Space

An alien recently passed through our Solar System, moving so fast we couldn’t catch it, but next time we should be ready for a chase.

Earth’s first interstellar visitor

ASTRONOMY IS IN a golden age. Hot on the heels of the observed collision of two distant neutron stars (AG 142) comes another first for the science of the Universe. This time, though, it’s much closer, and an alien – the extraordinary cigarshaped asteroid called ‘Oumuamua.

The object’s Hawaiian name means ‘first messenger from afar’. It’s known to originate from another solar system and is the first such visitor to have been seen passing through our own family of planets.

Deducing ‘Oumuamua’s interstellar origin was easy. Soon after it was picked up by the Pan-STARRS1 telescope in Hawaii, in October 2017, calculations based on its observed motion told us it had come from a long way away. It was moving too fast to have originated here.

Likewise, its elongated shape was easy to deduce, once measurements had been made of brightness fluctuations as it tumbled along its trajectory. Estimates of its size (35 x 35 x 230m) also came from brightness measurements. And its deep red colour, similar to that of icy Solar System asteroids beyond Neptune, suggests its rocky surface has been bombarded with cosmic rays for many millions of years.

So where did ‘Oumuamua come from? It entered our Solar System from the direction of the bright northernhemisphere star Vega. However, ‘Oumuamua would have taken about 600,000 years to reach us from there, and back then,Vega was in a different position. New research suggests ‘Oumuamua came from a star associated with the Pleiades star cluster, possibly ejected from its solar system after colliding with another asteroid, producing its jagged shape.

‘Oumuamua is now moving away too fast for us to investigate it directly with robotic spacecraft. But evidence suggests it is just one of many interstellar visitors that traverse our Solar System every year, and we will certainly detect more as bigger telescopes improve our ability to scan the skies. Next time, we might be better prepared to give chase.

This artist’s impression of interstellar asteroid ‘Oumuamua, which is unlike anything normally found in the Solar System, is based on observations from the European Southern Observatory’s Very Large Telescope in Chile and other observatories around the...