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, art, nature and of course space and science. For
example, "Planet Hunters: the Search for Earth's Twin" talks about the search for a planet just
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claim your discounted annual membership today. '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. My thanks as always to my Patreon supporters
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stay healthy, and stay curious, my friends.
Spoiler alert - part of an exo-planet. I doubt the SOLVED nomenclature is warranted.
And by "solved", they mean a hypothesis has been proposed that fits the (meager) observations.
Holy crap, it's not someone being edgy and saying it's aliens
TLDW: Its "weird" behavior can be explained by it being made of nitrogen ice.