This video is sponsored by Magellan TV. Welcome to Launch Pad, I'm Christian Ready, your friendly
neighborhood astronomer. In 2017, 'Oumuamua became the first observed interstellar object to
pass through our Solar System. That alone would have made it one for the books, but it didn't
behave like anything we'd seen before. Several ideas were put forward to explain its origin,
including as a chunk of hydrogen ice, a giant fractal snowflake, and even a discarded lightsail.
But none of these satisfactorily explained Omamo's origin.
Especially the alien lightsail. Sorry. But now a team of astronomers may have finally
figured it all out. 'Oumuamua wasn't exotic at all, it was just a fragment of nitrogen
ice that was shattered from an icy Pluto-like dwarf planet in a young star system in the
Perseus arm of the Galaxy. and ejected into interstellar space half a billion years ago.
OK, that does sound kind of exotic. But as we'll see, this new analysis not only explains
'Oumuamua's origin and strange behavior, but suggests that these kinds of objects may be more
common than previously thought. In other words, 'Oumuamua, maybe the first detection of a sample
of an exoplanet brought to us. So today we're going to talk about how 'Oumuamua's origin can
be explained as a chip off the old exo-Pluto, but first I would like to thank Magellan TV, who are
very kindly sponsoring today's video. Magellan curates award-winning documentaries on history,
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'Oumuamua was discovered in October 2017. It had already made its closest
approach to the Sun and was heading out from the inner Solar System.
Working backward, it was clear that it was on a hyperbolic orbit.
In other words, it came from interstellar space and buzzed the Sun, never to return. What an
exciting observation that was! I mean, it was our first recorded interstellar object! But everything
about 'Oumuamua just seem to defy explanation. Its light curve suggested it was shaped either like
a cigar or a pancake with axis ratios between 5:1 all the way up to 10:1. Even the hamburger-shaped
Ultima Lobe of Arrokoth has an axis ratio of about 2:1, and that was considered surprisingly flat.
'Oumuamua's access ratio, at the very least, was more than twice as extreme.
When the Spitzer Space Telescope tried to observe 'Oumuamua in the infrared. It didn't see anything,
and that meant 'Oumuamua couldn't have been more than a couple of 100 meters across,
or else Spitzer would have picked up its thermal emission.
But in order to be that small and still be visible in optical telescopes,
it had to be very reflective. That is, it needed to have a high albedo.
But the weirdest thing about 'Oumuamua was that it actually
gained acceleration as it was leaving the Solar System.
Now to be clear, 'Oumuamua was slowing down as it was escaping the Suns gravity,
but it wasn't slowing down enough. And, it was also moving off of its predicted course
as if something were pushing on it. This extra push is called "non-gravitational acceleration".
It's fairly common with comets, which emit jets of gas and dust when warmed by the Sun.
These jets, act aslow thrust rockets and give comets a little extra push.
But as if to demonstrate that 'Oumuamua was not a comet,
a second interstellar visitor - 2I/Boriov was discovered in August of 2019. This object was
very much a comet. The only thing that gave away its interstellar origin was its hyperbolic orbit.
The fact that these two objects were discovered within two years of each other
implies they must be part of a population of perhaps millions of interstellar objects
passing through the Solar System. But while Borisov was very much a comment,
'Oumuamua was very much not, and it wasn't an asteroid either. It was a thin, broad,
shiny, reddish, tumbling object that somehow accelerated on its way out of the Solar System.
So, researchers considered alternative hypotheses to explain its strange behavior.
Shmuel Bialy and Avi Loeb showed that 'Oumuamua's tumbling, acceleration, and lack of outgassing
could be explained if it wasn't a natural object at all, but rather a discarded lightsail.
Made by aliens.
Now I would love it if that was the case. I mean aliens, right? How cool would that be?
But it's not a very convincing hypothesis for a number of reasons. Among them is that it seems
pretty unlikely that the very first interstellar object we discover just happens to be an
alien lightsail. Besides, the door was still wide open for natural, if unconventional explanations.
For example, Amaya Moro-Martín showed that sunlight pressure
would accelerate 'Oumuamua if it were a fractal snowflake.
Such flakes are thought to form in the outer regions of protoplanetary disks where it's cold
enough, but it's not clear if fractal snowflakes could survive ejection from their parents systems,
or collisions with interstellar dust grains, or even just spinning up to
'Oumuamua's observed rotation rate. So other researchers considered a different approach:
perhaps. 'Oumuamua was a large fragment of ice?
When ice is exposed to sunlight in dry conditions, it sublimates,
going straight from a solid to a gas space is very dry. As ice sublimates off the surface,
it would act as a weak thruster and 'Oumuamua would accelerate in response.
Darryl Seligman and Gregory Laughlin considered hydrogen ice.
Hydrogen sublimates very rapidly, so even a little bit gives a lot of acceleration.
