, but the latest -- dubbed OGLE-2016-BLG-1928 -- is the smallest rogue planet scientists have found.

"Our discovery demonstrates that low-mass free-floating planets can be detected and characterized using ground-based telescopes," said Andrzej Udalski, principle investigator on the OGLE project.

Astronomers estimate rogue worlds were initially formed around stars inside protoplanetary disks, but were ejected by gravitational interactions with other young planets. By studying the size distribution of rogue planets, scientists expect to gain a greater understanding of planetary formation and evolution.

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