Since its surprise arrival in the solar system in 2017, the interstellar object Oumuamua has left the entire scientific community puzzled. A pair of American astronomers now believe they have solved the mystery. Oumuamua is not an alien spacecraft!

The story of Oumuamua

At first, the mysterious object was classified as an asteroid. It was later reclassified as a probable comet, while for some, Oumuamua was even considered a possible alien spacecraft. The 200-meter long space rock raced through the Solar System at the end of 2017. During its passage, the rock approached Earth at a distance of 24 million kilometers, 62 times the distance between Earth and the Moon, before disappearing after a few weeks of discovery.

The brief observations made during that period showed that Oumuamua traced a “hyperbolic” orbit. The boomerang-shaped trajectory indicates that the space rock is not native to our solar system, but was just passing through before disappearing forever. First interstellar body ever observed, of course, pushed astronomers around the world to bury themselves in the available data to discover everything possible about this mysterious object.

One of the questions that scientists struggled to answer was about Oumuamua’s speed, which seemed to increase as the rock rotated around the Sun. Large bodies, such as planets and stars, can give smaller objects a gravitational push that accelerates them. But Oumuamua was traveling at 87 kilometers per second, about three times faster than a common comet. This speed cannot be justified by a gravitational push.

The Strange Acceleration

For this reason, many scientists concluded that it could not be an asteroid. Oumuamua must have been a comet. These objects, when they come into the solar system, receive additional momentum from the water that evaporates from their icy nuclei as they heat up near the Sun. However, comets are famous for their spectacular tails as a result of the degassing process. Oumumua, on the other hand, did not show any tail, so this hypothesis was also discarded.

Researchers then tried to explain the mechanism behind Oumuamua’s acceleration, but all the proposed ideas always had significant gaps. In a new study published in Nature (ref.), Jennifer Bergner, a professor of chemistry at the University of Berkeley, and Darryl Seligman, of the US National Science Foundation at Cornell University, propose a new theory that could finally put an end to the question.

“Several years I’ve been trying to explain the outgassing” Seligman said. “At first, I thought maybe there wasn’t enough dust in the outgassing to form the coma. Then we thought maybe it was made of a more volatile material than what we see in typical comets, like hydrogen or nitrogen. But there were theoretical problems with each of these explanations”.

The answer we were looking for

Hydrogen, for example, requires extremely low temperatures to freeze into objects the size of Oumuamua. Scientists don’t expect these temperatures inside the dense molecular clouds where these objects form, and nitrogen is not abundant enough in the Milky Way. In fact, Seligman and Bergner propose that there may be nothing particular about the chemistry of Oumuamua.

The object was probably subjected to processes outside of the solar system that we don’t observe in our typical comets. “A comet traveling through interstellar space is basically cooked by cosmic radiation, forming hydrogen as a result” Bergner said. “If this process happened, is it possible to trap the hydrogen so that it is released when Oumuamua entered the solar system?”.

The team’s calculations showed that, in theory, the strength of this hydrogen outgassing could explain Oumuamua’s strange acceleration. Astronomers have even found experimental studies more than 40 years old that show that high-energy particles in cosmic rays can break apart molecular hydrogen from water ice but trap it within a block of ice.

“The nice thing about Jenny’s idea is that it’s exactly what should happen to interstellar comets” Seligman said. “We had all these stupid ideas, like hydrogen icebergs and other crazy things. Oumuamua is not an alien spacecraft at all”. Although Oumuamua has disappeared forever, Seligman hopes that soon new interstellar visitors will be discovered that will help astronomers find further explanations. New telescopes coming online in the next decade should enhance this search and provide new answers.

Sofia Bianchi

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