The Absolute Cosmos: Smallest rogue planet candidate spotted in Milky Way
Scientists have spotted the smallest rogue planet candidate ever been observed. This free floating planet not bound to any star is possibly smaller than our Earth.
The rogue planet candidate has a mass somewhere between Earth and Mars. It is just ten percent as hefty as Earth.
Currently, more than four thousand exoplanets have been found and many of them do not resemble with those in our solar system. Yet, they all have one thing in common- they all are in orbit around a star.
The existence of rogue planets floating in Milky Way has already been predicted by theories of solar system formation and evolution.
It is hard to directly observe exoplanets. Mostly, scientists use transit method to detect extrasolar planets. When a planet passes in front of its parent star, it causes a dip in its star light, this enables the scientists to perform various calculations.
Whereas, rogue or free floating planets effectively emit no radiation and do not orbit any star. So they can not be located using traditional methods of planet detection.
Instead, to detect a rogue planet scientists use an astronomical phenomenon called gravitational microlensing. It states that- any massive object (acting as lens) may bend the light from a bright background object (acting as source).
The gravity of lens acts as a massive magnifying glass that bends and enlarges the light from the distant stars.
When a massive object- a star or a planet, passes between an observer on Earth and a faraway source star, its gravity could deflect and focus light from the source.
To observe a microlensing all three objects- source, lens and observer need be perfectly aligned. This makes observing microlensing a rare phenomenon.
This is the reason why sky surveys looking for gravitational microlensing events are actively observing hundreds of millions of stars in our galactic centre- where the possibility of microlensing is highest.
The Optical Gravitational Lensing Experiment (OGLE) survey- one of the longest and largest sky survey carries out such experiment. OGLE started operations twenty eight years ago in 1992.
Gravitational microlensing enables the scientists to study faint/dark objects (such as planets), as it is not dependent on the lens’ brightness.
The time duration of these microlensing events rely on lensing object’s mass. The less massive the object (lens) is, the shorter the microlensing event is.
Microlensing events which typically last for several days are caused due to stars. Events linked with rogue planets lasts barely for few hours.
By observing the duration and shape of the event’s light curve, scientists can measure the mass of the lensing object.
The team reported the shortest timescale microlensing event till date, lasting for about only forty two minutes. The event has been named OGLE-2016-BLG-1928.
And since the time duration of the event was short, additional observations collected by KMNet (Korea Microlensing Telescope Network) were also used.
As soon as scientists spotted this event, they were pretty clear that it was caused by an extremely tiny object.
Models of the event suggest that the lens (object) possibly must have been as massive as Mars and is most likely a free floating (rogue) planet. If confirmed, this is will be the smallest rogue planet found till date.
The lens (object) is not orbiting any star, at least not within eight astronomical units. In fact, if it were orbiting a star, its presence could have been detected in the light curve of the event.
According to OGLE team, the study result shows that low mass rogue planets can be detected and characterized by using ground based observatories.
Scientists believe that these rogue planets were formed in protoplanetary disks around stars and later got ejected from their host planetary systems due to gravitational interactions with other objects in the system.