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ʻOumuamua: The interstellar rolling stone

In October, astronomers working with the Pan-STARRS1 telescope came across a highly unusual object. The object had been wandering between the stars for millions of years before encountering the Solar System entirely by chance. While scientists had hypothesised the existence of such objects, they had never actually directly observed one.

In International Astronomical Union had to rush to come up with a new naming convention for the unique object. The object was named ʻOumuamua, based on the recommendations of the team that made the discovery. For the pronounciation, just add an “h” sound before the O. The name means a messenger from the distant past. Apart from its extrasolar origins, the shape of the object was extremely elongated, about ten times as long as it was wide. Such an object had never before been seen in the Solar System.

The path of ʻOumuamua through the Solar System. Image: ESO/K. Meech et al.

The discovery and the shape of ʻOumuamua, was however very similar to a mysterious interstellar object in the Arthur C Clarke science fiction novel, Rendezvous with Rama. Olivier Hainaut from the European Southern Observatory said in a press release, “This felt very much like the beginning of the famous science fiction story, Rendezvous with Rama.” Astronomer Jason Wright from Penn State noted on his blog, AstroWright, “Its discovery closely tracks the opening chapter of the book Rendezvous with Rama, by Arthur C. Clarke, about the discovery of an interstellar spaceship on a similar trajectory to ‘Oumuamua.”  

One paragraph that equally applies to ʻOumuamua is “It was a lonely wanderer between the stars, making its first and last visit to the Solar System – for it was moving so swiftly that the gravitational field of the sun could never capture it. It would flash inwards past the orbits of Jupiter, Mars, Earth, Venus and Mercury, gaining speed – as it did so, until it rounded the sun and headed out once again into the unknown.” The designation for the object before it got the name of ʻOumuamua was 1I/2017 U1. The initial designation for the Rama cylinder in the Arthur C Clarke novel was 31/439.

Artist’s impression of ʻOumuamua. Image: ESO/M. Kornmesser.

The asteroid appears to have come from the direction of the star Vega in the constellation of Lyra. However, when the asteroid was in that location 3,00,000 years ago, the constellation appeared in a different place on Earth. The first interstellar visitor was deflected by the Sun, and was moving at speeds of about 87.3 kilometers per second. At the time of the discovery, ʻOumuamua had already crossed the point closest to the Sun, and was heading towards the constellation of Pegasus.

Unlike the book, astronomers actually interrupted the planned observation campaigns of telescopes to take a closer look at the curious interstellar nomad. There was very limited time on hand, it is highly unlikely that the object would ever visit the Solar System again, and researchers can only guess when an opportunity for studying an interstellar asteroid would arise again. For all these reasons, astronomers jumped at the chance to observe the asteroid. A group of astronomers used three telescopes in Hawaii and two telescopes in Chile to conduct follow on observations of ʻOumuamua, in an effort to study its properties, and reveal its mysteries. Another group of astronomers studied the object through telescopes in Hawaii and Spain. A third team of international astronomers studied the object from a telescope in New Mexico. NASA’s Hubble and Spitzer space telescopes also tracked the asteroid. Yan Fernandez from the University of Central Florida said, “On very, very short notice they were able to rework the telescope’s schedule — which is set months in advance — to get us 4 hours of time”. Director of the Gemini Observatory, Laura Ferrarese said, “Needless to say, we dropped everything so we could quickly point the Gemini telescopes at this object immediately after its discovery.”

The findings were published in a paper titled “A brief visit from a red and extremely elongated interstellar asteroid” by Meech et al, in Nature. The object was at least 400 meters long, and was spinning rapidly, once every 7.3 hours. The surface had acquired a reddish hue from basking in the cosmic rays for millions of years. The dense asteroid was rocky or metallic. As the colour matches that of the Saturnian moon Iapetus, the composition of the asteroid is considered to be similar, a blend of minerals, carbon, iron and organic compounds. There is little or no water on ʻOumuamua. The visitor is similar to objects in the outer solar system, the trans neptunian objects or the remote and distant worlds in the Kuiper belt. The colour of ʻOumuamua suggested that it had characteristics similar to comets as well as the Trojan asteroids. While similar in composition to nearby celestial objects, the shape is nothing like anything in the Solar System. The most elongated known objects are only about three times as long as they are wide.

Another paper, published in the The Astrophysical Journal Letters, titled “Colors of the Interstellar Planetesimal 1I/`Oumuamua” by Bannister et al, does not entirely agree with the findings with the paper published by Meech et al. The significant differences are in the observed colours of the object, and how elongated the object is. According to the Bannister paper, the length of the axial ratio of the object is at least 5.3:1 (as against 10:1 in the Meech paper). Another paper titled “Time-resolved Color Photometry of Highly Elongated Interstellar Object 1I/’Oumuamua” by Bolin et al, published in The Astrophysical Journal Letters also found the colouration of ʻOumuamua consistent with Trojan asteroids in the solar system. This paper gave a range of between 3.5 to 10.3:1 for the axial ratio. The discrepancies in observations are because the window available for observing the object was very small.

The elongated nature of ʻOumuamua raised another question. An interstellar alien spaceship is most likely to have an elongated, streamlined shape, similar to a cigar, to reduce exposure to damage from interstellar dust. Breakthrough Listen and an initiative backed by Russian billionaire physicist Yuri Milner both scanned the object, to see if it was transmitting any signals. The scans were in line with the observations from the astronomers. There were no extraterrestrial signals found, and ʻOumuamua was confirmed to be a natural object. The elongated shape is believed to be because of a tidal distortion event before ʻOumuamua was ejected from the system that it was born in.

Another paper, titled “`Oumuamua as a messenger from the Local Association” by Fabo Feng and Hugh RA Jones published in The Astrophysical Journal Letters used computers to simulate the motion of the asteroid. The study found 109 encounters with nearby stars, at a distance of less than 16 light years, or 5 parsecs. While the asteroid had a reddish hue, it was not very red, which indicates that ʻOumuamua is relatively young. The asteroid was traced back to a group of young stars in the Pleiades moving group, located in the constellation of Taurus. The cluster is visible to the naked eye. The asteroid was likely ejected from a binary pair of stars, from the warmer part of the system, where the asteroid belt is located within our Solar System. An interesting point raised by the paper is that astronomers on alien worlds studying the object in the distant future may trace `Oumuamua back to our Solar System.

Our Solar System ejects comets and asteroids because of the interactions with the larger outer planets. Researchers believe that a similar mechanism leads to asteroids and comets being ejected from other systems as well. So far, the estimate for the frequency of such interstellar visits have been ridiculously low. Researchers now believe that between one to ten such objects pass through the Solar System every year. These have not been detected so far considering how faint they are, as well as how brief their visits last. Because of ʻOumuamua, there there are plans to modify telescopes to make them more capable of detecting such interstellar objects in the future. Studying such objects will increase the understanding of our solar system, as well as the process of planet formation. Tracking hypervelocity asteroids is also important considering the serious destruction that can be caused by a collision with Earth. As the speeds of these interstellar objects are much greater than that of local asteroids, the devastation by an impact event would also be greater.

 

Aditya Madanapalle

Aditya Madanapalle

An avid reader of the magazine, who ended up working at Digit after studying journalism, game design and ancient runes. When not egging on arguments in the Digit forum, can be found playing with LEGO sets meant for 9 to 14-year-olds.