What Is Oumuamua? How Is It Different From Other Comets?

Oumuamua or 1I/2017 U1 is the first interstellar object that was detected while crossing the solar system. It was observed on October 19, 2017.

On October 19th, 2017, the University of Hawaii’s Pan-STARRS1 telescope observed the first known interstellar object crossing the solar system. It was named Oumuamua (pronounced oh-moo-ah-moo-ah), which means ‘scout from the distant past.’

It has been wandering through the Milky Way for millions of years. However, scientists were able to collect data about this unusual object for only a short period. They were unable to detect it earlier because it was too far from the Sun to reflect enough light to be visible.

Discovery of Oumuamua

Oumuamua first caught astronomers’ eyes as it zipped past the Sun at an unnaturally high speed—about 196,000 miles per hour, or 87.3 kilometers per second. That means it was traveling too fast to have originated in the solar system. Objects from within our solar system typically move at an average of 12 miles per second.

Oumuamua comet, interstellar object passing through the Solar System

An artist’s rendition of Oumuamua (Photo Credit : Dotted Yeti/Shutterstock)

Another clue that led scientists to believe that it was a foreign visitor was its trajectory. It was first observed a month after its closest approach to the Sun. This was when it was within the orbit of Mercury. It hurtled past the Sun from above the plane of the planets on an inclined orbit. It was moving fast enough to escape the Sun’s gravity.

Additionally, it showed powerful non-gravitational acceleration. Its motion showed that there was something other than gravity dictating its path. Ultimately, scientists concluded that the most likely reason for this acceleration was outgassing.

A final weird feature of Oumuamua was its shape. It appeared to be elongated, like a cigar, up to 400 meters long and about 40 meters wide. This aspect ratio is higher than that of any other object that has been observed in the solar system.

Is Oumuamua A Comet Or An Asteroid?

At first, astronomers assumed that Oumuamua was a comet. A comet is a small body of frozen gases, rocks and dust that orbits the Sun. Due to the Sun’s heat, it can spew dust and gas into a sizable glowing head. The dust and gases develop a tail that stretches away from the Sun. However, Oumuamua appeared in telescopic images as a single point of light, without a tail. Hence, astronomers concluded that it was an asteroid.

a swarm of asteroids in front of the Milky Way galaxy

Asteroids (Artist’s rendition) (Photo Credit : Dotted Yeti/Shutterstock)

Further observations revealed that it was accelerating slightly. Scientists realized that the tail and the jets simply might not be visible to their telescopes. They hypothesized that ‘outgassing’, the spurting of volatile materials, might be the reason behind its acceleration. It was causing a slight change in its trajectory from earlier predictions. This acceleration was a sign that it was behaving more like a comet. As a result, scientists reverted to their original conclusion. Oumuamua was a comet, albeit an unusual one.

High quality star and comet universe on black background

This is what a comet with a tail generally looks like (Photo Credit : KPP/Shutterstock)

What Do We Know about Oumuamua So Far?

Scientists had very little time to investigate this interstellar visitor (roughly a year). As a result, they were able to collect limited, yet significant information about it. They know for sure that it came from outside our Solar System. Its trajectory is hyperbolic, and it came into the Solar System from above the ecliptic plane. Due to the Sun’s gravity, it made a sharp turn under the Solar System and began to move outward. It passed from under the Earth’s orbit, then above the orbits of Mars and Saturn. On its outward journey, it again traveled above the ecliptic orbit. Eventually, after December of 2018, it traveled too far for scientists to continue observation.

Scientists witnessed dramatic variations in its brightness over time. Thus, they concluded that it was highly elongated, with a convoluted shape. These variations also suggested that it was rotating about two different axes. We also know from its trajectory that it will not be coming back.

Its composition is similar to asteroids. It is dense and either rocky or metallic. The surface has a reddish hue due to irradiation from cosmic rays. It is completely inert, without any dust, water or ice.


There are still many things that we don’t know about this object. We don’t know where it originated, and scientists don’t know much about its composition. We have very little information about what it is doing now, and we aren’t likely to ever see it again.

Earlier, the division between asteroids and comets used to be very clear. Asteroids had circular orbits. They were made of rock and metal and lived in the asteroid belt between Mars and Jupiter. Comets had tilted elliptical orbits. They were made of rock and ice and came from outside the solar system.

However, in recent years, scientists have found asteroid families all over the solar system. They have also found comets with both long and short orbits. They have discovered unusual features on objects they thought they knew very well. For example, the asteroid Ceres has salts on its surface. These salts were left behind as ice vaporized by sublimation. This process is typical of comets, not asteroids!

The asteroid Ceres

The asteroid Ceres (Photo Credit : Justin Cowart/Wikimedia Commons)

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In the past few years, Oumuamua and many other objects seem to be displaying this kind of ambiguity. For this reason, scientists believe it may be time to think beyond the binary of asteroids and comets. It may be time to come up with a variety of new categories. The discovery of such objects shows, very clearly, that we have not yet uncovered all the objects or secrets of our Solar System. There is still a great deal to discover!

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