'Oumuamua' space mystery may have been solved

New research suggests the interstellar visitor may be a comet in disguise.

by Tereza Pultarova, Space.com / Dec.19.2017 / 6:45 PM ET

An artist's impression shows the mystery space object 'Oumuamua.ESO / M. Kornmesser

Although it looks like an asteroid, the first interstellar object spotted passing through the solar system, called 'Oumuamua, may be more like a comet in disguise.

When astronomers first spotted the oblong, tumbling interstellar object 'Oumuamua passing through the solar system in October, they were surprised — not only did it come from outside the solar system, according to its trajectory, it seemed to be an asteroid, rather than the comet researchers thought was more likely for an interstellar visitor.

However, a new paper suggests 'Oumuamua may be made of ice, like a comet, just disguised with a protective crust.

 This view of the interstellar object 'Oumuamua was captured by the 4.2-meter William Herschel Telescope in La Palma in Spain's Canary Islands. A. Fitzsimmons, QUB/Isaac Newton Group, La Palma.

According to professor Alan Fitzsimmons from the Queen's University Belfast, Northern Ireland, there is much more "icy stuff than rocky stuff" in the solar system, making it more likely for emissaries from other systems to also be icy, if other solar systems evolved in the same way.

"We know that our solar system has ejected many more icy bodies then rocky bodies," Fitzsimmons, lead author of the paper published today (Dec. 18) in the journal Nature Astronomy, told Space.com.

As the solar system formed, planets made of gas and ice near the outer edges of the solar system ejected trillions of objects, Fitzsimmons said. In addition, the mass of small icy bodies at the outermost reaches of the solar system, known as the Oort cloud, has lost objects over billions of years due to gravitational disruption from other stars. It was therefore logical for astronomers to expect that the first interstellar visitor they would see should be a comet.

"Given that this object passed relatively close to our sun as it was travelling through our solar system, one would expect any ices on the surface to basically be heated and it should behave like a comet," Fitzsimmons said. "We should see gas streaming off the surface, we should see dust particles being ejected in the cometary atmosphere, perhaps even a tail."

But astronomers observing 'Oumuamua with their telescopes have seen no signs of such a behavior. They concluded that the object must be rocky in nature — an asteroid. However, when Fitzsimmons and his colleagues examined data on the surface of the object more closely, they found it doesn't look like a typical asteroid either.

"We didn't see any signs of typical spectroscopic signatures that you would expect from the minerals on the surface of an asteroid we see in our solar system," Fitzsimmons said. "It rather seems to resemble the [icy] objects that are there in the outer solar system. That kind of got our head scratching. If the object had, originally at least, ice in it, what's happened to it?"

Fitzsimmons and his colleagues looked at older studies and laboratory experiments that tried to find out what happens to icy bodies, such as comets, that are exposed for a long time to energetic particles and cosmic rays. These studies suggest that the ice from the surface layers of such bodies evaporates because of the cosmic environment.

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