It's official: the Asteroid 1I/2017 U1, aka "‘Oumuamua", which screamed through the solar system in October 2017 is an interstellar object. And a very strange one at that.*
The 400 metre long asteroid is moving fast – 38.3 km/second is its current sun-relative velocity – and has already passed Mars' orbit after sling-shotting around the Sun on September 9th. NASA has announced it will pass Jupiter in May 2018, Saturn in January 2019, and when it exits the solar system it will be heading for the Pegasus constellation.
Its top speed during its visit was 87.3 km/second, reached as it hurtled around the Sun (even before astronomers spotted it).
The telescopes that observed the asteroid since its discovery – Pan-STARRS, the European Southern Observatory (ESO), and in space, Hubble and Spitzer among them – have since gathered sufficient observations that the International Astronomical Union last week confirmed the interstellar origins of the object, which has been named.
Astronomers have also said the object is ten times as long as it is wide, based on how its brightness changes as it spins.
ESO observations made using its FORS (FOcal Reducer and low dispersion Spectrograph) instrument and images from other large telescopes, a team led by the Institute for Astronomy in Hawaii's Karen Meech found “‘Oumuamua varies in brightness by a factor of ten as it spins on its axis every 7.3 hours” – a variation greater than any known solar system asteroid or comet.
“This unusually big variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape,” Meech said in the NASA announcement. “We also found that it had a reddish color, similar to objects in the outer solar system, and confirmed that it is completely inert, without the faintest hint of dust around it.”
NASA says preliminary orbit calculations suggest 300,000 years ago the asteroid was in “the approximate direction of the bright star Vega”. However, that's only an apparent location, as Vega has moved a long way in that time.
Australian National University astrophysicist Dr Brad Tucker explained to Vulture South that the object could tell researchers about planetary formation.
The shape is interesting, he said: “it could have been that it's been weathered and destroyed, having travelled through space for so long.”
Alternatively, “it could have been ejected from a collision, or it formed on the edge [of another star system] and just kept going”, he added.
"‘Oumuamua's highly metallic content is more interesting, Tucker continued. “The composition is more interesting, that it is very metal-rich, or appears to be”.
That, he noted, is similar to a number of asteroids in our solar system, which suggests “that it's born in a similar way”.
Alternatively, it might be that other star systems astronomers have already observed, which have exoplanets with metallic atmospheres (titanium, for example), might be more common than we expect. That could mean our rocky solar system is an outlier.
Knowing there's one interstellar asteroid, Tucker added, will mean the search for others becomes a higher scientific priority. He pointed out that the age of all-sky surveys means observers are looking in places astronomy traditionally ignored (such as “boring” areas away from the solar system plane, or away from the galactic plane). ®
* No, "‘Oumuamua" is not a typo. The word is Hawaiian and that language includes a character called an "okina", drawn as the character "‘" that indicates a glottal stop. ‘Oumuamua translates as "first visitor from the distant past". We'll leave it to readers to pronounce it.