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What exactly was this ‘interstellar visitor’?

In October, a cigar-shaped interstellar visitor nicknamed ‘Oumuamua was spotted tumbling through our solar system at 85,700 miles per hour. Astronomers rushed to study it before it disappeared from view.

The first detailed report about ‘Oumuamua, which loosely means “a messenger that reaches out from the distant past,” was released in November. But we’ve learned a lot since then, and a study published in the journal Nature on Wednesday shared even more evidence that it’s an entirely new class of object.

Of course, ‘Oumuamua is not to be confused with the first interstellar immigrant that has taken up permanent residence in our solar system, which was announced in May. The exo-asteroid, known as 2015 BZ509, was hidden around Jupiter until its recent discovery.

Oumuamua, the first observed interstellar visitor to our solar system, is shown in an artist’s illustration.
Full Credit: European Southern Observatory/ESO/M.Kornmesser

Here’s what we’ve learned about ‘Oumuamua.

A cigar-shaped ‘oddball’

“What we found was a rapidly rotating object, at least the size of a football field, that changed in brightness quite dramatically,” study author Karen Meech of the University of Hawaii’s Institute of Astronomy, leader of the research team, said in a statement.

The long and rocky cigar-shaped object has a burnt dark-reddish hue due to millions of years of radiation from cosmic rays. This hue is similar to that of objects found in the Kuiper Belt, in the outer part of our solar system, but its orbit and shape firmly place it in the category of interstellar origin. It most likely has a high metal content and spins on its own axis every 7.3 hours.

But the shape, 10 times as long as it is wide, has never been seen before. This complex and convoluted shape means the object varies incredibly in brightness.

Researchers first called it a comet and then an asteroid, and then it was deemed the first of its kind: a new class of “interstellar objects,” officially designated A/2017 UI by the International Astronomical Union, which created the category after it was discovered.

The surface of it looked like a comet’s core, but it didn’t have a “coma,” the atmosphere and dust around comets as they melt and release gases.

Ground- and space-based telescopes, like Hubble and Spitzer, continued to track ‘Oumuamua for as long as they could. In November, it was 124 million miles from Earth — the same as the distance between Mars and Jupiter — but its trajectory took the object past Mars’ orbit, and it passed Jupiter in May. It will go beyond Saturn’s orbit in January 2019 and then leave our solar system, bound for the Pegasus constellation.

Astronomer Marco Micheli and his colleagues used the observations of ‘Oumuamua’s motion as it traveled through our solar system to study it. Micheli is with the ESA SSA-NEO Coordination Centre In Frascati, Italy.

Their new study suggests that ‘Oumuamua is a cometary interstellar object. They were able to determine this by accounting for the motion of the object itself.

The arc it’s traveling can’t be solely due to the gravitational force of the sun, planets and large asteroids. Instead, its acceleration, which has been directed away from the sun, was relying on something else. Comets can propel themselves by releasing gas, and that’s what the researchers believe is happening here — even if we can’t see it.

“The gases we think are responsible for the activity are difficult to observe from the ground, especially on an object as faint as ‘Oumuamua,” Micheli wrote in an email. “They would not have been visible in the optical images we obtained. We also don’t see any dust in our images in the form of a coma or a tail, but if it were made of grains larger than typical dust of our solar system comets we would not see it.”

A new frontier

This doesn’t change the fact that ‘Oumuamua is still a new class of interstellar objects. And the fact that it acts like a comet matches with the models of what many interstellar objects would be, Micheli said. Star systems like our own solar system would be more likely to spit out comets than asteroids.

“This means that the original predictions that the first interstellar visitor should be a comet were correct,” University of Hawaii’s Meech said in an email. “It seems that each time we look at new data from ‘Oumuamua we get some new surprises. The fact that we conclude that it is a comet makes it more interesting to me — because this comet has some unusual characteristics — and is a bit different from comets in our solar system. It seems to be lacking small dust (and this may be related to its journey through space), and its chemistry may be different from comets in our solar system, which gives us a glimpse of the chemical make up in another planetary system.”

Though the dark red color, or its motion, wasn’t unusual compared to asteroids and comets seen in our solar system, the elongated cigar shape is. If the star system that ‘Oumuamua came from has giant gas planets and it got close to those planets, it might have been torn into pieces as it was ejected, hence its long, thin shape.

Astronomers know that when our solar system was forming, it effectively spit out comets and asteroids because of the orbits of the largest planets. So it would stand to reason that other planetary systems are sending the same remnants our way.

That means this visitor could carry the secrets to how other solar systems have formed.

There are most likely between one and 10 of these types of “visitors” per year in our solar system — but they move so fast that we’ve never been able to see or study them until ‘Oumuamua.

“Unfortunately ‘Oumuamua is now too far from Earth to be observable, so any new study will need to rely on data collected over the past few months,” Micheli said. “However, we are ready for the next one that will hopefully be discovered.”