They found that of hydrogen ice covered just 6% of 'Oumuamua's surface, it would provide
the observed acceleration. However, hydrogen freezes at 14 Kelvin. Such temperatures are
only reached in the cores of dense molecular clouds where starlight never penetrates. It's
not clear how an object gets ejected from such an environment, or even how it retains its ice once
exposed to the interstellar medium, but perhaps other kinds of ices would explain 'Oumuamua.
To that end, Alan Jackson and Steven Desch developed a sophisticated model to calculate the
rates of mass loss and corresponding acceleration on a pancake-shaped, oblate spiroid. As an aside,
many astronomers think, 'Oumuamua was probably pancake-shaped because a cigar 'Oumuamua would
have to have had a very specific spin orientation to reproduce the observed light curve. A pancake
on the other hand, can create the observed light curve over a wider range of orientations.
So chances are that 'Oumuamua is likely pancake-shaped, with access ratios of about 6:1.
Jackson and Desch calculated the acceleration for different kinds of ices over a range of
size and albedo combinations. In other words, 'Oumuamua could have been small and shiny,
or it could have been larger and darker. Only, not too large or else Spitzer would have seen it.
Then they compared the computed accelerations to 'Oumuamua's observed acceleration.
Water, CO2, ammonia, and oxygen ices were immediately ruled out because
they couldn't accelerate 'Oumuamua fast enough. By contrast, hydrogen ice provides so much
acceleration that 'Oumuamua would have had to have been very small with an albedo close to 1.
Neon sublimates at just 9 kelvin,
so 'Oumuamua would have lost its neon ice long before it arrived in the solar system.
Carbon monoxide is ruled out because its sublimation temperature is high
enough that Spitzer likely would have seen it.
Methane ices are a thing and we've detected them on Pluto
in trace amounts. And the methane we did find on Pluto was dissolved in nitrogen ice.
But nitrogen ice, on the other hand, exists in large quantities on Pluto.
In fact, the large Sputnik Basin is a seaC of rolling dunes of nitrogen ice. It's also found
on Neptune's moon, Triton, and in other Kuiper belt objects. And it can give 'Oumuamua the right
acceleration in two circumstances. The first is if 'Oumuamua had a slightly larger radius,
but a low albedo of 0.1, the second is a smaller radius and a higher albedo of 0.64.
Remarkably, this higher albedo is very close to those of Pluto and Triton,
our Solar System's very own nitrogen ski resorts! But all of this raises
an important question: would nitrogen ice survive a close flyby of the Sun?
The find out Jackson and Desch modeled 'Oumuamua's close encounter. At perihelion,
'Oumuamua closed to within 0.255 AU. That's a quarter of the distance from the Earth to the Sun.
At that distance, 'Oumuamua would have lossed nitrogen ice at a very high rate.
However, evaporating ice carries away heat and actually cools the ice left behind.
This is a phenomenon called "evaporative cooling. Despite being closer to the Sun than Mercury.
'Oumuamua's surface temperature remained closer to Pluto's. but keeping cool came at a very high
cost. By the time 'Oumuamua was discovered, it was down to just 8% of its original mass.
However, this dramatic loss of ice is what gave, 'Oumuamua its extreme shape.
Prior to the encounter, 'Oumuamua would have been larger and thicker,
with an axis ratio of around 2:1. As it lost mass it shrank and its axis ratio became more extreme,
going all the way up to 6:1. In other words, it became smaller
and thinner, much like the way a bar of soap turns into a thin sliver as it's used over time.
Even after it past the Sun, it continued to lose mass,
albeit at a slower rate, just enough to give 'Oumuamua its observed acceleration.
Jackson and Ash even wondered if 'Oumuamua might have lost some of its ice before the encounter
while it was still in interstellar space. They found that Galactic Cosmic Rays
would erodes the ice by a significant amount. Cosmic Rays are charged particles like hydrogen
and helium nuclei that are accelerated to the 10 to 100 mega electronvolt range.
That's enough to erode nitrogen ice at an average rate of 6 1/2 meters per billion years.
However, that's just the rate of Cosmic Ray erosion in the Sun's neighborhood today.
Currently the Sun lives between two spiral arms of the Galaxy. Spiral arms are where stars form
and star forming regions can generate about four times as many Cosmic Rays.
Over a billion years, 'Oumuamua likely would have passed through at least one,
perhaps two spiral arms. During those periods, 'Oumuamua would have lost ice twice as rapidly.
On top of that, star formation was much higher in the past than it is today.
The Galaxy experienced multiple surges of star formation over the last 8 billion years,
probably due to repeated collisions with the Sagittarius dwarf Galaxy.
Each wave of star formation would have increased the flux of Cosmic Rays
throughout the Galaxy before settling down to the present day rate.
Our Solar System formed four and a half billion years ago. If 'Oumuamua were the same age,
it would have lost about 260 meters of nitrogen ice along the large axes.
