Is Oumuamua really an alien spacecraft? Scientists have put forward the latest answer

Is Oumuamua an alien spaceship? Scientists have put forward the latest answer

Observatories including NASA's Hubble Space Telescope have discovered that Oumuamua has strange non-gravitational accelerations that may come from comet-like jets of gas. Video credit: NASA's Goddard Space Flight Center/Katrina Jackson Subtitle production: Wowzha

Strange Visitors from Space

In 2017, scientists discovered a mysterious small celestial body that slingshot-like around the sun, through the inner solar system, and then flew out into interstellar space. This is the first visitor from outside the solar system, with a strange shape, between a cigar and a pancake. Astronomers named it 'Oumuamua (officially known as 1I/2017 U1), which means "the first distant messenger" in Hawaiian.

‘Oumuamua has characteristics that are different from any known asteroid or comet: it does not have a coma or tail like most comets, and is very small in size, more like an asteroid; but as it accelerates away from the sun and flies into interstellar space, contrary to all expectations, it has non-gravitational acceleration.

This has led some people to suspect that Oumuamua might be an alien spaceship.

That brief flyby gave researchers just a few weeks to study the mysterious object, leaving us with even more puzzles before it disappeared forever. Even more than five years after its discovery, researchers are still struggling to make sense of the few observations they managed to get.

This composite image shows the interstellar object ‘Oumuamua. As telescopes tracked the mysterious object as it moved, it was surrounded by faint trails of stars. Image credit: ESO/K. Meech et al.

What exactly is ‘Oumuamua made of? What force is accelerating it outward? Is it a comet, an asteroid, or, as some claim, an alien spacecraft powered by a light sail? These questions have sparked endless speculation and imagination among scientists and the public.

Now, in a paper published March 22 in the journal Nature, an astrochemist at the University of California, Berkeley and an astronomer at Cornell University suggest that ‘Oumuamua’s deviation from its mysterious hyperbolic trajectory around the sun could be explained by a simple physical mechanism.

This mechanism may be common in many icy comets, that is, as the comet heats up in the sunlight, solid hydrogen turns into gas, and the release of gas pushes the comet to accelerate. If this hypothesis is true, it will strengthen the argument that Oumuamua is not some extremely strange and unique celestial body, and certainly not an abandoned alien spacecraft, but a natural comet-like body.

Too small, too far, too mysterious

Unlike all the well-studied comets in the solar system, ‘Oumuamua is so small that the tiny thrust created by the release of hydrogen from its ice and ejection would slightly change its gravitational deflection around the sun.

Most comets are essentially "dirty snowballs" that periodically pass close to the Sun from the outer regions of the Solar System. When heated by sunlight, comets eject water and other molecules, producing a bright coma and often a tail of gas and dust. The ejected gas acts like thrusters on a spacecraft, giving the comet a tiny push that causes its orbit to deviate slightly from the elliptical orbits of other Solar System objects, such as asteroids and planets.

Artist's concept of 'Oumuamua. Its 10:1 aspect ratio is unlike anything we've seen in our solar system. Image credit: ESO/M. Kornmesser

When 'Oumuamua was discovered, it had no coma or tail and was too small and too far from the Sun to have released enough water, leading astronomers to speculate about its composition and the possibility of being propelled by external forces. Is it a solid piece of hydrogen ice that is spewing hydrogen gas? Or is it a large, fluffy "snowflake" pushed forward by the pressure of sunlight?

The most common explanation

Jennifer Bergner, an assistant professor of chemistry at the University of California, Berkeley, thought there might be a simpler explanation. She investigated the question with her Cornell colleague Darryl Seligman, now a National Science Foundation postdoctoral fellow at Cornell.

"Comets are basically being baked by cosmic radiation as they travel through the interstellar medium, creating molecular hydrogen. Our idea was: if that happens, could this hydrogen be stored inside the comet and then released as the comet warms up as it enters the solar system?" Bergner said. "Could that quantify the amount of thrust needed to account for non-gravitational acceleration?"

Bergner, who studies chemical reactions on cold rocks in the frigid vacuum of space, found experimental studies published in the 1970s, 1980s and 1990s showing that large amounts of molecular hydrogen are produced and locked up in ice when it is hit by high-energy particles like cosmic rays, which can penetrate tens of meters of ice and convert a quarter or more of the water into hydrogen.

For comets a few kilometers long, the source of the hydrogen release is too small relative to the size of the entire comet, so it is difficult to detect the corresponding changes in composition and acceleration. But Oumuamua is so small that Bergner and Seligman believe that the hydrogen release actually generated enough thrust to drive its acceleration.

