Rogue planet found floating through our galaxy, scientists say

Astronomers have found a “rogue” planet floating through our galaxy, untethered to any sun.

The “free-floating” world is slightly smaller than Earth, making it the smallest such planet ever to be found.

But it could be one of many such rogue planets in the Milky Way, and scientists have suggested they may even outnumber the stars in our galaxy.

Such planets are impossible to see using traditional techniques: since by definition they do not orbit around a sun and so cannot be spotted by the dimming they cause when passing in front of stars, and they do not give off any radiation.

Instead, the scientists took advantage of an astronomical phenomenon known as "gravitational microlensing" to spot the planet, which revealed itself by bending the light of more distant stars.

The result was an effect that is much like a giant magnifying glass, which brightens light from a background "source" star to reveal the presence of the massive object.

Gravitational microlensing is only possible when an astronomer's telescope lies in almost perfect alignment with the observed object and the source star. That is rare: if astronomers were only looking at one star, they would only have one chance every million years to see it using such a technique.

Przemek Mroz, a postdoctoral scholar at the California Institute of Technology and a lead author of the study published in Astrophysical Journal Letters, said: "Chances of observing microlensing are extremely slim because three objects - source, lens and observer - must be nearly perfectly aligned."

The researchers examined data collected from microlensing surveys of the Galactic Bulge, which is the central part of the Milky Way.

They used the 1.3-metre Warsaw Telescope at Las Campanas Observatory in Chile to gather the data.

The researchers said the newly detected planet is the smallest rogue world ever found, with models indicating it may have a mass somewhere between Earth and Mars.

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

"We can rule out the planet having a star within about eight astronomical units."

Astronomers believe free-floating planets may have formed in the rotating discs of dense gas and dust around stars and were ejected from their parent planetary systems after gravitational interactions with other bodies.

They say studying these objects could enable astronomers to learn more about the turbulent past of planetary systems such as the Solar System.

An article describing the findings, 'A Terrestrial-Mass Rogue Planet Candidate Detected in the Shortest-Timescale Microlensing Event’, is published today in Astrophysical Journal Letters.

Additional reporting by Press Association