Photo by European Southern Observatory / M. Kornmesser
In October 2017, the Pan-STARRS1 telescope in Hawaii detected an enigmatic object from interstellar space, dubbed ‘Oumuamua, which means “scout” or “messenger” in Hawaiian. This strange visitor baffled astronomers and sparked wild speculations due to its unusual elongated cigar-like shape, lack of bright coma or dust tail, and extraordinary speed of 196,000 miles per hour (315,431 kilometers per hour). However, a group of astronomers from the University of California, Berkeley, believe they have found a simple answer to the mysteries surrounding ‘Oumuamua.
Jennifer Bergner, a UC Berkeley assistant professor of chemistry, explained that as a comet travels through the interstellar medium, it is bombarded by cosmic radiation, which forms hydrogen as a result. She proposed that if this hydrogen were trapped within the comet’s body, it could release the gas upon entering the solar system and getting warmed up. This outgassing of hydrogen could then produce the force required to explain the observed non-gravitational acceleration of ‘Oumuamua.
As the first known interstellar object to pass through our Solar System, ‘Oumuamua was estimated to have a size of 71 x 69 x 12 feet (115 x 111 x 19 meters), which made it unique among well-studied comets. Its small size meant that its gravitational deflection around the sun was altered by the tiny push created when hydrogen gas erupted from the ice.
While most comets exhibit a bright halo or coma and tails of gas and dust upon approaching the sun, ‘Oumuamua lacked these features and was too small and distant from the sun to capture sufficient energy to eject much water. This led to a myriad of theories about its composition and propulsion, including being a hydrogen-releasing iceberg, a fluffy snowflake pushed by the sun’s light pressure, an alien-designed lightweight sail, or a spacecraft with its own propulsion.
Bergner’s research builds upon experiments from the 1970s to 1990s, which showed that when ice is bombarded by high-energy particles like cosmic rays, an abundance of molecular hydrogen is produced and trapped within the ice. She found that high-energy electrons, protons, and heavier atoms could convert water ice into molecular hydrogen and that the comet’s snowball-like structure could trap the gas within the ice. When heated by the sun’s light, the ice would change from an amorphous to a crystalline structure, releasing hydrogen gas in the process.
Together with colleague Darryl Seligman, Bergner calculated that the outgassing of ice on the surface of a comet could emit enough gas to affect the orbit of a small comet like ‘Oumuamua. This finding suggests that ‘Oumuamua could be a standard interstellar comet that has undergone heavy processing, with models consistent with what is observed in the solar system from comets and asteroids.
In conclusion, the mystery of ‘Oumuamua might have a simple explanation after all – the release of trapped hydrogen gas due to the heating by the sun. This finding challenges the more speculative theories about the object’s origins and propulsion and reinforces the idea that ‘Oumuamua is a natural object with a unique shape and composition.