Our solar system is a big place. We sent the Voyager probes on a one-way trip out of the sun’s domain in 1977. It took the two crafts more than 35 years to reach the edge of the heliosphere, the point where they entered interstellar space. And they only had to travel approximately the distance of the system’s radius, not the diameter, which is 18.75 trillion kilometers long!

Though most of this region is an empty vacuum, an exorbitantly high number of bodies inhabit the solar system. In addition to Sol, we have eight planets (or nine, according to Pluto fanatics), five dwarf planets (or four, according to Pluto fanatics), at least 226 moons, at least 4,500 comets, and at least 1.1 million asteroids. From there, the numbers get whacky. Scientists estimate that as many as 25 million meteoroids hit the Earth every day, to the tune of adding 48.5 tons of mass during each rotation. The number of infinitesimal objects out there that don’t come anywhere near our planet must be unfathomably large.

And, of course, our solar system is just a speck on a speck on a speck when it comes to the scope of the observable universe. We cannot reasonably examine how many objects exist in other systems, galaxies, or interstellar space. Astrophysicists postulate that interstellar objects – entities not bound gravitationally to just one star – exist in numbers that approach infinity and likely zip from one solar system to another. Despite the vastness of space, odds are that foreign bodies pass through our neighborhood with regularity.

Yet, until 2017, every single object we have observed within our solar system originated within our solar system. That’s when an astronomer in Hawaii discovered the first alien body, which they named ʻOumuamua.

The Haleakala Observatory - photo by dronepicr

Using the Pan-STARRS telescope at Haleakalā Observatory on Maui, Robert Weryk noticed a strange object on 19 October 2017. The body was relatively small and non-luminous, making it difficult to detect and study. By the time Werky spotted ʻOumuamua, it was already on its way away from the sun. Other telescopes got into the game to study the body, including the Hubble. Its trajectory indicated it had come from where the star Vega currently resides, though that sun would not have been there when ʻOumuamua visited those coordinates.

Interstellar objects can take many forms, mostly similar to what we find in our solar system. Rogue planets that were somehow ejected from a system. Rogue comets that don’t orbit a star. Asteroids that originated somewhere else. Most scientists believed, if an interstellar object reached our system, it would likely be a comet. Planets don’t tend to move like asteroids or comets. Asteroids don’t tend to have the long-haul attribute or staying power of their bigger, icier comet cousins. So, when ʻOumuamua showed up near the sun, many astronomers pegged it as a comet.

The more we peeked at it, however, the odder the story became. ʻOumuamua is bizarrely shaped. Its length is approximately ten times as long as its width, making it appear like a cigar or a pancake, depending on your viewpoint. Because it’s relatively dark and tiny, it’s hard to know with certainty its dimensions, but its width is somewhere between 35 and 167 meters (115 to 548 feet) and its length is between 100 and 1,000 meters (300 and 3,000 feet). This topology is rather unorthodox for a comet.

An artist's rendering of what ʻOumuamua might look like - NASA

Though dim, ʻOumuamua’s brightness varied greatly, by as much as a factor of 10 as it spins on its axis. According to NASA, no “known asteroid or comet from our solar system varies so widely in brightness, with such a large ratio between length and width.” The biggest ratio they have detected is about three times as long as it is wide.

Further, ʻOumuamua lacked a telltale marker for comets: it had no tail or coma. Comets typically have dust or gas tails that spew off them as they hurtle through space, often enlarged when the body nears a star’s melting power. So, perhaps it was an asteroid, instead. Then astronomers noted it was actually accelerating and moving too quickly to be an inert rock. Comets, thanks to the ejection of gases, lie in the domain of acceleration, but not asteroids.

If it’s not a comet and it’s not an asteroid, what exactly is it?

Some serious astronomers floated the notion that ‘Oumuamua is alien technology. The chair of the Harvard Astronomy Department wrote, “one possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as a debris from an advanced technological equipment.” 

This viewpoint was skewered by the majority of space scientists. Radio astronomers pointed the best human technology at the entity to see if any broadcasts might be coming from extraterrestrial intelligence. Alas, the speakers played no alien prog rock. ‘Oumuamua was most likely just boring space rock.

The question remained: what is ‘Oumuamua? The alien theory might sound absurd, but it elucidates a point that we framed the query in human terms. We know comets to be like X and asteroids to be like Y. But do they need to be like X or Y everywhere in the universe? Perhaps a rogue interstellar object might display novel characteristics. Or perhaps ‘Oumuamua is not a comet or an asteroid, but a new type of object completely.

Scientists pondered other forms. Some wondered if ‘Oumuamua were a “cosmic dust bunny,” since it seemed to be light, non-dense, and “fluffy.” Others wondered if it were a massive iceberg of frozen hydrogen or nitrogen that had been encased with a shell thanks to cosmic radiation.

'Oumuamua is circled in this combined image of many satellite snapshots - NASA/ESO/K. Meech et al.

‘Oumuamua didn’t lend scientists much time to study it, as it races away at 59,000 miles per hour. Its size and hyperbolic destination meant we might never know what the first detected interstellar object actually was.

Recently, however, some scientists believe they might have uncovered the mystery of the body’s acceleration. If ‘Oumuamua were a “cosmic dust bunny” made of frozen water, space ice might allow its structure to hide gaseous hydrogen internally. The nature of space ice might create “fluffiness” within an object, with air pockets sort of like a sponge. When ‘Oumuamua went around the sun, it’s possible that the heat it experienced altered the structure a bit, allowing some of the gas to escape. Calculations done by the University of Hawaii show that hydrogen escaping in this way could explain the acceleration detected in ‘Oumuamua.

Since we caught the potential cosmic fluff ball on its way outward, there’s no way we’ll ever really know if this proposed solution is correct. Some scientists believe ‘Oumuamua is just an ordinary comet that discharged enough water and gas to accelerate it, but it’s so dark and small that we didn’t detect those discharges.

The trajectory of 'Oumuamua - graphic by Tomruen

Either way, the era of interstellar objects is now begun.

ʻOumuamua (oh MOO-uh MOO-uh) is a Hawaiian word that translates directly to “a messenger from afar arriving first.” In Hawaiian, ou means “reach out for” and mua means “first” or “in advance of.” The diacritical mark that opens the moniker is not an apostrophe but an ‘okina, a Hawaiian character that prompts a guttural stop. Mua was officially doubled for emphasis, but it doesn’t hurt that it formed a name that rolls off the tongue. Its official designation is 1I/2017 U1. The “I” stands for Interstellar Object. 1I is the first in what will likely be a long string of newly classified bodies.

In 2019, telescopes detected a second interstellar object, designated as 2I/Borisov. This body left no mystery: it is an interstellar comet. Though we’ve only detected two, this reality has less to do with the rarity of interstellar objects than it does with our detection methods. Moving forward, however, scientists predict we’ll start to find at least one per year. This prospect has already prompted some astronomers to plan for missions to intercept an interstellar object, which would lead to a treasure trove of universal information.

Barring the discovery of faster-than-light travel or wormhole shortcuts, we’ll likely never know exactly what ʻOumuamua is. The smart money seems to be on comet, but asteroid and spacecraft can never be disproved! The first interstellar scout currently rockets toward the constellation Pegasus.

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