Oumuamua is gone now. It is past Neptune’s orbit, and is far enough out that no telescope
can study it ever again. Its origins and nature remain a mystery that still stokes
hot debate in the academic community to this day. Was it a fragment of a broken planet,
flung our way from a distant star? Was it a piece of alien technology, either accidentally
or deliberately sent to our solar system by an enigmatic intelligence? Surprisingly, or perhaps
unsurprisingly, there are still academics who put forward both theories as plausible.
Oumuamua’s anomalous nature makes it difficult to classify as either a comet, an asteroid,
or something artificial. But it might be important to reach such a classification, as 6 new objects
have been detected in our solar system that share Oumuamua’s most bewildering trait – its
ability to accelerate, with no clear explanation for where such acceleration is coming from.
I’m Alex McColgan, and you’re watching Astrum. And today we will be exploring Oumuamua and these 6
new fittingly named “dark comets”, and exploring the theories that have arisen in the last few
years as scientists attempt to explain something that has so far defied clear explanation.
As a quick recap: Oumuamua is the first recognised interstellar object to arrive in our solar system.
It did so on the 19th of October 2017, and was spotted by the Haleakalā Observatory in Hawaii.
Scientists quickly noticed that there was something strange about this object. It was
small – perhaps between 100 and 1000m long, and was believed to have an unusual shape;
perhaps a cigar or a flat dish. Its trajectory and speed made it clear that it would eventually
leave our solar system, so scientists reasoned that it must not have come from here. The object
was thus given the Hawaiian name “Oumuamua”, meaning “scout” or “first distant messenger”.
As the first of its kind that we know about (although admittedly definitely not the first
interstellar object to pass through our solar system, as there are trillions of possible
candidates out there that likely have done the same), scientists were excited to study Oumuamua,
to learn what characteristics it might have that made it similar or dissimilar to objects found in
our own solar system. As they did so they began to notice certain abnormalities. While Oumuamua was
initially classified as a comet, it became apparent that Oumuamua lacked a coma, or cometary
tail, making it more similar to an asteroid. However, this lack of coma became a puzzling issue
when Oumuamua was seen accelerating away from the Sun, on its slow way out of our solar system.
The rate of acceleration was minor – only about 17m per second when it was nearest the Sun – and
yet this was enough to cause a stir in academia. Oumuamua was not doing what physics said it should
do. In the few months scientists were able to observe it, Oumuamua was deviating from its
path . In physics, objects can only accelerate when they are pushed. And so, scientists began
to try and explain what was pushing Oumuamua. A few initial hypotheses were quickly ruled out.
This did not seem to be simple solar winds giving a small nudge. While it is a recorded phenomenon
for the small trace particles fired off from the Sun to push at objects in space, this small force
was not enough to explain Oumuamua’s acceleration, assuming it was an ordinary asteroid.
I should note that this is an assumption, as even the best photos of Oumuamua only
show a tiny speck, making it difficult to say for sure what it looks like.
Most theories about its shape come from the variations in its light curve, the brightness
of which rose and fell uniformly as Oumuamua travelled. This wouldn’t happen for a
round object, but would happen for a tumbling irregularly shaped object like a disk, or a cigar.
Scientists reached for another example in our solar system of accelerating objects – comets.
As comets travel close to the Sun, the ice within them warms and sublimates,
turning into gas and spouting off from the comet’s main body. This outpouring of gas and dust forms
the comet’s signature tail, but it also gives the comet a little push, acting like a little thruster
on the side nearest the Sun that accelerates the comet away from the source of all that heat.
But as I mentioned, scientists could not detect all that dust and gas.
They looked, but it didn’t seem to be there. This absence gave rise to more exotic theories.
Let’s take a look at an argument between two theorists, with two theories.
The first theory was the most headline-catching. Harvard professor Avi Loeb promoted in numerous
papers that Oumuamua could represent alien technology. He argued in 2018 that solar winds
could provide the acceleration seen with Oumuamua, but only if Oumuamua was actually much thinner
than scientists originally assumed: between 0.3 and 0.9mm thin. As a 1000m long, 1mm thin object
was unlikely to appear in nature, Loeb argued that this had to mean it was artificial – a light sail,
created to catch solar winds and use them to accelerate through space from one star to another.
This theory met resistance from other members of the academic community. Darryl Seligman,
our second theorist, and a postdoctoral researcher at Cornell University, countered by co-authoring a
paper in 2020 that said that perhaps the reason no outgassing was detected from Oumuamua was because
Oumuamua was emitting an invisible gas such as hydrogen. This would not have been detectable
using the telescopes that were trained on Oumuamua. Seligman proposed that Oumuamua
was entirely or largely made of such hydrogen – a hydrogen iceberg, that was sublimating thanks to
the warmth of the Sun and it was that sublimation that was creating the push. Loeb disagreed. A few
months later he co-wrote a paper asking where exactly this hydrogen iceberg could have come
from? He showed mathematically that the starlight in the interstellar vaccum was warm enough that
any hydrogen iceberg that formed in even the nearest dense molecular clouds would have melted
before they got here. Loeb was still convinced that an alien explanation was the most probable.
