The shard-like asteroid from deep space which shot through the solar system last years, known as ‘Oumuamua, set many an astronomer’s heart racing. The peculiar body was determined to be the first confirmed interstellar asteroid to have been observed (1). It’s possible, though, that other comets which pursue so-called hyperbolic orbits (moving fast enough to escape the solar system) also have an interstellar origin, rather than having originated from the Oort Cloud. A team of Spanish astrophysicists, who have more than a passing interest in the topic of Planet X, have performed powerful computer simulations to build up a picture of the trajectories and spatial origins of various hyperbolic comets (2). The objects they chose to consider have inbound velocities greater than 1km/s
Following adjustment for the Sun’s own movement through space towards the Solar Apex, interstellar visitors would likely have a more or less random distribution to their radiants (the position in the sky from which they came, rather like meteor showers striking the Earth’s atmosphere). The Spanish team carried out statistical analysis on the emerging sky maps of these radiants, and looked for patterns or clusters of these origin points. Statistically significant patterns did indeed emerge from the data. A particularly large source was located in the zodiacal constellation Gemini. Such a clustering might indicate a number of possibilities, which the astrophysicists explore in their paper.
One possibility is a close flyby of a star in the past which could have disrupted the outer edges of the distant Oort Cloud, sending comets in-bound towards the Sun. Looking at the tracking of candidate flybys in the (by Cosmic standards) relatively recent past, Carlos de la Fuente Marcos, Raul de la Fuente Marcos & S. J. Aarseth argue that there is a possible correlation between this cluster of hyperbolic orbit radiants in Gemini, and a close flyby of a neighbouring binary red dwarf system known as Scholz’s star some 70,000 years ago (2). At a current distance of about 20 light years, Scholz’s star may be a close neighbour to the Sun relatively speaking, but even so it took a while for it to be discovered. This was probably because of a combination of factors: Its proximity to the Galactic plane, its relative dimness, and its slow relative movement across the sky (3). Its distance was less than a light year 70,000 years ago, and its rapid movement away from us in the intervening time helps to explain why it was difficult to detect as a neighbouring binary star: Its retreating motion is mostly along our line of sight, making it difficult to differentiate from background stars. Read More…
The interstellar asteroid 1I/2017 U1 (otherwise known as 1I/’Oumuamua) is fast receding into the distance, towards the constellation of Pegasus (1). The existence of this rocky visitor from the stars was announced last October (1). Its trajectory was too fast for it to be a solar system comet – even one from the furthest reaches of the Oort Cloud. That was an exciting discovery, because that meant that 1I/2017 U1 was the first confirmed observation of an object arriving in the solar system from deep space.
Although 1I/2017 U1 was initially considered to be an interstellar comet, that thinking changed when it failed to emit any gases as it performed its perihelion transit around the Sun (3). This barren rock, confirmed as an interstellar asteroid (4), is now speeding away from the Sun. It spent a relatively short time in the observation zone of professional telescopes, thanks to its great speed, but this was enough to reveal more weirdness (5). It is an elongated object spinning head over tip, doing cartwheels through the solar system. Some wondered whether it might be artificial, given the lack of coma as it traversed past the Sun. But attempts to pick up signals from the object came up blank (6). Still, its shape is nothing like any known body in our Solar System. If solar system asteroids resemble rocky potatoes, then 1I/2017 U1 is more like an interstellar carrot, spinning haphazardly through our system. To remain intact under these conditions, its internal structure must be robust (7).
The colour of our interstellar carrot is neutral with a reddish hue. The colouration may be patchy across its surface. Solar system minor bodies (asteroids, Kuiper Belt Objects, Trojans) vary in colour, often dependent upon which population group any particular object belongs to. Continuing my daft vegetable analogy, solar system potatoes come in different varieties. Many are neutral in colour, some are reddish, others distinctly red. Like comparing a Maris Piper to a King Edward. If we compare 1I/’Oumuamua’s colouration to those of various classes of solar system objects, then it seems to most resemble those of the dynamically excited populations of Kuiper Belt Objects. However, it is less red than the scattered Trans-Neptunian objects whose orbits extend beyond the heliopause (7).