Supermassive black hole bursts
Artist's impression shows the atmosphere of a Neptune-like planet being swept backwards by powerful radiation from an outburst in the center of the Milky Way Galaxy M. Weiss/CfA

The centre of our Milky Way galaxy is filled with rocky alien worlds but how were these exoplanets formed? According to a new study, powerful outbursts from supermassive black holes may have converted Neptune-like exoplanets at the heart of the Milky Way into rocky super-Earths.

Most large galaxies are believed to have supermassive black holes residing at the centres, feeding on surrounding matter and in turn, producing bright flares of ultraviolet radiation and x-ray. Scientists examined the closest supermassive black hole to Earth – Sagittarius A* – which has a mass four million times that of the sun.

"We wondered what these outbursts from Sagittarius A* would do to any planets in its vicinity," John Forbes from the Harvard-Smithsonian Center for Astrophysics (CfA), a co-author of the new study, said in a statement. "Our work shows the black hole could dramatically change a planet's life."

The researchers found that, in the case of planets located around 70 light years away from Sagittarius A*, explosions can strip away a massive amount of planets' atmospheres, leaving behind a bare, rocky core.

"It's pretty wild to think of black holes shaping the evolutionary destiny of a planet, but that very well may be the case in the center of our Galaxy," Howard Chen of Northwestern University, the lead author of the new research, said.

Scientists believe that impact from such supermassive black hole outbursts may be one of the most common ways for super-Earths to form at the centre of the Milky Way.

"These super-Earths are one of the most common types of planet that astronomers have discovered outside our Solar System," said Avi Loeb of the CfA, co-author of the study.

Although it is possible that some of planets to be located within the habitable zone of stars, it would still be challenging for life forms to exist. Any life-supporting atmospheric qualities remaining would likely be destroyed by gamma-ray bursts and supernova explosions. Black hole bursts, when combined with these bombardments to such exoplanets, could end up completely eroding such super-Earths' atmosphere.

What is more, these exoplanets would also face the threat of being flung far away from its life-sustaining host star by the gravitational disruptions of a star flying by. In the case of Sagittarius A*, which is packed with planetary objects, such disruptions may be more common.

"It is generally accepted that the innermost regions of the Milky Way is not favorable for life. Indeed, even though the deck seems stacked against life in this region, the likelihood of panspermia, where life is transmitted via interplanetary or interstellar contact, would be much more common in such a dense environment," said Loeb. "This process might give life a fighting chance to arise and survive."

The findings of the new research has been published in The Astrophysical Journal Letters and is available online.