His refutation was strong enough to send Seligman back to the drawing board,
who dropped the hydrogen iceberg idea. However, Seligman continued to play around
with the idea that Oumuamua had been moved by escaping pure hydrogen gas.
Initially, he didn’t have an explanation for how this could be, until in 2023 he met with
University of California assistant professor Jennifer Bergner, who pointed to experiments in
labs where water ice in extremely cold conditions hit with radiation could trap pockets of hydrogen,
only to release it later when warmed up as the ice structure rearranged itself. As it happened,
water ice is much more plentiful in space, and so is radiation – cosmic radiation could be enough
to provide the pre-baking that would be needed. Between the two of them, Seligman and Bergner
wrote a paper arguing that Oumuamua needed a new category entirely. It wasn’t a regular comet,
or an asteroid, but rather a “dark comet” – one with a coma that was invisible, but present. Their
explanation accounted for Oumuamua’s acceleration, and also for the lack of dust, as dark comets
would not need to release dust as they were simply reconfiguring their structures and releasing the
pockets of invisible gas, rather than blasting gas from out its surface like a small, gassy volcano.
While this was not enough to convince Loeb, who co-authored two more papers in the next month
that accused Seligman of bad maths, while also continuing to push his alien spaceship model,
Seligman was already considering the next step in his own logic. He began to wonder – if Oumuamua
represented a dark comet, could there be other dark comets out there? He, Bergner, and others
began pouring through the data of objects already existing in our solar system. They might not
be interstellar, but was anything else in the solar system accelerating when it shouldn’t be?
Sure enough, they found 6 that matched their criteria. 6 objects that showed non-gravitational,
non-solar-wind-based acceleration that couldn’t be explained by any known mechanism.
These objects were small - some as tiny as just 3 metres across. They looked like asteroids, and
didn’t have remarkable features. They were near Earth objects – all orbiting close enough to Earth
that missions to them were extremely viable. And they were all exhibiting signs of acceleration.
To be clear, such acceleration was very minor – small enough to have been overlooked previously.
These objects aren’t zipping around the solar system from planet to planet like spaceships.
They are not interstellar objects. But just like Oumuamua, science cannot currently account for
their motion, especially given that they do not have visible signs of outgassing.
And intriguingly, one of them is already scheduled to be visited by 2031.
1998 KY26 is due to be visited by the Japanese Hayabusa2 probe, an asteroid sample-return mission
that was launched in 2014 and finished its primary mission 6 years later, but since has been given a
mission extension to visit other asteroids in the Near Earth Apollo group. 1998 KY26 is rotating
quickly – once every 10 minutes – and Hayabusa2 will aim to perform a flyby to learn more about
this water-rich tiny object for the benefit of future human missions to Mars. Once it’s there,
perhaps it will become clearer what the source of 1998 KY26’s strange acceleration might be.
It’s still not obvious who among all the various scientists out there is right about Oumuamua.
Given that it’s now out of our reach, perhaps we will never know. But it’s undeniably
intriguing that more objects exhibiting strange acceleration have been detected, and highly
likely that they will shed further insights into Oumuamua’s possible nature and origins.
If they are found to accelerate through invisible outgassing of hydrogen, Seligman will stand
validated. But if a little hatch opens up and a small alien form peeks out to wave at us, before
accelerating off out of the solar system, then we might regret not listening more closely to Loeb.
I think the former is more likely than the latter, as there is no way 1998 KY26 is a light sail;
we have excellent imaging of this one. But either way, it will be fascinating to study.
Of course, in the end, it might prove that neither theory is correct. That is the wonder of science.
The more we explore the universe, the more we encounter strange and unexpected phenomena,
and as we learn more about them the better our theories become. Perhaps one day we will encounter
more objects like Oumuamua from outside our solar system, which may lend further weight to a
particular explanation. The search, even for dark objects that currently act under invisible forces,
always fills me with a marvellous curiosity. Once 2031 rolls around, perhaps that itch for answers
will be scratched. Or perhaps we will be simply faced with more questions. Only time will tell.
Can you throw a ball powerfully enough that you could turn around
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This somewhat tongue-in-cheek question is actually a valid thought experiment that lets you explore
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Thanks for watching! If you missed the first Oumuamua video, check it out here. And thanks
to my patrons and members for your support. If you want access to these perks and have
your name added to this list, check the links below. All the best and see you next time.