That's a lot of ice! But how long was 'Oumuamua really wandering through space? Well,
nobody knows for sure, but its velocity suggests it probably wasn't that long.
Given 'Oumuamua's exit velocity from our Solar System, astronomers were able to backtrack its
path and work out that while it was traveling through interstellar space, it couldn't have been
moving at more than about 9 kilometers per second. That's comparable to the velocities of young stars
within the Galaxy. So, 'Oumuamua was likely ejected out of a young planetary system, which
means it couldn't have been traveling through the Galaxy for more than 2 billion years at the most.
Two billion years is long enough for 'Oumuamua to have made several
spiral arm crossings and lose a lot of mass in the process.
So much that it would have had to have lost 90% of its mass just to reach the Solar System.
And that's not impossible, but it seems kind of unlikely. On the other hand, Jackson and
Desch found that if 'Oumuamua were ejected half a billion years ago, it only would have
needed to have lost roughly half its mass. That's still a lot, but it's a little more plausable.
So a fragment of nitrogen ice that shrank down to 45 by 44 by 7 1/2 meters at the time of discovery
would explain 'Oumuamua's albedo, its non-gravitational acceleration,
and its lack of carbon monoxide, CO2, or dust.
And nitrogen ice is not particularly rare. We have plenty of it right here
in the Solar System. 'Oumuamua might be uncommon, but it's hardly exotic.
It's probably just a fragment of an exo-Pluto.
But how does a fragment of an exo-Pluto get chucked into interstellar space in the 1st place?
Well, Jackson and Desch published a second paper that investigates this in considerable detail.
Overachievers.
They started by considering the history of our Solar System. Models show that billions of years
ago, the primordial Kuiper belt was much more massive, totaling about 35 Earth masses. But
interactions with giant planets - I'm looking at you, Jupiter - ejected most
of this mass into the Oort cloud, and even into interstellar space. Today's Kuiper Belt is around
1/10th of 1% of its original mass. That means the primordial Kuiper belt would have had about
3000 Pluto sized objects and millions more regular Kuiper belt objects, or "KBOs".
The migration of the giant planets turned the Kuiper Belt into a demolition Derby,
with Plutos and KBOs hurtling into each other, shattering into small fragments of ice. We have
evidence of these kinds of collisions. Pluto's Sputnik plains - you know, the one filled with
nitrogen ice - is likely an impact basin that formed in a collision with another small body.
That impact would have liberated large chunks of nitrogen ice.
Many of the fragments would have collided with other bodies and shattered into even
smaller pieces that would just evaporate in the sunlight as they swung past Jupiter.
But most of the survivors would be ice fragments around 50 meters in diameter,
half of which were made of nitrogen and the other half water ice. The rest would be larger comets
and they would settle into the ORT cloud or be ejected into interstellar space, never to return.
Now that's just the early history of our Solar System,
but we've seen evidence of planetary migration and scattering in other systems. For example,
a "Planet Nine" was discovered in the outskirts of a protoplanetary disk,
presumably after a close encounter with the binary star system at its center.
Such migrations and the resulting scattering are likely very common in young planetary systems.
Jackson and Desch estimate that our Solar System likely ejected around 100 trillion ice fragments
alone. That would mean interstellar space is teeming with ice fragments and comets
thrown out of their planetary systems. Nitrogen ice fragments would be the most vulnerable to
Cosmic Rays; most of them might only survive for about a billion years, whereas
water ice fragments fare a little better, surviving for around 3 billion years.
Now given their lower survival rate,
nitrogen fragments might make up just 10% of the fragments reaching our Solar System. But
nitrogen ice has a much higher albedo than water, so it's easier to detect.
Comets are even more rare because they are more massive, an harder to toss out
of their home systems. But they're also larger and brighter and are therefore
easier to detect. So perhaps then, it's not surprising that the first
two interstellar objects we discovered was a nitrogen ice fragment and a comet.
If 'Oumuamua were ejected half billion years ago and was traveling at a rate of nine kilometers
per second, then it would've traversed over 3.6 kiloparsecs of the Galaxy. Given that distance and
its direction of approach, it likley originated somewhere in the Perseus arm of the Galaxy,
which just happens to host a number of young star systems.
Granted, all of this is based on the model. But it's a detailed model that so far appears
to be a very plausable explanation for 'Oumuamua's strange behavior.
Unfortunately, 'Oumuamua is gone. We'll probably never know just what it was, but this analysis
predicts that there should be many more such fragments of both nitrogen and water ice
passing through our Solar System. The Vera C. Rubin Survey Telescope will be able to test
this prediction by scanning the entire night sky,
night after night. It will catch anything moving all the way down to 27th magnitude.
That should be fine enough to detect both nitrogen and water ice fragments.
If Jackson and Desch right, then we'll soon be studying bits of exoplanets as they float on by.
Or alien hardware. Whatever.
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