Selections from 2020: No Hydrogen Ice for ‘Oumuamua

Editor’s note: In these last two weeks of 2020, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 (‘Oumuamua)

Published August 2020

Main takeaway:

One more potential explanation for the strange behavior of interstellar asteroid 1I/2017 U1 (‘Oumuamua) has been dashed thanks to a study by Thiem Hoang (Korea Astronomy and Space Science Institute; Korea University of Science and Technology) and Abraham Loeb (Harvard University). Hoang and Loeb show that an ‘Oumuamua-sized body made out of molecular hydrogen ice could not have survived the interstellar journey from its birth site to our solar system.

Why it’s interesting:

outgassing 'Oumuamua

Illustration of outgassing from the surface of interstellar asteroid ‘Oumuamua. [ESA/Hubble, NASA, ESO, M. Kornmesser]

A number of mysteries were raised by the 2017 discovery of ‘Oumuamua, the first interstellar object detected visiting our solar system. Chief among these: what caused ‘Oumuamua’s unexpected non-gravitational acceleration as it sped away from the Sun and off into space? One posited theory was that this interstellar asteroid was actually a hydrogen iceberg. A composition of hydrogen ice could explain the asteroid’s unusual, elongated shape, and sublimation of this ice could drive its extra acceleration. But Hoang and Loeb’s analysis shows that an ‘Oumuamua-sized hydrogen iceberg would be destroyed within 10 million years by starlight heating. Collisional heating would also rapidly wear at such an object in its birthplace, potentially destroying it before it could escape into the interstellar medium and journey to our solar system. And even the formation of such a hydrogen iceberg seems unlikely: Hoang and Loeb show that it’s difficult to form icy grains rich in molecular hydrogen in the dense clouds that could birth an interstellar asteroid.

What this means for dark matter:

The implications of this hydrogen challenge extend beyond ‘Oumuamua. Primordial snowballs — theorized bodies made of molecular hydrogen ice in the early universe — have been proposed as one potential component of dark matter. But Hoang and Loeb’s work shows that such snowballs are unlikely to have been able to form, and even if they had, they would be unable to survive to present day. This suggests we must look elsewhere — both for an explanation for dark matter and for an explanation for ‘Oumuamua’s mysteries.


Thiem Hoang and Abraham Loeb 2020 ApJL 899 L23. doi:10.3847/2041-8213/abab0c