'Oumuamua: Invader From Beyond the Solar System

Interstellar asteroid 'Oumuamua as imagined by an artist.
Interstellar asteroid 'Oumuamua appears to be an elongated icy object with a metallic crust. This is an artist's concept of what it might look like up close. ESO/M. Kornmesser

It's not often that an interstellar visitor shaped like a cigar whizzes through the inner solar system. But that's exactly what happened in mid-2017 when the object 'Oumuamua blazed past the Sun on its way back to interstellar space. The strange shape set off a flurry of speculation and wonder. Was it an alien ship? An errant world? Or something even stranger?

Some suggested it resembled a berserker-type machine featured in an early episode of "Star Trek" or a similar interstellar ship featured in one of Sir Arthur C. Clarke's books, "Rendezvous with Rama." Yet, as bizarre as its shape is — which some planetary scientists attribute to a long-ago catastrophic event such as a collision — 'Oumuamua appears to be an otherwise normal icy asteroid blanketed with a metallic crust. In other words, it's another rocky-looking space object passing by for astronomers to study.

Finding 'Oumuamua

An observation of 'Oumuamua made by the William Herschel Telescope on i late October, 2017. 'Oumuamua is the stationary dot in the center; the long trailed lines are stars that were smeared as the telescope tracked the asteroid. Alan Fitzsimmons (ARC, Queen's University Belfast), Isaac Newton Group

By the time 'Oumuamua was discovered on October 19, 2017, it was about 33 million kilometers from Earth and had already passed very close to the Sun on its trajectory. At first, observers weren't sure if it was a comet or an asteroid. In telescopes, it appeared as a tiny point of light. 'Oumuamua is very small, only a few hundred meters long and about 35 meters wide, and appeared through telescopes as just a tiny point of light. Still, planetary scientists were able to figure out its direction and speed (26.3 kilometers per second or more than 59,000 miles per hour).

Based on observations made by telescopes and specialized instruments based in Hawai'i, La Palma, and elsewhere, 'Oumuamua has a darkened crust similar to bodies in our own solar system that are icy but have been irradiated by cosmic rays and ultraviolet radiation from the Sun over long periods of time. In this case, cosmic rays have zapped the surface for billions of years as 'Oumuamua traveled through space. That bombardment created a carbon-rich crust that protected the interior from melting as ‘Oumauma passed by our star.

The name 'Oumuamua is the Hawaiian word for "scout", and was chosen by the team operating the Pan-STARRS telescope located on Haleakala on the island of Maui in Hawai'i. In this case, it's on a scouting mission through the solar system, didn't pose any threat to Earth (some asteroids do), and will never be seen again.

'Oumuamua's Origins

'Oumuamua trajectory as seen from EArth.
This is 'Oumuama's apparent path through the sky as seen from Earth. It appears to have originated in the direction of the constellation Lyra, and is moving toward Pegasus. Tom Ruen, via Wikimedia, Creative Commons Attribution-Share Alike 4.0.

As far as we know, this odd little worldlet is our first visitor from outside of our solar system. No one is quite sure exactly where 'Oumuamua originated in our neighborhood of the galaxy. There's speculation about some relatively young star groupings in the constellations Carina or Columba, although they are no longer along the path the object has traveled. That's because those stars, too, are moving through the galaxy.

Based on its trajectory and makeup, it's likely that our solar system is the first one the object has encountered since it was "born." Like our own Sun and planets, it formed in a cloud of gas and dust billions of years ago. Some astronomers suspect it might have been part of a planet that was broken apart in another star system when two objects collided early in the history of a star system.

Which star was its birth parent, and what happened to create 'Oumuamua are mysteries that remain to be solved. In the meantime, there's a wealth of data to be studied from all the observations made of this strange little world.

As for whether the object is really an alien spacecraft, some radio astronomers aimed the Robert C. Byrd Greenbank Telescope in West Virginia at 'Oumuamua to see if it could detect any intelligent signals that might be emanating from it. None were observed. However, from the studies of its surface, this little object is more similar to icy worlds in our own solar system than it is to an alien ship. That similarity actually tells astronomers that conditions for forming worlds in other solar systems are similar to the ones that created our own Earth and Sun, more than 4.5 billion years ago.