Dept. of Physics and Astronomy, Western University, London ON CANADA
Institute for Earth and Space Exploration (IESX), London ON CANADA
7 November 2019
In 2017, the first asteroid to enter our Solar System from interstellar space was discovered: it was given the Hawaiian name 'Oumuamua which means Messenger from Afar. Now a second visitor, comet Borisov has appeared. Tracing the origins of these far-travelling bodies is part of a project undertaken at Western University by Tim Hallatt and Paul Wiegert.
"Our Solar System is big: it contains all the planets and asteroids, everywhere we've ever been or sent a spacecraft to. But our Galaxy is truly vast. Over one hundred thousand times bigger than our Solar System, our Milky Way Galaxy contains all the stars we can see on a clear night, and *their* solar systems. When a visitor from the wider Galaxy passes through our Solar System, we know we have an unprecedented opportunity to study something special." says Hallatt, who started this work while an undergraduate at Western and who is now doing graduate work at McGill.
Determining the origin point of interstellar guests is no easy matter. Exactly how they form is unknown, but by tracing their motion back in time it is possible, at least in principle, to determine where they came from. But our Galaxy has over 100 billion stars, stellar nurseries, gas clouds and other possible sources for these objects, and they're all moving about too, as the whole Galaxy rotates about its centre.
This project started with the study of our first guest, 'Oumuamua. Because of its relatively low speed with respect to our Galaxy, 'Oumuamua could be young. "Young here means less than 100 million years old. Not young in human terms, but this is a short time compared to the age of our Galaxy, which is estimated to be 100 times longer at 10 billion years" says Hallatt. Comet Borisov became part of the study after its unexpected appearance earlier this year, though it is likely much older, making it that much harder to trace.
Though the precise movements of all of our Galaxy's stars and other denizens is not well-known enough yet to determine which one is Oumuamua's origin point, Hallatt and Wiegert were able to calculate that the origin should be within our local Galactic neighbourhood, and relatively easy to study telescopically, if only we can find it. Though 'Oumuamua and its parent system may both have travelled tens of thousands of parsecs (a parsec is 3.26 light-years, and our nearest neighbour stars are a little over one parsec away) since that asteroid set out, they remain relatively close together, like two ships headed across the ocean on similar courses. Thus they remain relatively close to each other, though both may have travelled halfway across the Galaxy since their paths separated.
"Being able to study the origin system of such travellers would provide a wealth of clues as to their nature" says Wiegert. "Though their origin remains elusive, we're gradually drawing in the net. Its only a matter of time before they reveal their secret."
About the simulated fly-throughs
The fly-throughs above include over 7 million stars from the European Space Agency's Gaia DR2 catalog, but this is only a fraction of the one hundred thousand million (=100 billion) stars in our galaxy. Many of these stars are visible to the naked eye from Earth, though some are too faint. Passages within 5 parsecs (one parsec or pc is 3.26 light years) of Gaia DR2 stars are indicated in purple. The stars are coloured according to their temperature, as measured in Gaia DR2. Cooler (usually smaller) stars are in red, up through orange, yellow, while and blue-white, though hotter larger stars are quite rare. Stars are shown much larger than life: the radius of all stars, regardless of their actual size, in these animations is 20,000 Astronomical Units (or AU: 1 AU is about the average Earth-Sun distance), intended as a rough measure of the size of their Oort clouds (if any). The checkerboard pattern lies on the Galactic Plane, which slices our galaxy into north and south halves in much the same way the Earth's equator divides our planet in two. Each square is 100 parsecs (326 light-years) on a side. Our Sun, Earth and the rest of our Solar system are 25 parsecs north of the Galactic Plane, and approximately 85 checkerboard squares (8,500 pc) from the centre of our Milky Way Galaxy, which itself is approximately 200 squares (20,000 pc) in diameter. The Earth, the Sun and the rest of the planets, asteroids, etc. of our Solar System, are all inside and very near the centre of the yellow sphere labelled 'our Sun'. Note that some nearby stars are too bright for the Gaia DR2 catalog (e.g. alpha Centauri) and do not appear here.
About the names
Interstellar asteroids and comets are designated by a special prefix. 'Oumuamua's full designation is 1I/'Oumuamua, where the '1' indicates that it was discovered first, and the 'I' indicates it is interstellar (that is from beyond our Solar System). 1I/'Oumuamua was discovered by Rob Weryk at the PanSTARRS project. Comet Borisov's full designation is 2I/Borisov, and was discovered by Gennadiy Borisov at the MARGO observatory in Crimea.
Hallatt, T. & Wiegert, P., 2019, "The dynamics of interstellar asteroids and comets within the Galaxy: an Assessment of Local Candidate Source Regions for 1I/'Oumuamua and 2I/Borisov", submitted to American Astronomical Society (AAS) journals [link to the preprint on arxiv.org]
Additional videos 1I/'Oumuamua and 2I/Borisov
Here are some illustrative videos to give you more of a sense of the system. Note that these are illustrations only, not all details are correct (e.g. stars are shown larger than their true size for visibility, among other things).
This one shows the path of these two into, and then back out of our Solar System. Click here for a larger version.
This one shows them moving back in time, and eventually switches to the frame of our Milky Way galaxy, because that is ultimately the context we need to think of these in. Click here for a larger version.
Finally here is a snapshot of our local galactic neighbourhood, with the stars (mostly in white and green) within 25 parsecs of the Sun only, and some local star-forming regions, again just for context. Click here for a larger version.
© Copyright 2019-2021 by Paul Wiegert