Astronomers suspect that the recently discovered object 1I/2017 U1 has been crossing interstellar space for hundreds of millions of years. It comes from the direction of the bright star Vega in the constellation Lyra. However, given its tremendous speed of 95,000 km/hr, it would have passed the point where Vega is now some 300,000 years ago. As Vega was nowhere near its current position in space at that time our Sun may be the first star it has encountered in a very, very long time.
The scales of time and distance stagger the imagination. Where did this lonely wanderer originate and what titanic forces caused it to be nudged away from its parent star…wherever that object might be? Arriving from afar this surprise visitor speaks of time, wonder, age, and mystery. It embodies, in a sense, what many feel when they look up at the night sky. Indeed, the Hawaiian name bestowed upon 1I/2017 U1 by its discoverers captures this sense of wonder perfectly. They call the object ‘Oumuamua which means “a messenger from afar that arrives first.”
Given its speed astronomers did not have much time to track this object. By the time it was discovered it had already passed its trajectory’s closest point to the Sun. As it receded light would drop off quickly and so would the ability to gather useful observations. Discovered on October 19th by the Pan-STARRS 1 telescope in Hawaii, astronomers there plotted its orbit over the course of several nights and it quickly became apparent that the new object was passing through our neighborhood from somewhere beyond the Solar System. This exciting initial report was quickly followed by additional observations from astronomers all around the world.
The Very Large Telescope at the European Southern Observatory was quick to spring into action. Located at Cerro Paranal high in the Atacama Desert of northern Chile the VLT consists of four identical, 8-meter wide telescopes. These telescopes can be positioned to serve as an interferometer in which the distance between them makes for an even larger instrument. These were used across several nights to observe 1I/2017 U1.
The details of this observation campaign were revealed yesterday in an article in the journal Nature. The investigators found many surprises and some confirmations regarding what was once considered a hypothetical scenario regarding extrasolar objects. The visitor swept quite close to the Sun at just a little under 40 million kilometers. This is roughly one-quarter of the Earth’s distance from the Sun. In astronomical distances this was just a little inside the distance that the planet Mercury orbits, although at a much steeper angle compared with the ecliptic plane.
What is interesting about this close pass is that ‘Oumuamua seems to be purely asteroidal in nature, with no behavior or evident out-gassing as might be displayed by a comet. Indeed, a comet passing so close to the Sun would have developed both coma and tail as volatiles on its surface sublimated before the Sun’s heat. ‘Oumuamua appears to be a large, rocky object with a high metal content. Spectroscopic observations show its surface to be ruddy in hue and that is suggestive of organic molecules. This is similar to the surface of many asteroids found in the Solar System.
‘Oumuamua has a “day” that lasts about seven hours. This was noted by measuring a light curve that showed a variation in the intensity of reflected sunlight. That change in brightness varies by an order of magnitude (10x) every seven hours. This weird light curve is not seen in local asteroids, and is suggestive that ‘Oumuamua is quite elongated, almost like a long rugged column of rock.
Other measurements showed few if any traces of dust or ice on the surface. This is indicative of great age as surface compounds were likely swept away by irradiation and cosmic particle bombardment across millions of years. It is also an extremely dark object. Unlike local asteroids ‘Oumuamua absorbs almost 96% of the light that strikes its surface. The visitor is extremely elongated and has an average radius of 100 meters and is thought to be at least 400 meters in length.
An important revelation regarding ‘Oumuamua is that given the fact that it is the first asteroid to be identified as having formed around an alien (albeit unknown) star, it can serve as a model to test a number of theories regarding planetary formation models. In a sense this visitor provides us a sample of material from another star system. Observations and analysis of the new data indicate that it is in many ways similar to some asteroids in our own neighborhood. This is of special note. It helps confirm the idea that planetary compositions as we know them in our solar system could be similar to others throughout the galaxy. Given that there are organic compounds on Oumuamua ‘s surface, the concept that these materials are passed from star system to star system across eons has also been strengthened.
In the last few weeks the estimate for the number of interstellar asteroids that might pass through our solar system has been revised upward. One study has suggested that as many as 1000 objects might fly by our home star every year. That equates to over three per day either entering near-space or departing. Like ‘Oumuamua these objects would have speeds which would make it impossible for the Sun’s gravity to ever capture them.
It is intriguing to think that so many objects with exotic, extrasolar origins might be passing through our solar system. Astronomers are already calling for new observation campaigns and improved instruments to observe such objects. Although Oumuamua might be the first observed and recorded messenger to arrive from afar, it certainly may not be the last.