A Rogue Planet Discovered in The Milky Way

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Scientists have recently discovered a rogue planet in our Milky Way Galaxy, which is a rare find.


  • Rouge planets are gravitationally unbound to any stars. Detecting them is very difficult.
  • But, Polish astronomers from OGLE Survey have recently discovered a rogue planet in our Milky Way Galaxy.
  • This particular rogue planet is the first one detected to be of Earth size.
  • Till now, 4000 extrasolar planets have been discovered.
  • A common thing that underlines planets in our solar system and extrasolar planets are that they orbit a star.
  • But, planet formation and evolution theories predict free-floating or rogue planets.
  • These planets are gravitationally unattached to any star.

How Exoplanets are Discovered?

  • Exoplanets orbiting a star are generally discovered using the light shift in the planet’s host star.
  • When an exoplanet passes in front of its host star’s disk, the star’s light drops periodically in small amounts causing the so called transits.

Detecting Rogue Planets

  • Free-floating planets are hard to find using the transit method as they exhibit no radiation as do not have a host star.
  • Instead, astronomers detect rogue planets using an astronomical phenomenon called gravitational microlensing.
  • Microlensing emanates from Einstein’s theory of general relativity.
  • According to the theory, a mass object can bend the light of a background object. Here, the mass object is the lens and the background object can be a star.
  • When a mass object passes between an obeserver on earth and the distant star, the object acts as a magnifying lens where the gravity of the object deflects the light from the background star for a brief period resulting in brightening of the source star’s light.
  • According to astronomers, chances of observing microlensing is extremely rare as there must be perfect alignment between the observer, the lens and the source star.
  • Thus, when one is hunting for microlensing events, if only one source star is observed, one has to wait for another million years for microlensing to happen.
  • Thus, surveys such as OGLE survey observes millions of source stars at the centre of the  Milky Way Galaxy where microlensing events will be highest.
  • As microlensing does not depend on lens’ brightness both faint and dark objects such as planets can be detected.
  • The duration of microlensing depends on the mass of the lensing object – less massive objects will have shorter lensing duration.
  • Microlensing events associated with stars can be observed for a few days. But, lending events associated with free-floating planets can be observed only for a few hours.
  • Astronomers determine the mass of the object by measuring the duration of the lensing event and the shape of the light curve.

OGLE Survey

  • Polish astronomers of the OGLE survey are using a 1.3-meter Warsaw Telescope located at Las Campanas Observatory, Chile to observe microlensing events.
  • The rogue planet discovered has one of the shortest microlensing event recorded to date.
  • The event is named as OGLE-2016-BLG-1928 and the duration was 42 minutes.
  • This is not the first time the OGLE survey had detected a rogue planet.
  • A few years ago OGLE astronomers provided the first evidence for a large population of rogue stars in the Milky Way galaxy.
  • But, the new planet discovered is the smallest rogue planet discovered to date.
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