Starwatch: First interstellar visitor's name is nod to Hawaiian sighting

Astronomers finally have a name for the first known object from interstellar space to visit our solar system. The International Astronomical Union announced last week that it is to be called 1I/‘Oumuamua where “1I” designates it as the first interstellar object and ‘Oumuamua is a Hawaiian word that is said to reflect the way that this is akin to a scout or messenger reaching out to us from the past.

The name was chosen by the astronomers who used the Pan-STARRS 1 telescope in Hawaii to discover the dim visitor on 18 October. It had already swept through perihelion within 38m km of the Sun on the 9th and 24m km from the Earth on the 14th. Its path is shown on our diagram where it is plotted in yellow when it lies above the Earth’s orbit of the Sun, the ecliptic, and in blue as it passed below.

The initial reaction was to name it as the comet C/2017 U1 (PANSTARRS) but it soon became clear that it was no ordinary comet. The more than 5,000 comets that have been analysed to date have all been members of the Sun’s family. Some follow closed elliptical paths, returning to the Sun periodically, while others plunge towards the Sun along almost parabolic orbits from the Oort cloud, a postulated spherical reservoir of billions of icy bodies at the edge of the solar system.

The new object was clearly in a hyperbolic orbit that had brought it from beyond the Oort cloud at a relative speed of 26km per second. Having passed the Sun, it is now speeding away in a very different direction, towards the Square of Pegasus, never to return. More observation also failed to detect any trace of a tail or coma, the diffuse envelope of gas and dust that we expect around every comet, at least while they are in the inner solar system. This makes it look asteroidal in nature and led to its relabelling as A/2017 U1, the first time this had been done.

But all known asteroids follow elliptical orbits and are firmly in the Sun’s grip. The time was ripe for the newly conceived title of 1I/‘Oumuamua. The name itself is a surprise, and not only because of its leading Hawaiian punctuation, called an ‘okina. Hitherto, asteroids have usually been named only after they have been tracked for several years. In fact, because of its faintness, there is every prospect this object will be lost from view very quickly.

It is thought to be less than 200 metres wide, rocky in nature with a reddish surface and it probably escaped from an alien star system millions or billions of years ago, perhaps being flung out by a Jupiter-sized planet during the process of star formation.

It approached the solar system from a direction close to that of Vega in Lyra, the bright star 25 light years away and high in our W sky this evening. Vega, though, is probably not the parent star since it was elsewhere when ’Oumuamua passed that point about 300,000 years ago.

Although there are attempts to trace it back to its origin, the chance of success is slim since that star may be many thousands of light years away, or even on the far side of our Milky Way galaxy. And it is worth remembering that if ‘Oumuamua is ever spotted by alien astronomers in the distant future they would be mistaken if its motion led them to regard it as an escapee from our Sun.