Astronomers will be able to figure out what kind of stars interstellar objects like ‘Oumuamua come from, and thus something about their composition, based on their velocity as they enter our solar system, new research suggests.
So far, astronomers have discovered only two confirmed interstellar objects (ISOs) in our solar system, ‘Oumuamua And 2I/Borisov. They couldn’t be more different: ‘Oumuamua had no comet tail, while Borisov looked like a typical comet.
However, the properties of their own planetary systems are imprinted on both, said graduate student Matthew Hopkins of the University of Oxford in England, who conducted the new research and presented it at Britain’s National Astronomy Meeting in early July.
Related: ‘Oumuamua: The Solar System’s First Interstellar Visitor Explained in Pictures
“Because they come from other stars, their properties will correlate with those stars,” Hopkins told Space.com.
While we’ve only seen two ISOs so far, our solar system at any given time, most too far away from us to be detected. However, most or all of those ISOs probably started out as comets around other stars, before being thrown into interstellar space by an encounter with a Jupiter-sized planet, or perhaps a passing star.
In our solar system, “for every comet that Jupiter [and Neptune] pushed in Oort cloudit completely ejected 10 of them, and there are a trillion objects in the Oort cloud,” Hopkins said. If you do the math, it’s easy to jump to the conclusion that ISOs are “the most numerous objects in the galaxy.”
Moving groups of interstellar objects
Each star moves through the galaxy at its own pace and together they form moving groups related to their point of origin, which in turn corresponds to their intrinsic chemistry.
The stars with the heaviest elements, like ours Sun, live in the galaxy’s “thin disk,” a plane in the spiral arms about 400 light-years thick. Surrounding it is the “thick disk”, which can extend up to 1,000 light-years above the plane of the galaxy and contains mostly older stars with less heavy elements.
The populations of stars belonging to each disk have different velocity distributions. Because the ISOs they eject have a similar velocity to their parent star relative to the Sun, they tend to stick to the same moving groups, but these moving groups cross the path of the Sun all the time.
“The sun is essentially hitting it,” Hopkins said. This means that we should preferably expect ISOs to come from the “solar apex”, which is the direction of the Sun’s motion relative to other nearby stars.
“‘Oumuamua was very close to the top of the sun,” Hopkins said. “Borisov was a little further away, but still quite close [to the solar apex]and that’s where we expect most of them to come from.”
Coming from this direction means they are closest to the sun, where they are easiest to detect, as they are in the southern hemisphere sky – the same sky as the new Vera Rubin Observatory will measure. Vera Rubin is expected to do just that discover hundreds of new ISOs.
Related: Vera Rubin: The astronomer who uncovered dark matter
Slower ISOs contain less water
The lower the relative speed of an ISO compared to the sun, the more likely it is to fall in the inner solar system where we can detect it; the fast ones will just speed through without necessarily being much attracted to the sun’s gravity. The relative velocity of an ISO is related to the relative velocity of its parent star, which strongly depends on whether that star originates from the thin disk with more heavy elements, or from the thick disk with less heavy elements.
“My results show that the speed of an ISO correlates with its composition, and this allows us to get to grips with what type of stars they may be coming from,” Hopkins said.
The lower speed ISOs (relative to the sun) are expected to come from the thin disk, where stars and their associated planetary systems are formed from gas and dust containing heavier elements. The more heavy elements there are in the disc of gas and dust that make up planets and comets, the smaller the fraction of water an ISO will have.
This is because a protoplanetary disk rich in heavier elements contains a lot of carbon, and carbon (as well as iron, magnesium, silicon and sulfur) is adept at picking up all the free oxygen atoms, two at a time, to form molecules of carbon dioxide. Water can only be formed from all of the remaining oxygen atoms, meaning that ISOs that form within these disks generally contain a lower fraction of water.
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Could this lack of water explain why ‘Oumuamua showed no comet tail?
“Because it had a slower speed relative to the sun, it probably came from a thin disk star with heavier elements,” Hopkins said. However, he’s keen to point out the caveat that we don’t know the history of ‘Oumuamua – it could have lost its water and other volatile elements in some other way. For example, maybe they were wiped out by cosmic rays as they traveled through interstellar space, or by too many short passes to the parent star before it was ejected.
Borisov, on the other hand, was in the mid range for water content based on spectral observations of its tail.
With only two examples of ISO currently, it’s hard to draw too many conclusions. However, when the Vera Rubin Observatory is up and running later this decade, the hundreds of ISOs it should find will be able to provide a more complete picture of where they come from and what their chemical properties are.
“If there is a preference for ISOs moving in the same way as the sun falling into the inner solar system, then we would expect more ISOs from the thin disk,” Hopkins said.
That could mean we’ll see more objects similar to ‘Oumuamua instead of Borisov. Only time will tell how accurate that prediction is.