Sorry, E.T. fans: Interstellar visitor 'Oumuamua isn't an alien spacecraft. It's just passing gas.

An artist's depiction of the interstellar comet 'Oumuamua, as it warmed up in its approach to the sun and outgassed hydrogen (white mist), which slightly altered its orbit.
Leaking hydrogen may be speeding up the mysterious interstellar object 'Oumuamua. (Image credit: NASA, ESA and Joseph Olmsted and Frank Summers of STScI)

Since its surprise arrival in the solar system in 2017, the interstellar object 'Oumuamua has puzzled scientists. A duo of American astronomers now think they have solved one of the space rock's lingering mysteries. 

First thought to be an asteroid, later recast as a likely comet, and by some even considered a possible alien spaceship, the 650-foot-long (200 meters) 'Oumuamua zoomed through the central solar system in late 2017. During its brief visit, the rock approached Earth within 15 million miles (24 million kilometers), about 62 Earth-moon distances, and disappeared a few weeks after its discovery

Observations made within this short period of time soon proved that 'Oumuamua was on what astronomers call a "hyperbolic" orbit, a boomerang-shaped trajectory that indicated the rock is not native to our solar system but was only passing through the sun's neighborhood and would never be seen again. 

Related: Interstellar object 'Oumuamua still puzzling scientists 5 years after discovery

The first interstellar body ever observed, 'Oumuamua was a sensation, prompting astronomers all over the world to bury themselves in the available data to learn all there was to learn about the object.

One of the questions that scientists struggled to answer had to do with Oumuamua's speed, which seemed to increase as the rock rounded the sun. Large bodies, such as planets and stars, can give smaller objects, including comets and asteroids, a gravitational kick that speeds them up. But in the case of 'Oumuamua, which at 54 miles per second (87 kilometers per second) cruised three times faster than an average solar system comet, these gravitational kicks couldn't explain the observed acceleration. 

This acceleration swayed many scientists to conclude that, rather than an asteroid, 'Oumuamua must be a comet. Comets in the solar system receive additional momentum from the water and dust that evaporates from their icy nuclei as they warm up close to the sun. These comets, however, are famous for their spectacular tails that light up as a result of the outgassing process. And 'Oumumua showed no signs of a tail. 

Many scientists have tried to explain the mechanism behind 'Oumuamua's acceleration, but all of the proposed ideas have had significant gaps. In a new study, Jennifer Bergner, an assistant professor of chemistry at the University of California, Berkeley and Darryl Seligman, a U.S. National Science Foundation postdoctoral fellow at Cornell University, propose a new theory, which they think might finally put the issue to rest. 

"I have been trying to explain the outgassing for several years now," Seligman told Space.com. "First, I thought that perhaps there just wasn't too much dust in the outgassing [to form the coma]. Later, we thought that perhaps it was made of some more volatile material than what we see in usual comets, such as hydrogen, nitrogen or carbon monoxide. But there were theoretical issues with each of these explanations."

Hydrogen, for example, would require extremely cold temperatures to freeze into objects the size of 'Oumuamua, and scientists don't expect such temperatures inside the dense molecular clouds where these objects form, Seligman said. Nitrogen is not ubiquitous enough in the Milky Way to account for the expected number of such bodies in the galaxy, he added. 

Seligman and Bergner now propose that there might be nothing extraordinary about 'Oumuamua's chemistry. Instead, the object was subject to processes outside the solar system that astronomers don't know about from our observations of domestic comets. 

"A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result," Bergner said in a statement. "Our thought was: If this was happening, could you actually trap it in the body, so that when it entered the solar system and it was warmed up, it would outgas that hydrogen?" 

Related'Oumuamua: The solar system's 1st interstellar visitor explained in photos

The team's calculations showed that, theoretically, the force of this outgassing hydrogen could explain the strange acceleration of 'Oumuamua. The astronomers even found experimental studies more than 40 years old that demonstrated that high-energy particles, such as those present in cosmic rays, can split molecular hydrogen from water ice but keep it trapped inside an ice block. 

"What's beautiful about Jenny's idea is that it's exactly what should happen to interstellar comets," Seligman said in a statement. "We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it's just the most generic explanation."

Some scientists, however, are not satisfied with this explanation. Harvard University's astrophysics professor Avi Loeb in his response to Seligman's and Bergner's study claimed that the two "miscalculated the surface temperature of `Oumuamua," resulting in an incorrect estimate of the available amount of hydrogen in the ice berg. Leob, who had previously suggested that 'Oumuamua may be an alien space ship, thinks that the outgassing of the available hydrogen could not have produced the observable acceleration.

With 'Oumuamua gone forever, we will never know for sure what its chemical composition was. Seligman hopes that new interstellar visitors will be discovered soon that will help astronomers find any remaining answers, and also provide a window into other star systems in our Milky Way galaxy. New telescopes that are expected to supercharge this research are expected to come online within the next decade.

The new study was published online in the journal Nature on Wednesday (March 22). 

