An international collaboration of astronomers have discovered a peculiar signature in space. They believe they have discovered an Earth-sized rogue planet, a world untethered to any stars and floating freely in the Milky Way.

The detection is reported in The Astrophysical Journal Letters. The team observed a microlensing event, which is when the light of a distant star is distorted and brightened by the gravity of another object. The object in question appears to be a planet between the size of Mars and Earth.

The team could not gauge how far away the object is from us, so they couldn't confirm whether or not the planet is free-floating or orbits quite far from its star. If object OGLE-2016-BLG-1928, as it is known, is a rogue planet, then it would be the smallest candidate observed so far.

“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. The observer will measure a short brightening of the source star,” lead author Dr Przemek Mróz, from the California Institute of Technology, said in a statement. “Chances of observing microlensing are extremely slim because three objects – source, lens, and observer – must be nearly perfectly aligned. If we observed only one source star, we would have to wait almost a million years to see the source being microlensed."

Since the odds are very much against us, the solution is to observe many stars at once. The OGLE survey has been running for 28 years and monitors hundreds of millions of stars at the center of the Milky Way. Astronomers look for small variations in starlight that usually only last for a few hours. The length of the event is proportional to the lensing object’s mass, so the team was excited to witness a microlensing event lasting just 42 minutes.

“When we first spotted this event, it was clear that it must have been caused by an extremely tiny object,” added Dr Radosław Poleski from the Astronomical Observatory of the University of Warsaw.

Rogue planets are extremely difficult to study. They are tough to find because they tend to be very cold, so they do not appear bright enough in infrared. However, astronomers are undeterred by the challenged. One of the science goals of the upcoming Nancy Grace Roman Space Telescope is to search for rogue planets.