Mysterious cloud at solar system’s edge found to host strange ‘mini galaxy’

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The mysterious cloud of space rocks and debris at the edge of the solar system may have spiral arms, making it resemble a mini galaxy, a new study says.

The Oort cloud is a theoretical debris shell at the gravitational edge of the solar system whose origin could shed more light on the source of comets as well as the origin of our neighbouring planets.

It is estimated to exist 2,000 to 5,000 astronomical units away, with 1 AU being the distance between the Earth and the Sun.

The giant cloud of detritus is thought to be the source of strange comets and meteorites flying past Earth such as the mysterious cigar-shaped rock known as Oumuamua that was spotted in October 2017.

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Previous research has suggested that the Oort cloud contains remnants of the solar system's planets, which were formed over 4 billion years ago, but its exact shape and structure have been a mystery.

A new, yet-to-be peer-reviewed study suggests this cloud of debris may look like a mini galaxy with spiral arms.

Researchers used a supercomputer to model the structure of the Oort cloud based on the trajectories of comets as well as the gravitational forces within and beyond our solar system.

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They particularly tried to model the pull on objects at the edge of the solar system by the "galactic tide". This is the gravitational pull exerted by objects like stars and the monster black hole at the galaxy's centre on the Oort cloud's objects, but not on the solar system’s planets, which are pulled more strongly by our Sun.

When researchers used Nasa's Pleiades supercomputer to simulate these forces, it modelled the Oort cloud as containing an inner structure similar to the spiral arms of the Milky Way galaxy.

The spiral arms of the Oort cloud shown in the model stretched 15,000 AU end to end. “As the Galactic tide acts to decouple bodies from the scattered disk it creates a spiral structure in physical space that is roughly 15,000 AU in length,” the study says.

“The spiral is long-lived and persists in the inner Oort cloud to the present time,” it adds while noting that “direct observational detection of the Oort spiral is difficult”.