The debate over the origins and molecular structure of ‘Oumuamua continued today with an announcement in The Astrophysical Journal Letters that despite earlier promising claims, the interstellar object is not made of molecular hydrogen ice after all.

The earlier study, published by Seligman & Laughlin in 2020—after observations by the Spitzer Space Telescope set tight limits on the outgassing of carbon-based molecules—suggested that if ‘Oumuamua were a hydrogen iceberg, then the pure hydrogen gas that gives it its rocket-like push would have escaped detection. But scientists at the Center for Astrophysics | Harvard & Smithsonian (CfA) and the Korea Astronomy and Space Science Institute (KASI) were curious whether a hydrogen-based  could actually have made the journey from  to our solar system.

“The proposal by Seligman and Laughlin appeared promising because it might explain the extreme elongated shape of ‘Oumuamua as well as the non-gravitational acceleration. However, their theory is based on an assumption that H2 ice could form in dense molecular clouds. If this is true, H2 ice objects could be abundant in the universe, and thus would have far-reaching implications. H2 ice was also proposed to explain dark matter, a mystery of modern astrophysics,” said Dr. Thiem Hoang, senior researcher in the theoretical astrophysics group at KASI and lead author on the paper. “We wanted to not only test the assumptions in the theory but also the dark matter proposition.” Dr. Avi Loeb, Frank B. Baird Professor of Science at Harvard and co-author on the paper, added, “We were suspicious that hydrogen icebergs could not survive the journey—which is likely to take hundreds of millions of years—because they evaporate too quickly, and as to whether they could form in molecular clouds.”

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