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Tereza Pultarova
Senior Writer

Tereza is a London-based science and technology journalist, aspiring fiction writer and amateur gymnast. Originally from Prague, the Czech Republic, she spent the first seven years of her career working as a reporter, script-writer and presenter for various TV programmes of the Czech Public Service Television. She later took a career break to pursue further education and added a Master's in Science from the International Space University, France, to her Bachelor's in Journalism and Master's in Cultural Anthropology from Prague's Charles University. She worked as a reporter at the Engineering and Technology magazine, freelanced for a range of publications including Live Science, Space.com, Professional Engineering, Via Satellite and Space News and served as a maternity cover science editor at the European Space Agency.

  • PSH
    Logically speaking Oumuamua could be an intelligent rock. How do we know how alien life would appear to us. Now if you remember back in 1975 one of the most popular novelty items marketed in the U.S. was the Pet Rock. That was over 45 years ago. If our TV and radio stations broadcasted commercials for the Pet Rock then after 24 years of traveling at the speed of light these broadcasts would have reached dozens of star systems. What if one of these star systems had smart rocks monitoring our broadcasts. They might think we were imprisoning small slow moving helpless rocks to be our pets. They may also have monitored for even a longer period of time our rock music, and liked it. Now if they had advanced technologies that could defy all principles of modern physics (our physics not rock physics) and and they could move mass at the speed of light (requiring an infinite amount of energy), they could be here by 2017 which is when Oumuamua arrived in our Solar System. They probably wanted to visit, hang out and go to a few rock concerts but when we gave their vehicle that ridiculous name, they said these people just are not cool, Oumuamua is just not pronounceable for a rock, and they skipped town. If we called it Crick Crack Bonk we might be rock'n on with it now to some real heavy metal rock like nickel and iron meteors. If you ask me that is irrefutable evidence for smart rocks. :cool::D
    Reply
  • billslugg
    The hydrogen produced by cosmic rays will solidify if the temperature is below 10 K.
    Oxygen will solidify at 55 K. Maybe it outgassed too.

    This new theory explains why it had no tail.
    Reply
  • Pogo
    Occam’s razor . . . Sort of . . . the simplest explanation is frequently the correct one; in reference to the article’s statement that after reviewing exotic explanations for the acceleration, they settled on the simple one.
    Reply
  • Mars Tafts
    I don't believe for a minute that they didn't get at least a radar image of this object much less some good quality optical images from DoD satellites. If they didn't someone was sitting down on the job.
    Reply
  • mod
    I'm no scientist, but, sorry, I don't buy it. If this is the SIMPLEST answer, why wasn't it hypothesized right off the bat six years ago? It just doesn't make sense. I'm not saying it's DEFINITELY aliens like our friend on a certain History Channel TV show, but something with this thing is up if there was such a rush by the community to see if RADIO SIGNALS were being emanated by the object? Is this standard practice for all comets? C'mon people. :)
    Reply
  • billslugg
    Yes, it is the simplest answer but not the most obvious.
    We looked for radio signals because it was on a hyberbolic orbit and had an unusually large difference in brightness as it rotated.
    Reply
  • PSH
    PSH said:
    Logically speaking Oumuamua could be an intelligent rock. How do we know how alien life would appear to us. Now if you remember back in 1975 one of the most popular novelty items marketed in the U.S. was the Pet Rock. That was over 45 years ago. If our TV and radio stations broadcasted commercials for the Pet Rock then after 24 years of traveling at the speed of light these broadcasts would have reached dozens of star systems. What if one of these star systems had smart rocks monitoring our broadcasts. They might think we were imprisoning small slow moving helpless rocks to be our pets. They may also have monitored for even a longer period of time our rock music, and liked it. Now if they had advanced technologies that could defy all principles of modern physics (our physics not rock physics) and and they could move mass at the speed of light (requiring an infinite amount of energy), they could be here by 2017 which is when Oumuamua arrived in our Solar System. They probably wanted to visit, hang out and go to a few rock concerts but when we gave their vehicle that ridiculous name, they said these people just are not cool, Oumuamua is just not pronounceable for a rock, and they skipped town. If we called it Crick Crack Bonk we might be rock'n on with it now to some real heavy metal rock like nickel and iron meteors. If you ask me that is irrefutable evidence for smart rocks. :cool::D
    I just wanted to mention if this alien spacecrafts a rock'n don't come a knock'n.
    Reply
  • Pogo
    What I was referring to was that they hypothesized (is that a word?) some more exotic chemistry to produced the excess acceleration before settling on the more simple hydrogen gas from irradiated water.
    Reply
  • Pogo
    This was a body length of a few hundred meters or a thousand at the most. The close approach was about 33m km. Trig that and you get something like 0.0006 seconds of arc image, lucky if it’s a pixel on an imager. Just no way of an image more than a dot.
    Reply
  • Pogo
    SETI threw their antennas at it and said that any radio emissions would have to be less than 0.08 watts
    Reply