16.07.2023

Oumuamua may have lacked the water to grow a tail because it came from a planetary system rich in heavy elements.

An artist's depiction of the interstellar object 'Oumuamua. (Image credit: ESA/Hubble, NASA, ESO, M. Kornmesser)

Astronomers will be able to figure out what kind of stars interstellar objects such as 'Oumuamua come from, and therefore something about their compositions, 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 have been more different from one another: 'Oumuamua lacked any kind of cometary tail, whereas Borisov looked like a typical comet.

However, the properties of their home planetary systems are imprinted on both of them, said grad student Matthew Hopkins of the University of Oxford in England, who conducted the new research and presented it at the U.K.'s National Astronomy Meeting in early July.

"Because they come from other stars, their properties are going to correlate with those stars," Hopkins told Space.com.

Though we've only spotted two ISOs to date, it's expected that thousands of them are passing through our solar system at any given time, most too far away from us to be detected. However, most or all of those ISOs likely began life as comets around other stars, before an encounter with a Jupiter-sized planet, or perhaps a fly-by star, ejected them into interstellar space.

In our solar system, "for every one comet that Jupiter [and Neptune] pushed into the Oort Cloud, it completely ejected 10, and there are a trillion objects in the Oort Cloud," said Hopkins. Doing the math, it is easy to come to the conclusion that ISOs "are the most numerous objects in the Milky Way galaxy."

Moving groups of interstellar objects

Each star is moving around the galaxy at its own pace, and together they form moving groups that are related to their point of origin, which, in turn, corresponds with their intrinsic chemistry.

The stars with the most heavy elements, such as our sun, live in the "thin disk" of the galaxy, a plane in the spiral arms about 400 light-years thick. Surrounding it is the "thick disk," which can stretch as high as 1,000 light-years above the plane of the galaxy and contains mostly older stars with fewer heavy elements. 

The populations of stars belonging to each disk have different velocity distributions. Because the ISOs that they eject share a similar velocity as their parent star relative to the sun, they tend to stick to the same moving groups, but these moving groups criss-cross the sun's path all the time.

"The sun is essentially running into them," said Hopkins. This means that we should preferentially expect to see ISOs coming from the "solar apex," which is the direction of the sun's motion relative to other nearby stars. 

"'Oumuamua was very close to the solar apex," Hopkins said. "Borisov was slightly farther away but still quite near [to the solar apex], and that's where we expect most of them to come from."

Coming from this direction means that they'll make their closest approach to the sun, where they are easiest to detect, while they are in the Southern Hemisphere sky — the same sky that the new Vera Rubin Observatory will be surveying. It is expected that Vera Rubin will discover hundreds of new ISOs.

Slower ISOs contain less water

The lower an ISO's relative velocity compared to the sun, the more likely it is that it will fall into the inner solar system where we can detect it; the fast ones will just speed through without necessarily being attracted much by the sun's gravity. An ISO's relative velocity is related to the relative velocity of its parent star, which depends significantly on whether that star hails from the thin disk with more heavy elements, or from the thick disk with fewer heavy elements. 

"My results show that the velocity of an ISO correlates with its composition, and because of this we can get a handle on the types of star they may have come from," said Hopkins. 

The lower-velocity ISOs (relative to the sun) are expected to come from the thin disk, where stars and their accompanying planetary systems form from gas and dust that contain more heavy elements. The more heavy elements there are in the disk of gas and dust that forms 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 plucking up all the free oxygen atoms, two at a time, to form molecules of carbon dioxide. Water can only form from any oxygen atoms that are left over, meaning that ISOs forming within these disks generally possess a lower fraction of water.

Could this lack of water explain why 'Oumuamua did not display a cometary tail?

"Because it had a lower velocity relative to the sun, it probably did come from a thin disk star with more heavy elements," said Hopkins. However, he is keen to point out the caveat that we don't know 'Oumuamua's history — it could have lost its water and other volatile elements some other way. Perhaps they were eradicated by cosmic rays while traveling through interstellar space, for example, or by too many close passes to its parent star before it was ejected.

Borisov, on the other hand, was in the middle range for water content based on spectral observations of its tail.

With currently only two examples of ISO, it is difficult to draw too many conclusions. However, once the Vera Rubin Observatory is up and running later this decade, the hundreds of ISOs that it should find will be able to provide a fuller picture of where they are coming from and what their chemical properties are.

"If there's a bias towards ISOs moving similarly to the sun falling into the inner solar system, then we would expect to see more ISOs from the thin disk," said Hopkins.

That might mean we'll see more objects similar to 'Oumuamua rather than Borisov. Only time will tell how correct that prediction is.

Quelle: SC