Avi Loeb’s response on ‘Oumuamua
On March 22, 2023, EarthSky published word of a new peer-reviewed study – conducted by researchers at UC-Berkeley and Cornell – suggesting that the strange object known as ‘Oumuamua is not as strange as many had first thought. The new work suggested that ‘Oumuamua, which streaked past our sun in 2017, is simply a comet from another solar system. Meanwhile, for the past several years, Harvard astronomer Avi Loeb has been the most vocal scientist in putting forward theories that suggest ‘Oumuamua might be an alien spacecraft. Late in the day on March 23, EarthSky received an email from Avi Loeb suggesting what he called a (non-peer-reviewed) “correction” to the comet idea. Loeb wrote:
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.
‘Oumuamua by the numbers
In other words, Loeb is standing by his assertion for the possibility that ‘Oumuamua isn’t a natural object. He continued, with a numerical explanation:
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 the ‘Methods’ section to their new study], 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 …
Hydrogen and propulsion
And he explained why flawed numbers presented in the comet study would lead to a flawed conclusion:
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.
Interest in ‘Oumuamua
Loeb had said a day earlier in an essay he wrote for Medium on March 22:
Altogether, it is fantastic to see the continuing interest within the mainstream of astronomy to explain the anomalous acceleration of `Oumuamua, more than 5 years after its discovery.
Bottom line: A March 22, 2023, study suggests that the object known as ‘Oumuamua, which swept through our solar system in 2017, was a comet from another solar system. Harvard astronomer Avi Loeb – whose 2021 book Extraterrestrial suggests the possibility that ‘Oumuamua might be an alien spacecraft – emailed EarthSky with a “correction” to that study. Loeb’s new book Interstellar is due out in August of 2023.