This Rogue Planet Wandering Through Milky Way May Be The Smallest Yet Discovered

It's quite tricky to find rogue planets but a team of researchers may have discovered the smallest known "free-floating" planet to date.

It has been about 25 years since the first exoplanet was discovered by scientists. Since then, thousands of others have been found, most of them discovered using the transit method or by the dips in the brightness of their host stars. But there is a type of planet that cannot be discovered with the transit method: rogue planets. This is because rogue planets are "free-floating" planets that do not have a host star.

In other words, these rogue planets are gravitationally "unattached" to a star and simply float in space on their own, quite unlike the planets in our solar system and many of the known exoplanets, a news release from the University of Warsaw said.

In the new study, published in Astrophysical Journal Letters on Thursday, a team of researchers reports their discovery of one such rogue planet in the Milky Way and it may just be the smallest Earth-sized rogue planet candidate to be discovered to date. The researchers believe it is about the size of Earth or Mars. They have also ruled out the possibility of it having a host star within the 8 astronomical units.

Rogue planets are believed to be quite common but, without a host star to observe their "dips", it is quite difficult to detect them.

So how did the researchers find this Earth-sized planet when they don't have a host star and practically don't emit any visible radiation? The team used a method called gravitational lensing using the data gathered by the Optical Gravitational Lensing Experiment (OGLE) project, which currently uses the 1.3-meter Warsaw telescope in Chile.

"Although in practice such objects do not emit any light, they may be detected using gravitational microlensing via their light-bending gravity," the researchers wrote.

A massive object such as a planet may be observed by the way it bends the light of a bright background object. This can be observed in the simulation video from the University of Warsaw.

"Chances of observing microlensing are extremely slim because three objects – source, lens, and observer – must be nearly perfectly aligned," study lead Przemek Mróz of California Institute of Technology said in the news release. "If we observed only one source star, we would have to wait almost a million year to see the source being microlensed."

Under projects such as OGLE, however, millions of stars in the Milky Way are being surveyed, increasing the chances of such discoveries.

"Our discovery demonstrates that terrestrial-mass free-floating planets can be detected and characterized using microlensing," the researchers wrote.

So where did these rogue planets come from? Astronomers believe they may have formed within the systems around stars but are eventually booted out, possibly due to their interactions with other bodies in the system.

It's also possible that they were already isolated when they formed from clouds of dust and gas, NASA said in an earlier feature about a study, which detailed how the Nancy Grace Roman Space Telescope (Roman Space Telescope) could potentially reveal a "myriad" of rogue planets.

The Roman Space Telescope is set to begin operations within the 2020s, and one of its goals is to conduct a large-scale microlensing survey.

Through such surveys, whether ground- or space-based, scientists can figure out just how abundant such rogue planets actually are and also have a better picture of planetary formation in the galaxy.