Astronomers discover smallest rogue planet in the galaxy

To observe the smallest rogue planet spotted to date, the astronomers used an astronomical phenomenon called gravitational microlensing. Jan Skowron/Astronomical Observatory of the University of Warsaw

The smallest rogue planet in the galaxy has been discovered by Polish astronomers.

Smaller than the Earth and with a mass of about three times that of Mars, the planet is located several thousand light years from the Sun.

Because free-floating planets emit virtually no radiation and - by definition - they do not orbit any host star, they cannot be discovered using traditional methods of astrophysical detection. 

Instead, the team of international astronomers led by those from the University of Warsaw’s OGLE team at the Astronomical Observatory used an astronomical phenomenon called gravitational microlensing.

With neither sunrises nor sunsets, and with no star to tether to, rogue planets travel in solitary arcs around the core of the Milky Way. Public domain

Microlensing comes from Einstein's theory of general relativity - a massive object (the lens) may bend the light of a bright background object (the source). The lens’s gravity acts as a huge magnifying glass which bends and magnifies the light of distant stars.

OGLE project leader, Professor Andrzej Udalski said: “Our current discovery confirms that low-mass rogue planets are common in the Milky Way, there may be billions of them, and that they can be detected and characterized by observations from the Earth's surface.”

He added: “Rogue planets emit almost no light and - by definition - do not orbit any star. Their detection using classical methods is practically impossible.”

Astronomers believe that rogue originally formed around stars in the same way as ‘normal’ planets do, but were then ejected from their parent planetary systems after gravitational interactions with other planets in the system. Jan Skowron /Astronomical Observatory of the University of Warsaw

Lead author of the study Dr. Przemysław Mróz from the California Institute of Technology said: “If there is a massive object between an observer on Earth and a distant source star, for example another star or planet, its gravity may bend and focus the source's light. 

“An observer on Earth will then see a short-term brightening of the distant source.

“The chance of observing microlensing phenomena is extremely small, because three objects (the source, the lens and the observer) must be almost perfectly aligned. 

Dr Przemysław Mróz said the chances of observing microlensing phenomena is extremely rare as three objects (the source, the lens and the observer) must be “almost perfectly aligned.”University of Warsaw

“If we were looking at only one source star, on average we would have to wait almost a million years to observe microlensing.

“That is why modern experiments looking for microlensing phenomena observe hundreds of millions of stars located in the centre of the Milky Way, where the probability of this phenomenon is greatest.”

He added: “The size of the gravitational microlenses caused by stars is usually much larger than the size of the source star. 

Professor Andrzej Udalski said: “Our current discovery confirms that low-mass rogue planets are common in the Milky Way, there may be billions of them, and that they can be detected and characterized by observations from the Earth's surface.”FNP

“The light of the entire disk of the source star is focused by the gravitational lens. It is different with planetary lensing. Only a small part of the star surface brightens.” 

Astronomers believe that free-floating or ‘lonely’ planets originally formed around stars in the same way as ‘normal’ planets do, but were then ejected from their parent planetary systems after gravitational interactions with other planets in the system. 

The study of lonely planets helps scientists understand the turbulent formation of young planetary systems, such as our solar system.