We still don’t know what the interstellar object ‘Oumuamua is for certain, but we have one more piece of information about what it is not.
To catch you up on the story so far, back in October 2017, a strange object moving at a remarkable 196,000 mph was spotted zipping through our galaxy. While it originally was thought to be an interstellar asteroid, it’s motion was seen to change in ways that indicated something had to be outgassing, a behavior closer to what we expect from comets. This object didn’t have the carbon-based molecules we’re used to seeing from outgassing comets. This led to some people speculating it is an alien spaceship, others speculating that it is a hydrogen iceberg, and a lot of us admitting that the universe is far more creative in how it creates than we are in anticipating.
‘Oumuamua doesn’t have any of the infrared heat or energy signatures we’d expect from a spacecraft, so astronomers explored the possibility of it being a hydrogen iceberg.
I’m going to admit, prior to today, I had never heard of a hydrogen iceberg.
Apparently, some folks have theorized that deep in molecular clouds it is possible for hydrogen gas to freeze onto dust grains and build up over time to form large structures that could be the missing dark matter in our universe. If this is the case, these hydrogen icebergs should be scattered just about everywhere, and more should be plunging through our solar system for the future Rubin Observatory to observe.
A new paper by Thiem Hoang and Avi Loeb finds that the conditions in giant molecular clouds aren’t actually conducive to forming hydrogen icebergs, and if they could somehow form, then regular old starlight would be sufficient to eat away at any hydrogen icebergs that emerged from the dark depths of these clouds.
The idea of hydrogen icebergs is a bad idea, and we can all move on with the knowledge that they explain neither ‘Oumuamua nor dark matter. This means that we have no way to explain ‘Oumuamua.
I really hope we see a lot more ‘Oumuamua-like objects once the Rubin Observatory starts surveying the sky. This thing is weird, we don’t have enough data to sort it out, and this is exceedingly unsatisfying.
“Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 (‘Oumuamua),” Thiem Hoang & Abraham Loeb, 2020 Aug. 17, Astrophysical Journal Letters (Preprint on arxiv.org)