Unprecedented discovery of extra-galactic planets

Astrophysicists at the University of Oklahoma have discovered planets outside of our galaxy for the first time in history. Over the past two decades, it has been confirmed that planets are ubiquitous in the Milky Way galaxy. But up until now there has been no evidence as to whether they are also common in galaxies outside our own. Scientists have found it hard to test the hypothesis that extra-galactic planets are common; there are few methods to test whether planets exist elsewhere, and it is more difficult to separate extra-galactic planets from their host stars and galaxies.

The discovery of a population of planets ranging in mass from Earth’s Moon to Jupiter was achieved by Xinyu Dai, an assistant professor in the Homer L. Dodge Department of Physics and Astronomy at the University of Oklahoma, and his colleague Eduardo Guerras, a postdoctoral research student. This has significant implications for our understanding of the universe. Using data from NASA’s Chandra X-Ray Observatory, a telescope in space controlled by the Smithsonian Astrophysical Observatory, Dai and Guerras say it is likely that there are trillions of planets in distant galaxies.

Dai and Guerras say it is likely that there are trillions of planets in distant galaxies

In making their discovery, Dai and Guerras relied on the astronomical effect of microlensing- the only known method capable of detecting planets from a great distance – to determine the existence of bodies found some 3.8 billion light years away from Earth. Microlensing is an effect predicted by Einstein’s general theory of relativity. According to Einstein’s theory, light bends when pulled by the force of gravity. Light emanating from a very distant, bright object (such as a quasar – an active galactic nucleus with a swirling black hole) is seen to be bent by the gravity of an intermediary object (such as a star or black hole) when viewed from Earth. Because of the additional light being bent towards the observer, it can appear as though the star is brighter and the image is amplified. If the source is placed directly in line with the intermediary object when viewed from Earth, then this effect is multiplied. Light rays from the source pass on all sides of the intermediary object creating what is known as ‘Einstein’s Ring’.

In their paper published in The Astrophysical Journal Letters, Dai and Guerras reported that they had detected unusual line energy shifts in the light emitted from the quasar. According to the pair, the model that best explains these findings is that unbound planets, which do not orbit stars, are floating between the quasar’s stars. Dai explained,“this is the first time anyone has discovered planets outside our galaxy. These small planets are the best candidate for the signature we observed in this study using the microlensing technique.”

Microlensing is an effect predicted by Einstein’s general theory of relativity

The discovery has been greeted with general enthusiasm, although some experts have cautioned against taking the findings as read. David Bennett, a gravitational lensing expert at NASA, said that while the findings were interesting, there “might be ways to interpret the data that would not imply a large population of free floating planets in the lens galaxy.” These findings might instead indicate the presence of a type of star called a brown dwarf.