Since mid-October, the astronomy community has been buzzing about what could be our Solar System’s first interstellar host. An automatic telescope saw something that appeared to have descended on the West Road from above, an angle that suggested it came from somewhere else. Now, a team of astronomers has quickly published a paper describing the unusual features and naming it “1I/2017 U1 ‘Oumuamua.” In Hawaiian, ‘Oumuamua roughly means “first messenger,” and 1I refers to it as the first interstellar object.
‘Oumuamua was first heard on October 19 by the Pan-STARRS1 telescope automation. Pan-STARRS1 appeared to have taken pictures of the object earlier in the day, but automatic analysis software did not recognize it. Further images over the next few days will allow researchers to refine its journey through our Solar System, confirming that ‘Oumuamua is making the closest approach to the inner Solar System of any object seen before. Essentially, it appears that we have descended into the Solar System from above, entering between the Sun and the orbit of Mercury. It is also moving very fast.
The Solar System was formed from a disk of thin material, and all the planets orbit roughly in the plane of that disk. Smaller objects, such as dwarf planets and comets, may take more irregular approaches with orbits that depart from that plane, but they are still roughly aligned with it. We have never seen anything quite like ‘Oumuamua.
Comets tend to have larger orbits, but the image shows that, despite its close path to the sun, ‘Oumuamua did not emit any dust or particles as it heated up. Within the limits of our detection, there is 10 million times less matter than you would expect to see released by a comet. So, ‘Oumuamua appears to be more of an asteroid. Its red tinge is typical of substances that have many organic chemicals on their surface.
Further clues as to its composition come from the irregular shape and rotation. Over a 7.3-hour period, ‘Oumuamua’s light intensity showed changes by a factor of more than 2.5. Either the parts of its surface are fundamentally different in terms of the amount of light it emits, or the object has an oblong shape. The authors suggest that it is a cigar shape (technically a “triaxial ellipsoid”), with two of its axes about 80 meters across and the third about 800 meters. If that is the case, then it has to be stable. tight enough to hold the stress of changing your speed.
The authors of the paper could not rule out that ‘Oumuamua started at the outer edges of our Solar System but was thrown inward at an unfavorable angle by interactions with an unknown planet. But this would require the existence of a large planet at a great distance from the Sun, so they thought it impossible.
In addition, models of the formation of the Solar System suggest that large planets can throw large amounts of small objects into interstellar space, so it is not surprising that one of them has finally been found. One difference here, however, is that ‘Oumuamua is unlike anything we know of in our Solar System. Additionally, we expect most of the ejected objects to be comet-like, which ‘Oumuamua clearly does not. So, while it’s dangerous to do too much in one instance, it’s a little surprising how different from our expectations this visitor is.
While the recently formed exosolar system is roughly in the direction that ‘Oumuamua came from, it was not in that position back when ‘Oumuamua would have exited on its current path. So we don’t know where he came from.
Its approach to the Sun has turned ‘Oumuamua into a parabolic orbit. After dropping below the plane of the Solar System, the object is shot back above the plane somewhere between the orbits of Earth and Mars. It is now moving rapidly away from all our telescopes and will disappear from view rather quickly.
But researchers hope that we will have other visitors from outside our Solar System. ‘Oumuamua was discovered shortly after Pan-STARRS1 had a software upgrade in the pipeline intended to help identify more objects that would otherwise be dismissed as signals. If ‘Oumuamua is an indication that the software works, then we may have additional studies sooner rather than later.
Creation2017. DOI: 10.1038/nature25020 (About DOIs).