Rogue planets, planets that wander in interstellar space without orbiting a star have been theorised by astronomers for decades.
The first examples were observed by a team led by Astrophysicist Takahiro Sumi of Osaka University in Japan in 2011. It’s now thought that there could be billions or even trillions in our galaxy.
While they aren’t heated by an alien sun, it’s possible that they could retain some warmth from underground radioactivity or other processes
And now an Earth-sized rogue planet has been identified by researchers. The smallest object of its kind ever catalogued, the planet named OGLE-2016-BLG-1928 would have a gravitational field comparable to Earth’s.
Principal investigator Professor Andrzej Udalski said: “Our discovery demonstrates low-mass free-floating planets can be detected and characterised using ground-based telescopes.”
The Warsaw University team used a technique called gravitational microlensing to spot the earth-like object.
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.
OGLE-2016-BLG-1928, is the shortest-duration object 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 almost certainly 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.
The study was published in Astrophysical Journal Letters.