Interstellar objects like 'Oumuamua could kickstart planet formation
According to a new study, interstellar wanderers like 'Oumuamua may speed up the formation of planets in the gas clouds of newly-formed protostars. Research by Susanne Pfalzner of the Jülich Supercomputing Center, Germany, and Michele Bannister of the School of Mathematics and Physics, Queen's University Belfast, indicate that such roving objects might act as seeds that help accelerate the creation of new planets.
When 'Oumuamua briefly visited the solar system in late 2017, it sparked worldwide speculation as to what it was, where it came from, and where it was going as it plunged into the inner system before shooting back into interstellar space on a hyperbolic trajectory. Speculation ranged from it being a fragment hurled out of a binary star system to a visiting artificial probe sent by an alien civilization. It may be that one day its voyage will end billions of years from now inside a new planet.
The basic mechanism by which new stars and planetary systems form is fairly well-established. Put simply, what happens is that a great molecular cloud of hydrogen, helium, and traces of the heavier elements starts to contract or coalesce as molecules are attracted to one another by mutual microgravity.
It's a very slow process at first, since most of the cloud is made up of single gas molecules, but these eventually stick together in clumps that grow into larger and larger grains until a cascade effect starts and the planets form around the new star in a frighteningly large pinball game of collisions and ricochets.
The process is incredibly slow, but according to the new study, the formation of planets also leads to the formation of countless tiny skyscraper-sized planetesimals like 'Oumumua that are slingshot into interstellar space, where they number in the trillions per cubic parsec.
That may sound like a lot, but since a cubic parsec measures 3.3 light years on each side, these trillions average out to "empty." However, it also means that at least one to ten million of these objects are present when a new planetary system forms.
The two researchers say that these objects speed up the planet-forming process by acting a bit like seeds. They compare them to scattered dandelion seeds, but a better analogy might be a common chemistry class experiment where a salt solution is made that is so concentrated that the water can't hold another molecule. This is very stable, but when a single salt crystal is suspended into this supersaturated solution, as it's called, it acts as a focus for the dissolved salt molecules, which cause the crystal to rapidly grow.
The 'Oumuamua-like objects act similarly, though the process is gravitational rather than chemical.
"For decades it has been investigated how planets grow from millimeter-sized grit to planets as huge as Jupiters," says Pfalzner. "This growth process would be very slow, but observations tell us otherwise – some planets exist around very young stars. The presence of hundred-meter-sized objects within this grit could accelerate the planet-formation process considerably. Attracting the grit and gas of the surrounding disk, some of the former interstellar objects could grow into full planets. Once the idea dawned, it was so obvious. I hope that many other researchers will pick it up and test the model."
Another consequence is that as planetary systems form, the number of interstellar wanderers increase, causing the formation of later systems to accelerate.
"The consequences of this discovery might be far-reaching," says Bannister. "Across the Galaxy, the debris of past planetary systems would help build the next generation of planetary systems. Every new generation of stars would increase the abundance of interstellar objects through space. The hearts of some planets, including those in our own solar system, may have started from a tiny seed, a world like 'Oumuamua formed elsewhere in the Galaxy."
The research as published in The Astrophysical Journal Letters.
Source: Queen's University Belfast