`Oumuamua Was Not a Hydrogen-Water Iceberg

Avi Loeb
5 min readMar 23


Figure from Hoang & Loeb (2023). Left panel: comparison of heating and cooling rates when the object is located at 1.4 times the Earth separation from the sun. Evaporative cooling by H2 is dominant over radiative cooling. Right panel: Surface temperature at different distances, calculated for the case with and without evaporative cooling. Different ratios of molecular hydrogen to water are shown. The bottom line is that evaporative cooling by molecular hydrogen decreases the surface temperature by an order of magnitude.

In a new paper that I submitted this morning for publication in collaboration with Thiem Hoang, we show that the paper published today in Nature by Jennifer Bergner and Darryl Seligman miscalculated the surface temperature of `Oumuamua. Bergner and Seligman suggested that the peculiar acceleration of `Oumuamua can be explained if it was made of water ice which was partly dissociated into hydrogen by cosmic-rays in interstellar space.

However, their surface temperature calculation near the Sun ignored the crucial cooling effect of evaporating hydrogen. By adding the cooling from hydrogen evaporation, our new paper shows that the surface temperature of the iceberg is reduced by an order of magnitude.

The correct calculation of the surface temperature is straightforward. It balances heating by sunlight with radiative cooling from the surface and the additional losses from the energy invested in dislodging hydrogen atoms from the lattice. The latter component was omitted in the thermal model presented by Bergner and Seligman in their “Methods” section, leading to an overestimate of the surface temperature by a factor of 9.

As a result of the decrease in surface temperature, the thermal speed of outgassing hydrogen is reduced by a factor of 3. The original model required that about a third of the hydrogen atoms be separated from water by cosmic-rays, and hence the new result requires all the hydrogen to be separated from water. This makes the model untenable because a full-hydrogen surface resembles the hydrogen iceberg model proposed in a previous 2020 paper by Darryl Seligman. Following this original proposal, I wrote a paper with Thiem Hoang, showing that heating by interstellar starlight would quickly destroy pure hydrogen layers, not allowing them to reach the solar system as `Oumuamua did.

Moreover, the lower surface temperature further limits the thermal annealing of water ice, a key process that is appealed to by Bergner and Seligman as a mechanism for releasing molecular hydrogen.

By now, the Nature paper was celebrated by science journalists worldwide. When I informed one of them about the temperature miscalculation earlier today, he told me that his journal will not post a correction to its original report in order “not to confuse the readers.” This response is appropriate for political matters, when the truth is not easily discerned and multiple opinions are equally valid. However, the benefit of science is that a calculation can be shown to be right or wrong, and the oath of science reporters should be to adhere to a full disclosure of the scientific truth.

Here is the honest truth, as defined by Encyclopedia Britannica: “a comet is a small body orbiting the Sun with a substantial fraction of its composition made up of volatile ices. When a comet comes close to the Sun, the ices sublimate (go directly from the solid to the gas phase) and form, along with entrained dust particles, a bright outflowing atmosphere around the comet nucleus known as a coma. As dust and gas in the coma flow freely into space, the comet forms two tails, one composed of ionized molecules and radicals and one of dust. The word comet comes from the Greek κομητης (kometes), which means “long-haired.” Indeed, it is the appearance of the bright coma that is the standard observational test for whether a newly discovered object is a comet or an asteroid.”

The coma of thousands of solar system comets was always visible. Yet, the first reported interstellar object did not show a coma. This does not allow `Oumuamua to be a generic water iceberg because long-period comets from the Oort cloud of the solar system are also exposed to the same interstellar cosmic-ray environment.

In the first year after the discovery of `Oumuamua, most of the scientific papers written about it argued that this object is weird, unlike the asteroids or comets seen before in the solar system. However, after I proposed the possibility that `Oumuamua might be artificial in origin, there was a series of expert papers insisting that `Oumuamua is a generic object of natural origin. The experts disagreed with each other on what this generic object might be: a hydrogen iceberg, a nitrogen iceberg, a dust bunny, or a hydrogen-water iceberg in the paper that just appeared in Nature.

A co-author of an elaborate review paper on `Oumuamua, told me last year that he believes that `Oumuamua actually had a cometary tail when we did not look at it but did not show the tail when we looked at it. This is like saying that an elephant is a generic zebra that shows its stripes only when we look away from it.

A “dark comet” is a contradiction of terms since all known comets were observed to have a visible cometary tail of gas and dust. `Oumuamua did not exhibit any traces of carbon-based molecules or dust based on deep observations by the Spitzer Space Telescope. It also did not show jitter from jets as a result of uneven sublimation of ice on its surface, nor a substantial evolution in its spin period, as often witnessed for evaporating comets.

Including the other anomalies of `Oumuamua, such as its extremely elongated but flat shape, requires a complex storyline for it to be a generic comet. This is particularly true, given that the interstellar comet 2I/Borisov discovered two years later, appeared like the familiar solar system comets. When “the emperor has no clothes”, we better admit it. Otherwise, we would broadcast to the entire world that we lost our childhood curiosity.


Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, is scheduled for publication in August 2023.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".