First interstellar Solar System visitor discovered

An asteroid has been discovered that is the first known interstellar visitor to our Solar System.

An image of star cluster NGC 3293, taken by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. Our Sun formed in a tightly-packed cluster like this, where asteroid exchange can occur between star systems.
Credit: ESO/G. Beccari


Astronomers have discovered the first known permanent 'immigrant' to the Solar System.

The cosmic visitor is an asteroid currently situated in Jupiter’s orbit, and has been a resident for a long time.

In 2017, astronomers announced the detection of the object ‘Oumuamua, a body that had reached our Solar System from interstellar space, but that was a more recent visitor.

This latest body, called (514107) 2015 BZ509, has been in our Solar System for much longer.

It orbits the Sun in the opposite direction to the planets in the Solar System.

“How the asteroid came to move in this way while sharing Jupiter’s orbit has until now been a mystery,” says Dr Fathi Namouni, lead author of the study. 

“If 2015 BZ509 were a native of our system, it should have had the same original direction as all of the other planets and asteroids, inherited from the cloud of gas and dust that formed them.”


​Images of 2015 BZ509 obtained at the Large Binocular Telescope Observatory (LBTO) that established its retrograde co-orbital nature. The asteroid is circled in yellow.
Credit: C. Veillet / Large Binocular Telescope Observatory


The team behind this study used computer simulations to trace the location of 2015 BZ509 back to the birth of the Solar System, 4.5 billion years ago.

They found that the asteroid has always moved in a retrograde orbit, and concluded that it must have been captured from another system.

“Asteroid immigration from other star systems occurs because the Sun initially formed in a tightly-packed star cluster, where every star had its own system of planets and asteroids,” says Dr Helena Morais, another member of the research team.

The team hope to learn more about how and when the asteroid came to our Solar System, as this could provide clues about the formation of the planets and their orbit around the Sun.

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