A visitor from another star — that was the news a few days after Oct. 19, 2017, when researchers discovered an enigmatic cigar-shaped object screaming away from the solar system at nearly 57,000 mph (92,000 km/h). The entity was moving too fast to have originated from our own system, traveling on a U-shaped hyperbolic orbit that took it around the sun and sent it back out into interstellar space.
Astronomers using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS 1) telescope in Hawaii uncovered the odd object, which was at first designated as A/2017 U1. But soon, it was given the name 'Oumuamua, which in Hawaiian means "scout" or "visitor from afar arriving first," according to the International Astronomical Union. ['Oumuamua: The Solar System's 1st Interstellar Visitor Explained in Photos]
"It's long been theorized that such objects exist — asteroids or comets moving around between the stars and occasionally passing through our solar system — but this is the first such detection," Paul Chodas, manager of the Center for Near-Earth Object Studies at the NASA's Jet Propulsion Laboratory in Pasadena, California, said in a statement.
Researchers immediately trained as many telescopes on 'Oumuamua as possible in order to learn more about the object before it became too dim for any instrument to see. They found it to be between 1,300 feet (400 meters) and 2,600 feet (800 m) long and quite thin — perhaps six times smaller in width than in length. The visitor was also tumbling end-over-end and had a dark red color, similar to other cold, small objects in the outer solar system. Scientists determined that it had come from the direction of the constellation Lyra, though no one has yet figured out precisely what system it originated in.
So, what is it?
'Oumuamua was classified as a comet when it was first spotted. But the fact that it did not produce a large puffy coma that streams out behind as a tail, as comets normally do when they are heated by the sun, led astronomers to second guess themselves. Some researchers argued that it was a rocky asteroid or perhaps even a new class of interstellar object.
Others have speculated that 'Oumuamua is a planetesimal, or a chunk of rock normally incorporated into a larger planet that was flung from its native system by gravitational forces. And it might even be a zombie comet or, more precisely, "a monstrous, extremely fluffy aggregate of loosely bound dust grains," as a paper about the idea put it, left behind by a comet that disintegrated as it approached the sun.
By and large, the current consensus in the scientific community is that 'Oumuamua is a comet because it's moving slightly faster than expected if it were just being propelled by gravitational tugging from the sun. The most likely explanation is that the object is outgassing material, like a comet does, creating jets that push it along. Observations suggest that it is 10 times more reflective than our solar system's homegrown comets, perhaps because 'Oumuamua's outgassing has blasted dust from its surface, revealing bright ice underneath.
A visitor from outer space?
This jet-driven motion almost makes 'Oumuamua sound like a spacecraft that came from another star, took a quick pass through our system, and then sped away. Even astronomers have been intrigued by this possibility, and researchers at the Search for Extraterrestrial Intelligence (SETI) listened for any potential technological signals leaking from the object not once, but twice. Both campaigns heard nothing.
Along with his co-author Shmuel Bialy, Avi Loeb of the Harvard-Smithsonian Center for Astrophysics released his own conjecture that 'Oumuamua was a gossamer-thin lightsail (a type of propulsion using the sun's photons) from an advanced technological civilization. His reasoning centered on the fact that outgassing would have altered the visitor's rotation period — an effect that should have been easy to identify but wasn't seen.
Loeb suggested that 'Oumuamua could be a defunct piece of space junk that accidentally landed in our system or potentially an exploratory ship sent to check out our region. Few scientists have backed Loeb's claims, but he contends that dismissing the possibility of aliens out of hand eliminates useful hypotheses that should be acceptable parts of rational inquiry.
"Science should be open-minded," he told Space.com when first announcing his idea.
After seeing 'Oumuamua, researchers are now eager to spot other similar objects. Greg Laughlin, an astronomer at Yale University, has estimated that there could be trillions upon trillions of 'Oumuamua-like entities drifting through the Milky Way. This lends credence to the idea that microbes might have hitched a ride on 'Oumuamua-type objects in the past and spread throughout space — a theory known as panspermia.
Astronomers might not have to wait too long to find more interstellar objects. The Large Synoptic Survey Telescope (LSST) in Chile, scheduled to come online early next decade, will scan the night sky in unprecedented detail and could discover another 'Oumuamua or two.