By Chris Ciaccia | Fox News Scientists have discovered another rogue planet, but this one has even experts perplexed: It's slightly smaller than Earth and is floating through the Milky Way. The potential exoplanet likely has a mass similar to Mars, researchers said in the study published on the arXiv.org repository. Though rogue planets (those without stars) have been discovered before, they're extremely hard to spot. "Our discovery demonstrates that low-mass free-floating planets can be detected and characterized using ground-based telescopes," the study's co-author, Prof. Andrzej Udalski, principal investigator of the Optical Gravitational Lensing Experiment (OGLE) project, said in a statement. More than 4,000 exoplanets have been discovered by NASA in total, most using the transit method. Approximately 50 of them were believed to be potentially habitable as of September 2018, as they have the right size and the right orbit of their star to support surface water and, at least