An Earth-sized rogue planet that could harbour life is roaming the Milky Way, scientists have revealed.
The discovery backs a theory our galaxy is teeming with potentially habitable alien worlds wandering the darkness of space.
They are the hardest to spot - but are the most likely to be fertile. Some will have liquid oceans beneath their icy crust.
The newly detected 'free-floating' planet is the smallest ever found. It dramatically widens the horizon in the search for extra-terrestrial life.
Principle investigator Professor Andrzej Udalski said: "Our discovery demonstrates low-mass free-floating planets can be detected and characterised using ground-based telescopes."
They are unattached to any sun - meaning they drift aimlessly. The Warsaw University team used a technique called gravitational microlensing.
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The way a distant star's light changed when it was distorted by another star's gravity betrayed the planet's presence. It results from Einstein's theory of general relativity.
By measuring the duration of an event - and shape of the light curve - they can estimate the mass of the lensing object.
Named OGLE-2016-BLG-1928, it is the shortest ever recorded - lasting just 42 minutes. This shows it was caused by a very small object.
To put it in perspective most typically occur over several days - and identify stars. Those attributed to other rogue planets have elapsed over a few hours.
Co-author Dr Radoslaw Poleski said: "When we first spotted this event, it was clear it must have been caused by an extremely tiny object."
Indeed, models of the event indicate it has a mass between that of Earth and Mars - and is a rogue planet.
Dr Poleski said: "If the planet were orbiting a star, we would detect its presence in the light curve of the event. We can rule out the planet having a star within about eight astronomical units."
An astronomical unit is the distance between the Earth and the Sun.
The OGLE (Optical Gravitational Lensing Experiment) astronomers provided the first evidence for a large population of rogue planets in the Milky Way a few years ago.
They used the four foot Warsaw Telescope located at Las Campanas Observatory in Chile.
Each clear night, they point it to the central bulge of the Milky Way to study hundreds of millions of stars - seeking those which change their brightness.
Rogue planets are believed to have formed in protoplanetary disks around stars before being ejected by gravitational interactions with other objects in the system.
Earlier this year a US team estimated there are more than 100 billion rogue planets orbiting the Milky Way alone.
Theories predict they will typically be smaller than Earth. They shed light on the turbulent past of the universe - including our own solar system.
The Nancy Grace Roman Space Telescope will search for them when it launches in 2025.
OGLE began operations nearly three decades ago - making it one of the largest and longest sky surveys.