A fleeting visit: an asteroid from another planetary system just shot past Earth



The discovery of an unusual small object in the Solar system last month caught the imagination of the global astronomical community. Scientists around the world were asking “what is it?” and “where did it come from?”

Within days, they realised this tiny body was moving very quickly, and might not be bound to our Solar System. Astronomers swung telescopes towards the faint object, and soon confirmed it as the first interstellar object ever observed passing through the Solar system.


Read more: Water, water, everywhere in our Solar system but what does that mean for life?

Comets and asteroids - leftovers of creation



The regions of our Solar system (not to scale). Shutterstock/hydra viridis


They then live brief, chaotic lives. Flung around the Solar system as a result of the gravitational influence of the planets, they can end up on orbits that bring them close to Earth and the Sun.

Some will fall apart, while others will crash into one of the planets. The majority will eventually leave the Solar system, never to return. Such ejections are far from a new phenomenon as the Solar system has been shedding debris since it formed.

In 1994, fragments of comet Shoemaker-Levy 9 collided with Jupiter, leaving scars the size of the Earth. Hubble Space Telescope Comet Team and NASA

The Solar system is not unique

Over the past 20 years, we have learned that the majority of stars are accompanied by planets and their attendant debris. Observing stars at infrared wavelengths, we have learned that many are also accompanied by far greater quantities of debris than we see in the Solar system.


With so much material floating freely in space, it was always likely that some of that debris would swing close enough to the Sun for us to detect it – which brings us back to our newly discovered object.

Our first interstellar vagabond


As more observations were made, the only way that scientists could fit the object’s orbit to the data was if it was moving on a hyperbolic orbit – in other words, if it was not gravitationally bound to the Solar system.


Over the days that followed the object’s discovery, detailed observations revealed no evidence of any cometary activity. Long exposures using the world’s largest telescopes showed nothing more than a fast moving speck of light.


But where did it come from?



What we can say is that A/2017 U1 approached the Solar system from roughly the direction of the bright northern star Vega. We know the inbound direction to about one-fifth of a degree, and the path lies around five degrees from that star in the northern sky.

Unfortunately, we can’t go from this to tying A/2017 U1’s origin to any given star. To do that we would need to know the motions of every single star with exquisite precision, as well as how they affect one another (and our object).


The path of A/2017 U1 (dashed line) as it crossed the plane of the planets in our Solar system and then turned and headed back out. Brooks Bays/SOEST Publication Services/UH Institute for Astronomy

The future

What do we learn from A/2017 U1’s fleeting visit? The most important result is the confirmation of a long held expectation – that we would eventually discover comets and asteroids from distant stars sleeting through our Solar system.

Read more: Say hello to the Earth's nearest exoplanet neighbour: Proxima Centauri b


If we detect such objects with enough warning, more detailed observations could examine their chemical and isotopic compositions, allowing us to sample the makeup of planetary systems far from our own. The possibilities are endless, and hugely exciting!

But what of A/2017 U1’s fate? Its days near the Sun are over, and it is rapidly heading back to the cold depths of interstellar space.

