Oumuamua
There are two theories on the mysterious object Oumuamua. Scientist Avi Loeb claims it is extraterrestrial, another, after their first hydrogen iceberg claim was debunked, hydrogen bubbles forming in regular ice. The latter theory starts with the fact that H2 outgassing is not currently detected by any latest device, and hence it could be the culprit for the acceleration witnessed with Oumuamua. Both views are presented below.
Extraterrestrial
Extraterrestrial, Avi Loeb
For starters, prior to Oumuamua’s discovery, no confirmed interstellar object had ever been observed in our own solar system. That alone made Oumuamua historic, and it was enough to draw many astronomers’ attention..
And once we had our telescopes trained on Oumuamua, we looked for one bit of information in particular: how Oumuamua reflected sunlight. Our Sun acts like a lamppost that illuminates not only all the planets orbiting it but every object that comes close enough.. [A]ny two objects will rotate relative to each other when they pass... Anything other than a sphere, however, will reflect the Sun’s light by varying amounts as the object rotates. A football, for example, will reflect more light when one of its long sides faces the Sun and less light when, as it tumbles, its narrow sides face the Sun. For astrophysicists, an object’s changing brightness provides invaluable clues to its shape. In the case of Oumuamua, the object’s brightness varied tenfold every eight hours, which we deduced to be the amount of time that it took to complete one full rotation. This dramatic variability in its brightness told us that Oumuamua’s shape was extreme, or at least five to ten times longer than it was wide. This would make Oumuamua’s geometry more extreme.. than the most extreme asteroids or comets that we have ever seen...
[T]he object’s relatively small but extreme dimensions—with a length at least five to ten times greater than its width—allowed only two possible shapes. Our interstellar visitor was either elongated, like a cigar, or flat, like a pancake.. Either way, Oumuamua was a rarity. If it was elongated, we had never seen any naturally occurring space object that size and that elongated; if it was flat, we had never seen any naturally occurring space object that size and that flat.
And there was more.. In addition to being small and oddly shaped, Oumuamua was strangely luminous.. [with] levels of brightness similar to a shiny metal.
Oumuamua.. trajectory deviated from what was expected based on the Sun’s gravitational force alone. There was no obvious explanation for why. This, for me, was the most eyebrow-raising bit of data.. The universal laws of physics allow us to predict with certainty what a given object’s trajectory should be as it speeds around the Sun. But Oumuamua didn’t behave as we expected.
Comets [can] show a [similar] deviation.. The comet’s evaporating ice acts like a jet that pushes the comet. Because of that rocket effect, an outgassing comet can deviate from a path shaped by the Sun’s gravitation alone. Indeed, when astronomers observe such a comet, we can be precise. When we see an outgassing comet and measure the extent of its deviation, we can calculate how much of the comet’s mass was used up in giving it this extra push. But deep observations of the space around Oumuamua did not reveal any trace of water, carbon-based gases, or dust, ruling out the possibility that it was being pushed by cometary vapor or visible dust particles. Moreover, it did not change its spin rate as it should have if one-sided jets were pushing it sideways, as they often do in comets...
And then there is the evidence that the extra force acting on Oumuamua, the force that was causing it to deviate, declined in inverse proportion to the square of Oumuamua’s distance from the Sun. If the extra force were the result of outgassing, we would expect a faster deceleration of an object as it rapidly distances itself from the Sun.. When I read reports that the extra force on Oumuamua declined inversely with the distance from the Sun squared, I wondered what could be pushing it if not outgassing or disintegration. The only explanation that came to mind was the sunlight bouncing off its surface like wind off a thin sail.
Hydrogen Gas
[L]ab experiments done as far back as the 1970s showed that when ice is struck by high-energy particles, its molecules can break apart, leaving tiny bubbles of hydrogen gas trapped several meters deep in the ice. “A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result"...
"As the ice is warmed, it rearranges to a more stable and compact structure.. [t]his process.. leads to the collapse of these pockets and the formation of channels within the ice, through which trapped gas can escape... For a normal-size comet, this release of gas would have a negligible effect.. But because Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration"