Planets could be thrown out of solar system after nearby galaxy collides with Milky Way

The fate of Earth and its neighbouring planets seems grim - but humanity still has billions of years until catastrophe strikes

A rendering of the predicted merger between the Milky Way and the Andromeda galaxy. Nasa

Our solar system is expected to face a dramatic transformation because of an impending galactic event that will see a neighbouring galaxy collide with the Milky Way, potentially throwing planets into interplanetary space.

Life on Earth, however, will likely come to an end billions of years before that as the Sun, our life-giving star, becomes too hot for any living organism to survive.

Humans still have plenty of time, though, as these apocalyptic events are projected to unfold over the next five billion years.

Pauline Barmby, chairwoman of the department of physics and astronomy at the Western University in Ontario, Canada, has described what could be left of our solar system once the Andromeda galaxy eventually merges with it.

“A collision between the Milky Way and Andromeda, if it happens, won’t occur all at once,” she told The National.

“Most models of the galaxies’ orbits have them coming in for a close approach some time between five and eight billion years from now, separating for a few billion years, and then finally coalescing.”

The detailed fate of the solar system is not known with certainty, however, because the current orbits of the galaxies are difficult to measure precisely, according to Ms Barmby.

She said the part of the solar system that would most likely be affected by the Milky Way-Andromeda collision would be the Oort Cloud – a distant region of icy objects and comets surrounding the outer edges of the solar system.

“While nearby stars aren’t likely to themselves collide due to the galaxy collision, their orbits will change, and some will likely come near enough that they gravitationally perturb the Oort Cloud’s comets and eject some of them from the solar system,” said Ms Barmby.

Galaxy mergers can take place over hundreds of millions of years as gravitational forces slowly draw them together, distorting their shape and causing star formation, with the full merger process completing over an extended period.

The Sun holds the key

However, it is the dying Sun that could do the most damage to Earth and its neighbouring planets years before the merger.

It is expected to expand into a red giant mass – when it runs out of hydrogen fuel and swells to many times its original size – destroying Mercury and Venus in the process.

“Mars and the other outer planets are expected to survive, although their orbits will expand,” said Ms Barmby.

“Earth’s fate is unclear, because it’s difficult to predict exactly how much mass the Sun will lose in its red giant phase. If Earth is not destroyed, it certainly won’t be habitable.”

The remaining planets might not last much longer either, as they could be thrown out of the solar system entirely after the galaxy collision, when stars passing near the Sun cause gravitational influences between them.

"Over the very long term, 10 to 100 billion years from now, gravitational perturbations from stars passing near the Sun will cause gravitational interactions between the remaining planets - eventually they too will likely be ejected from the solar system," said Ms Barmby.

Dr Nidhal Guessoum, professor of physics and astronomy at the American University of Sharjah, echoed Ms Barmby’s thoughts and said that life on Earth “will have disappeared” well before the merger.

“As the Sun’s power continues to slowly increase it would make life on Earth impossible in about half a billion years from now,” he said.

While it may be a while until our galaxy merges with another, collisions in the universe are quite common, according to Dr Guessoum.

“We have a number of well-known and well-imaged cases of merged, distorted galaxies," she said. “Such mergers, which are due to gravitational pulls between galaxies, often lead to new stars being formed - and most likely new planets around them - by compression of galactic gas and dust.”

Some of the most well-documented galaxy mergers in the universe include the Antennae Galaxies, labelled as NGC 4038/NGC 4039), where two spiral galaxies are merging, creating tidal tails of stars and intense star formation known as a ‘starburst’.

Another example is the Cartwheel Galaxy, formed when a smaller galaxy passes through a larger one, producing a ring structure and triggering waves of star formation​.