“The existence of advanced extraterrestrial life is no more speculative than dark matter or extra dimensions,” says Harvard astronomer Avi Loeb about speculating about the possible origin of the first interstellar object to be detected in our Solar System. In fact, he says, “I think it’s the other way around.”
In November of 2018, Loeb suggested that it was unlikely that 1015 solar sails created by an advanced alien civilization are launched per star to make up a random population of ‘Oumuamua-like interstellar objects. This would require, he observed that “the unreasonable rate of a launch every five minutes from a planetary system even if all civilizations live as long as the full lifetime of the Milky Way galaxy. Instead, the required numbers could be reduced dramatically if ‘Oumuamua-like objects do not sample all possible orbits randomly but rather follow special orbits that dive into the innermost, habitable regions of planetary systems like our solar system.”
From Alien Spaceship to Hydrogen Iceberg
Fast forward to 2020 following observations by the Spitzer Space Telescope set tight limits on the outgassing of carbon-based molecules scientists Seligman & Laughlin 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 Harvard Smithsonian Center for Astrophysics and the Korea Astronomy and Space Science Institute (KASI) concluded in an announcement in The Astrophysical Journal Letters that a hydrogen-based object could not have actually have made the journey from interstellar space to our solar system.
Unlikely to Survive a Journey of Hundreds of Millions of Years
“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.”
Harvard’s Loeb, Frank B. Baird Professor of Science 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.”
Origin in Giant Molecular Clouds?
Traveling at a blistering speed of 196,000 mph in 2017, ‘Oumuamua was first classified as an asteroid, and when it later sped up, was found to have properties more akin to comets. But the 0.2km radius interstellar object didn’t fit that category, either, and its point of origin has remained a mystery. Researchers focused on the giant molecular cloud (GMC) W51—one of the closest GMCs to Earth at just 17,000 light years away—as a potential point of origin for ‘Oumuamua, but hypothesize that it simply could not have made the journey intact.
“The most likely place to make hydrogen icebergs is in the densest environments of the interstellar medium. These are giant molecular clouds,” said Loeb, confirming that these environments are both too far away and are not conducive to the development of hydrogen icebergs.
An accepted astrophysical origin for solid objects is growth by sticky collisions of dust, but in the case of a hydrogen iceberg, this theory could not hold together. “An accepted route to form a km-sized object is first to form grains of micron-size, then such grains grow by sticky collisions,” said Hoang. “However, in regions with high gas density, collisional heating by gas collisions can rapidly sublimate the hydrogen mantle on the grains, preventing them from growing further.”
Although the study explored destruction of H2 ice by multiple mechanisms including interstellar radiation, cosmic rays, and interstellar gas, sublimation due to heating by starlight has the most destructive effect, and according to Loeb, “Thermal sublimation by collisional heating in GMCs could destroy molecular hydrogen icebergs of ‘Oumuamua-size before their escape into the interstellar medium.”
The Mystery Endures
This conclusion precludes the theory that ‘Oumuamua journeyed to our solar system from a GMC, and further precludes the proposition of primordial snowballs as dark matter. Evaporative cooling in these situations does not reduce the role of thermal sublimation by starlight in the destruction of H2 ice objects.
‘Oumuamua first gained notoriety in 2017 when it was discovered screaming through space by observers at Haleakalā Observatory, and has since been the subject of ongoing studies. “This object is mysterious and difficult to understand because it exhibits peculiar properties we have never seen from comets and asteroids in our solar system,” said Hoang.
While the nature of the interstellar traveler is currently an unsolved mystery, Loeb suggests it won’t remain so for much longer, especially if it’s not alone. “If ‘Oumuamua is a member of a population of similar objects on random trajectories, then the Vera C. Rubin Observatory (VRO), which is scheduled to have its first light next year, should detect roughly one ‘Oumuamua-like object per month. We will all wait with anticipation to see what it will find.”
Source: Thiem Hoang et al. Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 (‘Oumuamua), The Astrophysical Journal (2020). DOI: 10.3847/2041-8213/abab0c
The Daily Galaxy, Max Goldberg, via Harvard-Smithsonian Center for Astrophysics