The reddish object is only about 115 x 111 x 19 meters in size, and because it is too small and too far away for telescopes to resolve, astronomers can only confirm its relative size. So far, all comets we have observed in the solar system have sizes between 1 kilometer and a few hundred kilometers, whether they are short-period comets from the Kuiper belt or long-period comets from the more distant Oort cloud.

This animation shows the trajectory of ‘Oumuamua as it travels through the inner solar system. Image credit: NASA/JPL-Caltech

Seligman said this is exactly what should happen with interstellar comets: "All these silly ideas we had before, like solid hydrogen ice and other crazy speculations about alien civilizations, the truth may just be the most common explanation."

Messenger from afar

4.5 billion years ago, when the solar system was first formed, many ice rocks were produced, which are comets in the solar system. They retain information about the evolutionary stages of the solar system. Studying them is a way to review the early formation stages of the solar system.

Distant planetary systems also seem to have comets, and many comets may be ejected due to the gravitational effects of other celestial bodies in the system, which has also happened in the history of the solar system. Some wandering interstellar comets will accidentally enter our solar system, bringing us more information about the planets outside the solar system.

In the past, astronomers published many papers about our inability to observe any interstellar comets. Then, along came ‘Oumuamua.

‘Oumuamua is so small that it appears as just a point of light in even the largest telescopes. But we know it must be a highly elongated object because its brightness varies greatly over a seven- to eight-hour period. Image credit: NASA/JPL-Caltech

On October 19, 2017, on the island of Maui, Hawaii, using the Pan-STARRS1 telescope of the Institute for Astronomy at the University of Hawaii in Manoa, astronomers observed for the first time a mysterious object that they believed to be a comet or asteroid. When they discovered the object's tilted orbit and high speed (87 kilometers per second), they realized that it came from outside the solar system and named it Oumuamua.

‘Oumuamua was the first interstellar object observed in the solar system other than a dust particle. The second, 2I/Borisov, was discovered in 2019, but it looked and behaved more like a typical comet.

As more and more telescopes focused on ‘Oumuamua, astronomers mapped its orbit and determined that it had passed beyond the sun and was heading out of the solar system.

How to speed up?

"We have never seen a comet without a coma in the solar system, so the non-gravitational acceleration of Oumuamua is really strange," Seligman said. This has triggered a series of speculations about the volatile molecules that may be contained in the comet that can cause the acceleration. Seligman himself has published a paper arguing that if the comet is composed of solid hydrogen (hydrogen icebergs), it will release enough hydrogen gas under the heating of the sun to explain the strange acceleration. Under the right conditions, comets composed of solid nitrogen or solid carbon monoxide can also release enough gas to affect the comet's orbit.

"Oumuamua is consistent with a standard interstellar comet, but it has undergone more changes," Bergner said. "The models we run are consistent with what we see in comets and asteroids in the solar system. So you can start with something that looks like a comet and have this happen." Oumuamua may have started out as an ordinary water-rich comet around a nearby star, and as it traveled through the interstellar medium, it received a "baptism" of cosmic rays, forming hydrogen. Then, as it entered the solar system and was heated, the hydrogen was released, producing the observed anomalous acceleration.

This idea also explains the lack of a dust coma.

"Even if there's dust in the ice matrix, the ice doesn't sublime but rearranges itself, allowing the hydrogen to escape. So the dust doesn't even get out," Seligman said.

How can a solid hydrogen object survive about 100 million years in interstellar space? Skeptical astronomers say the new theory may never be fully accepted, given the unbridgeable gaps in our knowledge of ‘Oumuamua. “It’s a good idea, I just don’t know if it can be proven,” said Karen Meech, an astronomer at the University of Hawaii who was not involved in the new research.

Michie also noted that the new paper doesn't explain 'Oumuamua's odd shape, a point Bergner agreed with, but Seligman's previous research suggests that if circles or disks of solid hydrogen ice exist, they could be eroded into elliptical shapes by cosmic rays.

"Dark" comet

Since 2017, Seligman, Bergner and their colleagues have identified six additional small bodies for which no coma has been observed that also have small nongravitational accelerations, suggesting that such "dark" comets are common. One of these dark comets, 1998 KY26, is the next target of Japan's Hayabusa2 mission, which is expected to be a "dark" comet in December 2022.

As for ‘Oumuamua itself, Bergner argues that with the icy interloper gone, there’s no way to verify the authenticity of this (or any other) new study’s findings.