The rocky composition of Oumuamua is throwing a wrench in what scientists know about planet formation

Observations of interstellar visitor, Oumuamua, is providing new insights into planet formation.
By Lliane Hunter | Apr 02, 2018
Oumuamua, an interstellar object that whizzed through our solar system last year, has a composition that suggests it should have formed close to its parent star, writes Elizabeth Howell for Space.com. Because it's hard to eject an object orbiting so close to its star, astronomers are finding it difficult to imagine how this object was able to make the trip to our solar system.

Oumuamua was discovered on October 19, 2017, using the NASA-funded Panoramic Survey Telescope and Rapid Response System. After looking at Oumuamua's high speed and highly inclined path through our solar system, scientists concluded the object was interstellar. In the journal Monthly Notices of the Royal Astronomical Society, astrophysicist Elisa Quintana describes what Oumuamua observations are revealing about the formation of "planetesimals," which are small, rocky objects that come together under gravity's pull to form planets.

"What's interesting is that just this one object flying by so quickly can help us constrain some of our planet-formation models," she said. Oumuamua zoomed past the sun at about 196,000 mph. While the object was traveling fast enough to escape our solar system, its speed was somewhat similar to that of a comet passing by the sun, NASA said. However, unlike a comet, Oumuamua did not leave behind a trail of gas and dust.

Solar systems such as our sun and its planets form out of vast clouds of gas, dust, and ice. Objects, such as comets, that form far away from their parent sun can remain icy. If the objects are close to the sun, it's too hot for ice to remain, so they coalesce into objects such as asteroids, explains Howell. Mysteriously, if Oumuamua formed as close to its star as an asteroid, it's difficult to imagine how it was ejected away from that zone.

"If we understand planet formation correctly, ejected material like Oumuamua should be predominantly icy," said Thomas Barclay, an astrophysicist at Goddard and the University of Maryland, Baltimore County. Sean Raymond, an astrophysicist at the French national Center for Scientific Research and the University of Bordeaux, added "It's these tiny, little, circular regions around stars. It's hard to imagine how Oumuamua could have gotten kicked out of its system if it started off as an asteroid."

Nevertheless, researchers have come up with a plausible scenario for how it was ejected. Based on simulations from other work, they suggest a gas giant planet, similar to our Jupiter, may have flung Oumuamua into interstellar exile. As gas giants plow by small objects such as asteroids, they exert intense gravitational forces on the objects. Perhaps, in the case of Oumuamua, such a gas giant planet's gravity exerted pressure on the object, forcing into the cigar-like shape observed today, and flinging it into interstellar space.

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