'Oumuamua is the first known object from an alien solar system detected passing through our own. Pronounced Oo-MU-ah-MU-ah, the name comes from the Hawaiian 'oumuamua, meaning "first distant messenger". The object paid a brief visit to the neighborhood in 2017, passing within 24 million miles (38.6 million km) of the sun on Sept. 9. Forty days later, it was discovered with the PanSTARRS telescope on Mt.. Haleakala in Maui, Hawaii. Spanning about 150 feet (45 meters) across and hellbent for parts unknown, astronomers tracked the celestial emissary until December 2017, when it became too faint to see.
'Oumuamua's orbit quickly revealed it didn't belong here. Bodies bound to the sun move in closed elliptical orbits, which resemble squashed circles. 'Oumuamua blew out of nowhere on an open, hyperbolic orbit. It dove toward the sun, zipped around it and never looked back.
Knowing that time was limited, astronomers aimed big telescopes at the interloper to determine its brightness, rotation rate and most importantly, its composition. We soon learned that 'Oumuamua appeared slightly red, had a bizarre elongated shape resembling a cigar and tumbled like a poorly thrown football. Depending on one's interpretation of the data, it might be a comet, an asteroid or even an alien spacecraft. Yet none of these really fit.
Had it been a comet we would have seen evidence of dust and gas in the form of a coma or tail. None was detected. 'Oumuamua was never more than a faint point of light in even the largest telescopes.
Asteroids are plentiful in our solar system, and they look like pinpoints because most are tiny. Presumably they're common in other star systems as well. But astronomers quickly ruled this out because 'Oumuamua did something very unusual. Normally, as a body gets farther from the sun it slows down. Mercury, the closest planet to the sun, orbits in just 88 days at a speed of around 106,000 miles an hour (170,600 km/hour). Compare that to Jupiter, which takes 12 years traveling at 30,000 mph (48,000 km/hour) to complete its orbit.
As 'Oumuamua moved farther from the sun, it sped up! Almost as if propelled by a rocket. Uh-oh, we're back to that alien spacecraft again, right? Not necessarily. The "rocket effect" happens routinely with comets. The sun's heat vaporizes dust-laden ice beneath the surface causing it to expand and blast into space as a narrow fountain of gas and dust called a jet. Jets push back against the comet and give it a kick that can alter its speed and direction. But wait a second. No comet-like material was seen at 'Oumuamua, so what's causing the acceleration?
Two astronomers at the University of Arizona think they have the answer. Alan Jackson and Steven Desch make a compelling case for 'Oumuamua as a shard of solid nitrogen ice. Half a billion years ago, an impact likely ejected it from the surface of a Pluto-like planet orbiting its host sun. The impact was powerful enough to send it reeling into galactic exile. Nitrogen ice vaporizing in sunlight provides just the right amount of kick to account for the observed acceleration as it left the sun's vicinity. Ice also explains 'Oumuamua's changing shape.
"‘Oumuamua likely wasn’t flat when it entered our solar system, but melted away to a sliver, losing more than 95 percent of its mass, during its close encounter with the sun," according to Jackson.
According to their study, the body arrived thicker and more rounded, but solar heating took its toll, vaporizing it into a flattened pancake. Galactic cosmic rays — high-speed atom fragments accelerated by explosive events like supernovae — chiseled away at it before that but much more slowly compared to the sun. Jackson compares 'Oumuamua to a bar of soap that starts out chunky but thins through constant use until only a sliver remains. No known object in our solar system is that flat... except for actual pancakes, of course.
There are lots of different kinds of ice: water, carbon dioxide, nitrogen and even hydrogen are few examples. Jackson and Desch calculated how each would affect the object's mass, shape, rocket effect and reflectivity. Only nitrogen matched the characteristics of 'Oumuamua. The clincher was the perfect fit between the light reflected from 'Oumuamua and the nitrogen ice found on the surfaces of Pluto and Neptune's moon Triton — all were equally shiny. Trace amounts of methane ice, also seen at Pluto, reddens when exposed to ultraviolet (UV) light and could easily explain the object's color.
Next they determined the likelihood of an exo-Pluto fragment arriving in the neighborhood. Four billion years ago, the planets were still jostling for position in the solar system, including Neptune. As it migrated away from the sun into the Kuiper Belt, a region rich in icy asteroids, the planet disrupted the orbits of these small objects. Collisions ensued with many of the resulting fragments ejected into interstellar space.
Assuming that collisions and expulsions are common during solar system evolution, it's estimated that at least one quadrillion (one followed by 15 zeroes) exo-Pluto fragments would be ejected per star in our galaxy. Once liberated, the broken pieces of these interplanetary clashes would wander the galaxy, slowly frittering away under cosmic ray bombardment, until a chance encounter with another star.
Astronomers estimate that about seven interstellar objects pass through the solar system every year. Desch and Jackson hope that future telescopes, like those at the Vera Rubin Observatory/Large Synoptic Survey Telescope in Chile, will find many more. Astronomers will use these instruments to conduct an intensive inventory of small bodies whirring around the sun, near and far.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.