Astronomers have discovered two new rogue planets—worlds that do not orbit stars—according to a study published in the preprint journal arXiv.

Przemek Mróz, a PhD student at the Astronomical Observatory of the University of Warsaw, led the research, which pinpointed two freefloating planets. They are named after the Optical Gravitational Lensing Experiment (OGLE), an astronomical survey operated from Las Campanas Observatory in Chile, which flags the gravitational fingerprints of rogue planets.

OGLE-2017-BLG-0560 was detected on April 16, 2017, and confirmed through follow-up observations from several other observatories. This planet could be anywhere between one to 20 times the mass of Jupiter—it’s hard to produce an exact estimate because the planet’s distance relative to Earth is not known.

Encouraged by the detection of this object in 2017, the team reexamined OGLE observations from previous years. That’s how they found OGLE-2012-BLG-1323, which was overlooked when it was first captured on August 21, 2012. Mróz and his colleagues estimate that this rogue planet is somewhere between Earth and Neptune in scale, making it one of the smallest starless worlds yet found.

Flung from their native star systems by gravitational encounters with other objects, rogue planets dwell in the cosmic dark. As a result, they are far more challenging to spot than planets illuminated by their parent stars. Where thousands of exoplanets have been detected within star systems, only about a dozen rogue planets have been discovered.

The vast majority of exoplanets are found when they pass in front of their host stars, causing a temporary dip in starlight that can be detected by astronomers.

Rogue planets have no star to cross, but they can produce a different light signature with a phenomenon called gravitational microlensing. When a foreground object, such as a rogue planet, passes in front of a distant light source, such as a star or galaxy, the gravitational field of the foreground object can distort the background light. This effect enables astronomers to spot the unique signature, and even make inferences about the mass and size of the object.

Read More: Astronomers Found a Rogue Planet 12 Times the Mass of Jupiter

The OGLE survey will continue to search for microlensing events that expose unbound worlds, which inform estimates of the broader rogue planet population. Starless planets may be even more common than stars in the Milky Way, according to Mróz’s team. That means there could be billions of orphan worlds drifting through the interstellar wilds of the galaxy—including, perhaps, lost siblings from our own solar system.